I have an ARM project, where I would like to keep certain unused variables and their data, until the time they are used.
I have seen prevent gcc from removing an unused variable :
__attribute__((used)) did not work for me with a global variable (the documentation does imply it only works on functions) (arm-none-eabi gcc 7), but putting the symbol in a different section via __attribute__((section(".data"))) did work. This is presumably because the linker's is only able to strip symbols when they are given their own section via -fdata-sections. I do not like it, but it worked.
So, I tried this approach, but the variables were not kept - and I think this is because something in that project enables -Wl,--gc-sections during linking. Here is a minimal example showing what I've tried to do (basically the main file only refers to the header where the variables to be "kept" are declared as extern - and other than that, main program has does not use these variables; and then those same variables are defined in a separate .c file):
test.c
#include <stdio.h>
#include "test_opt.h"
const char greeting[] = "Hello World - am used";
int main(void) {
printf("%s!\n", greeting);
return 0;
}
test_opt.h
#include <stdint.h>
extern const char mystring[];
struct MyStruct {
uint16_t param_one;
uint8_t param_two;
unsigned char param_three[32];
};
typedef struct MyStruct MyStruct_t;
extern const MyStruct_t mystruct;
mystruct.c
#include "test_opt.h"
const char __attribute__((section(".MYSTRING"))) mystring[] = "Me, mystring, I am not being used";
const MyStruct_t __attribute__((section(".MYSTRUCT"))) mystruct = {
.param_one = 65535,
.param_two = 42,
.param_three = "myStructer here",
};
Test with usual MINGW64 gcc
Let's first try without -Wl,--gc-sections:
$ gcc -Wall -g mystruct.c test_opt.c -o test_opt.exe
$ strings ./test_opt.exe | grep -i 'mystring\|mystruct'
Me, mystring, I am not being used
*myStructer here
mystring
MyStruct
MyStruct_t
mystruct
mystruct.c
mystruct.c
mystruct.c
mystruct.c
mystring
mystruct
.MYSTRING
.MYSTRUCT
.MYSTRING
.MYSTRUCT
Clearly, variables and content are visible here.
Now let's try -Wl,--gc-sections:
$ gcc -Wall -g -Wl,--gc-sections mystruct.c test_opt.c -o test_opt.exe
$ strings ./test_opt.exe | grep -i 'mystring\|mystruct'
mystring
MyStruct
MyStruct_t
mystruct
mystruct.c
mystruct.c
mystruct.c
mystruct.c
mystring
mystruct
Apparently, here we still have some symbol debugging info left - but there are no sections, nor data being reported.
Test with ARM gcc
Let's re-do same experiment with ARM gcc - first without -Wl,--gc-sections:
$ arm-none-eabi-gcc -Wall -g test_opt.c mystruct.c -o test_opt.elf -lc -lnosys
$ arm-none-eabi-strings ./test_opt.elf | grep -i 'mystring\|mystruct'
Me, mystring, I am not being used
*myStructer here
mystruct.c
MyStruct_t
MyStruct
mystruct
mystruct.c
mystring
mystruct.c
mystring
mystruct
.MYSTRING
.MYSTRUCT
Same as before, variables, content and section names are visible.
Now let's try with -Wl,--gc-sections:
$ arm-none-eabi-gcc -Wall -g -Wl,--gc-sections test_opt.c mystruct.c -o test_opt.elf -lc -lnosys
$ arm-none-eabi-strings ./test_opt.elf | grep -i 'mystring\|mystruct'
Note that, unlike the previous case, here there is neither any data content left, nor any debugging info/symbol names!
So, my question is: assuming that -Wl,--gc-sections is enabled in the project, and I otherwise do not want to remove it (because I like the functionality otherwise), can I somehow specify in code for some special variables, "keep these variables even if the are unused/unreferenced", in such a way that they are kept even with -Wl,--gc-sections enabled?
Note that adding keep to attributes, say:
const char __attribute__((keep,section(".MYSTRING"))) mystring[] = "Me, mystring, I am not being used";
... and compiling with (or without) -Wl,--gc-sections typically results with compiler warning:
mystruct.c:3:1: warning: 'keep' attribute directive ignored [-Wattributes]
3 | const char __attribute__((keep,section(".MYSTRING"))) mystring[] = "Me, mystring, I am not being used";
| ^~~~~
... I guess, because the variables are already declared const if I read that arrow correctly (or maybe because a section is already assumed to be "kept")? So attribute keep is definitely not the answer here ...
To inform linker that some variable needs to be preserved you should use the -Wl,--undefined=XXX option:
gcc ... -Wl,--undefined=greeting
Note that __attribute__((used)) is a compiler-only flag to suppress -Wunused-variable warning.
OK - I found something; not ideal, but at least its just a "syntax hack", and I don't have to come up with stupid stuff to do with the structs just so they show up in the executable (and usually even the code I come up with in that case, gets optimized away :)).
I first tried the (void) varname; hack used for How can I suppress "unused parameter" warnings in C? - I left it below just to show it doesn't work.
What ended up working is: basically, just have a static const void* where the main() is, and assign a pointer to the struct to it (EDIT: in the main()!); I guess because of "static const", the compiler will not remove the variable and its section, even with -Wl,--gc-sections. So test_opt.c now becomes:
#include <stdio.h>
#include "test_opt.h"
const char greeting[] = "Hello World - am used";
static const void *fake; //, *fakeB;
int main(void) {
fake = &mystruct;
(void) &mystring; //fakeB = &mystring;
printf("%s!\n", greeting);
return 0;
}
... and we can test with:
$ arm-none-eabi-gcc -Wall -g -Wl,--gc-sections test_opt.c mystruct.c -o test_opt.elf -lc -lnosys
$ arm-none-eabi-readelf -a ./test_opt.elf | grep -i 'mystring\|mystruct'
[ 5] .MYSTRUCT PROGBITS 00013780 013780 000024 00 A 0 0 4
01 .init .text .fini .rodata .MYSTRUCT .ARM.exidx .eh_frame
5: 00013780 0 SECTION LOCAL DEFAULT 5 .MYSTRUCT
379: 00000000 0 FILE LOCAL DEFAULT ABS mystruct.c
535: 00013780 36 OBJECT GLOBAL DEFAULT 5 mystruct
$ arm-none-eabi-strings ./test_opt.elf | grep -i 'mystring\|mystruct'
*myStructer here
mystruct.c
MyStruct_t
mystruct
MyStruct
mystruct.c
mystring
mystruct.c
mystruct
.MYSTRUCT
Note that only mystruct in above example ended up being preserved - mystring still got optimized away.
EDIT: note that if you try to cheat and move the assignment outside of main:
static const void *fake = &mystruct, *fakeB = &mystring;
int main(void) {
...
... then the compiler will see through your shenanigans, and greet you with:
test_opt.c:6:39: warning: 'fakeB' defined but not used [-Wunused-variable]
6 | static const void *fake = &mystruct, *fakeB = &mystring;
| ^~~~~
test_opt.c:6:20: warning: 'fake' defined but not used [-Wunused-variable]
6 | static const void *fake = &mystruct, *fakeB = &mystring;
| ^~~~
... and you're none the better off still.
Is there any way to dump the call stack in a running process in C or C++ every time a certain function is called? What I have in mind is something like this:
void foo()
{
print_stack_trace();
// foo's body
return
}
Where print_stack_trace works similarly to caller in Perl.
Or something like this:
int main (void)
{
// will print out debug info every time foo() is called
register_stack_trace_function(foo);
// etc...
}
where register_stack_trace_function puts some sort of internal breakpoint that will cause a stack trace to be printed whenever foo is called.
Does anything like this exist in some standard C library?
I am working on Linux, using GCC.
Background
I have a test run that behaves differently based on some commandline switches that shouldn't affect this behavior. My code has a pseudo-random number generator that I assume is being called differently based on these switches. I want to be able to run the test with each set of switches and see if the random number generator is called differently for each one.
Survey of C/C++ backtrace methods
In this answer I will try to run a single benchmark for a bunch of solutions to see which one runs faster, while also considering other points such as features and portability.
Tool
Time / call
Line number
Function name
C++ demangling
Recompile
Signal safe
As string
C
C++23 <stacktrace> GCC 12.1
7 us
y
y
y
y
n
y
n
Boost 1.74 stacktrace()
5 us
y
y
y
y
n
y
n
Boost 1.74 stacktrace::safe_dump_to
y
n
n
glibc backtrace_symbols_fd
25 us
n
-rdynamic
hacks
y
y
n
y
glibc backtrace_symbols
21 us
n
-rdynamic
hacks
y
n
y
y
GDB scripting
600 us
y
y
y
n
y
n
y
GDB code injection
n
n
y
libunwind
y
libdwfl
4 ms
n
y
libbacktrace
y
Empty cells mean "TODO", not "no".
us: microsecond
Line number: shows actual line number, not just function name + a memory address.
It is usually possible to recover the line number from an address manually after the fact with addr2line. But it is a pain.
Recompile: requires recompiling the program to get your traces. Not recompiling is better!
Signal safe: crucial for the important uses case of "getting a stack trace in case of segfault": How to automatically generate a stacktrace when my program crashes
As string: you get the stack trace as a string in the program itself, as opposed to e.g. just printing to stdout. Usually implies not signal safe, as we don't know the size of the stack trace string size in advance, and therefore requires malloc which is not async signal safe.
C: does it work on a plain-C project (yes, there are still poor souls out there), or is C++ required?
Test setup
All benchmarks will run the following
main.cpp
#include <cstdlib> // strtoul
#include <mystacktrace.h>
void my_func_2(void) {
print_stacktrace(); // line 6
}
void my_func_1(double f) {
(void)f;
my_func_2();
}
void my_func_1(int i) {
(void)i;
my_func_2(); // line 16
}
int main(int argc, char **argv) {
long long unsigned int n;
if (argc > 1) {
n = std::strtoul(argv[1], NULL, 0);
} else {
n = 1;
}
for (long long unsigned int i = 0; i < n; ++i) {
my_func_1(1); // line 27
}
}
This input is designed to test C++ name demangling since my_func_1(int) and my_func_1(float) are necessarily mangled as a way to implement C++ function overload.
We differentiate between the benchmarks by using different -I includes to point to different implementations of print_stacktrace().
Each benchmark is done with a command of form:
time ./stacktrace.out 100000 &>/dev/null
The number of iterations is adjusted for each implementation to produce a total runtime of the order of 1s for that benchmark.
-O0 is used on all tests below unless noted. Stack traces may be irreparably mutilated by certain optimizations. Tail call optimization is a notable example of that: What is tail call optimization? There's nothing we can do about it.
C++23 <stacktrace>
This method was previously mentioned at: https://stackoverflow.com/a/69384663/895245 please consider upvoting that answer.
This is the best solution... it's portable, fast, shows line numbers and demangles C++ symbols. This option will displace every other alternative as soon as it becomes more widely available, with the exception perhaps only of GDB for one-offs without the need or recompilation.
cpp20_stacktrace/mystacktrace.h
#include <iostream>
#include <stacktrace>
void print_stacktrace() {
std::cout << std::stacktrace::current();
}
GCC 12.1.0 from Ubuntu 22.04 does not have support compiled in, so for now I built it from source as per: How to edit and re-build the GCC libstdc++ C++ standard library source? and set --enable-libstdcxx-backtrace=yes, and it worked!
Compile with:
g++ -O0 -ggdb3 -Wall -Wextra -pedantic -std=c++23 -o cpp20_stacktrace.out main.cpp -lstdc++_libbacktrace
Sample output:
0# print_stacktrace() at cpp20_stacktrace/mystacktrace.h:5
1# my_func_2() at /home/ciro/main.cpp:6
2# my_func_1(int) at /home/ciro/main.cpp:16
3# at /home/ciro/main.cpp:27
4# at :0
5# at :0
6# at :0
7#
If we try to use GCC 12.1.0 from Ubuntu 22.04:
sudo apt install g++-12
g++-12 -ggdb3 -O2 -std=c++23 -Wall -Wextra -pedantic -o stacktrace.out stacktrace.cpp -lstdc++_libbacktrace
It fails with:
stacktrace.cpp: In function ‘void my_func_2()’:
stacktrace.cpp:6:23: error: ‘std::stacktrace’ has not been declared
6 | std::cout << std::stacktrace::current();
| ^~~~~~~~~~
Checking build options with:
g++-12 -v
does not show:
--enable-libstdcxx-backtrace=yes
so it wasn't compiled in. Bibliography:
How to use <stacktrace> in GCC trunk?
How can I generate a C++23 stacktrace with GCC 12.1?
It does not fail on the include because the header file:
/usr/include/c++/12
has a feature check:
#if __cplusplus > 202002L && _GLIBCXX_HAVE_STACKTRACE
Boost stacktrace
The library has changed quite a lot around Ubuntu 22.04, so make sure your version matches: Boost stack-trace not showing function names and line numbers
The library is pretty much superseded by the more portable C++23 implementation, but remains a very good option for those that are not at that standard version yet, but already have a "Boost clearance".
Documented at: https://www.boost.org/doc/libs/1_66_0/doc/html/stacktrace/getting_started.html#stacktrace.getting_started.how_to_print_current_call_stack
Tested on Ubuntu 22.04, boost 1.74.0, you should do:
boost_stacktrace/mystacktrace.h
#include <iostream>
#define BOOST_STACKTRACE_LINK
#include <boost/stacktrace.hpp>
void print_stacktrace(void) {
std::cout << boost::stacktrace::stacktrace();
}
On Ubuntu 19.10 boost 1.67.0 to get the line numbers we had to instead:
#include <iostream>
#define BOOST_STACKTRACE_USE_ADDR2LINE
#include <boost/stacktrace.hpp>
void print_stacktrace(void) {
std::cout << boost::stacktrace::stacktrace();
}
which would call out to the addr2line executable and be 1000x slower than the newer Boost version.
The package libboost-stacktrace-dev did not exist at all on Ubuntu 16.04.
The rest of this section considers only the Ubuntu 22.04, boost 1.74 behaviour.
Compile:
sudo apt-get install libboost-stacktrace-dev
g++ -O0 -ggdb3 -Wall -Wextra -pedantic -std=c++11 -o boost_stacktrace.out main.cpp -lboost_stacktrace_backtrace
Sample output:
0# print_stacktrace() at boost_stacktrace/mystacktrace.h:7
1# my_func_2() at /home/ciro/main.cpp:7
2# my_func_1(int) at /home/ciro/main.cpp:17
3# main at /home/ciro/main.cpp:26
4# __libc_start_call_main at ../sysdeps/nptl/libc_start_call_main.h:58
5# __libc_start_main at ../csu/libc-start.c:379
6# _start in ./boost_stacktrace.out
Note that the lines are off by one line. It was suggested in the comments that this is because the following instruction address is being considered.
Boost stacktrace header only
What the BOOST_STACKTRACE_LINK does is to require -lboost_stacktrace_backtrace at link time, so we imagine without that it will just work. This would be a good option for devs who don't have the "Boost clearance" to just add as one offs to debug.
TODO unfortunately it didn't so well for me:
#include <iostream>
#include <boost/stacktrace.hpp>
void print_stacktrace(void) {
std::cout << boost::stacktrace::stacktrace();
}
then:
g++ -O0 -ggdb3 -Wall -Wextra -pedantic -std=c++11 -o boost_stacktrace_header_only.out main.cpp
contains the overly short output:
0# 0x000055FF74AFB601 in ./boost_stacktrace_header_only.out
1# 0x000055FF74AFB66C in ./boost_stacktrace_header_only.out
2# 0x000055FF74AFB69C in ./boost_stacktrace_header_only.out
3# 0x000055FF74AFB6F7 in ./boost_stacktrace_header_only.out
4# 0x00007F0176E7BD90 in /lib/x86_64-linux-gnu/libc.so.6
5# __libc_start_main in /lib/x86_64-linux-gnu/libc.so.6
6# 0x000055FF74AFB4E5 in ./boost_stacktrace_header_only.out
which we can't even use with addr2line. Maybe we have to pass some other define from: https://www.boost.org/doc/libs/1_80_0/doc/html/stacktrace/configuration_and_build.html ?
Tested on Ubuntu 22.04. boost 1.74.
Boost boost::stacktrace::safe_dump_to
This is an interesting alternative to boost::stacktrace::stacktrace as it writes the stack trace in a async signal safe manner to a file, which makes it a good option for automatically dumping stack traces on segfaults which is a super common use case: How to automatically generate a stacktrace when my program crashes
Documented at: https://www.boost.org/doc/libs/1_70_0/doc/html/boost/stacktrace/safe_dump_1_3_38_7_6_2_1_6.html
TODO get it to work. All I see each time is a bunch of random bytes. My attempt:
boost_stacktrace_safe/mystacktrace.h
#include <unistd.h>
#define BOOST_STACKTRACE_LINK
#include <boost/stacktrace.hpp>
void print_stacktrace(void) {
boost::stacktrace::safe_dump_to(0, 1024, STDOUT_FILENO);
}
Sample output:
1[FU1[FU"2[FU}2[FUm1#n10[FU
Changes drastically each time, suggesting it is random memory addresses.
Tested on Ubuntu 22.04, boost 1.74.0.
glibc backtrace
This method is quite portable as it comes with glibc itself. Documented at: https://www.gnu.org/software/libc/manual/html_node/Backtraces.html
Tested on Ubuntu 22.04, glibc 2.35.
glibc_backtrace_symbols_fd/mystacktrace.h
#include <execinfo.h> /* backtrace, backtrace_symbols_fd */
#include <unistd.h> /* STDOUT_FILENO */
void print_stacktrace(void) {
size_t size;
enum Constexpr { MAX_SIZE = 1024 };
void *array[MAX_SIZE];
size = backtrace(array, MAX_SIZE);
backtrace_symbols_fd(array, size, STDOUT_FILENO);
}
Compile with:
g++ -O0 -ggdb3 -Wall -Wextra -pedantic -rdynamic -std=c++11 -o glibc_backtrace_symbols_fd.out main.cpp
Sample output with -rdynamic:
./glibc_backtrace_symbols.out(_Z16print_stacktracev+0x47) [0x556e6a131230]
./glibc_backtrace_symbols.out(_Z9my_func_2v+0xd) [0x556e6a1312d6]
./glibc_backtrace_symbols.out(_Z9my_func_1i+0x14) [0x556e6a131306]
./glibc_backtrace_symbols.out(main+0x58) [0x556e6a131361]
/lib/x86_64-linux-gnu/libc.so.6(+0x29d90) [0x7f175e7bdd90]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0x80) [0x7f175e7bde40]
./glibc_backtrace_symbols.out(_start+0x25) [0x556e6a131125]
Sample output without -rdynamic:
./glibc_backtrace_symbols_fd_no_rdynamic.out(+0x11f0)[0x556bd40461f0]
./glibc_backtrace_symbols_fd_no_rdynamic.out(+0x123c)[0x556bd404623c]
./glibc_backtrace_symbols_fd_no_rdynamic.out(+0x126c)[0x556bd404626c]
./glibc_backtrace_symbols_fd_no_rdynamic.out(+0x12c7)[0x556bd40462c7]
/lib/x86_64-linux-gnu/libc.so.6(+0x29d90)[0x7f0da2b70d90]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0x80)[0x7f0da2b70e40]
./glibc_backtrace_symbols_fd_no_rdynamic.out(+0x10e5)[0x556bd40460e5]
To get the line numbers without -rdynamic we can use addr2line:
addr2line -C -e glibc_backtrace_symbols_fd_no_rdynamic.out 0x11f0 0x123c 0x126c 0x12c7
addr2line cannot unfortunately handle the function name + offset in function format of when we are not using -rdynamic, e.g. _Z9my_func_2v+0xd.
GDB can however:
gdb -nh -batch -ex 'info line *(_Z9my_func_2v+0xd)' -ex 'info line *(_Z9my_func_1i+0x14)' glibc_backtrace_symbols.out
Line 7 of "main.cpp" starts at address 0x12d6 <_Z9my_func_2v+13> and ends at 0x12d9 <_Z9my_func_1d>.
Line 17 of "main.cpp" starts at address 0x1306 <_Z9my_func_1i+20> and ends at 0x1309 <main(int, char**)>.
A helper to make it more bearable:
addr2lines() (
perl -ne '$m = s/(.*).*\(([^)]*)\).*/gdb -nh -q -batch -ex "info line *\2" \1/;print $_ if $m' | bash
)
Usage:
xsel -b | addr2lines
glibc backtrace_symbols
A version of backtrace_symbols_fd that returns a string rather than printing to a file handle.
glibc_backtrace_symbols/mystacktrace.h
#include <execinfo.h> /* backtrace, backtrace_symbols */
#include <stdio.h> /* printf */
void print_stacktrace(void) {
char **strings;
size_t i, size;
enum Constexpr { MAX_SIZE = 1024 };
void *array[MAX_SIZE];
size = backtrace(array, MAX_SIZE);
strings = backtrace_symbols(array, size);
for (i = 0; i < size; i++)
printf("%s\n", strings[i]);
free(strings);
}
glibc backtrace with C++ demangling hack 1: -export-dynamic + dladdr
I couldn't find a simple way to automatically demangle C++ symbols with glibc backtrace.
https://panthema.net/2008/0901-stacktrace-demangled/
https://gist.github.com/fmela/591333/c64f4eb86037bb237862a8283df70cdfc25f01d3
Adapted from: https://gist.github.com/fmela/591333/c64f4eb86037bb237862a8283df70cdfc25f01d3
This is a "hack" because it requires changing the ELF with -export-dynamic.
glibc_ldl.cpp
#include <dlfcn.h> // for dladdr
#include <cxxabi.h> // for __cxa_demangle
#include <cstdio>
#include <string>
#include <sstream>
#include <iostream>
// This function produces a stack backtrace with demangled function & method names.
std::string backtrace(int skip = 1)
{
void *callstack[128];
const int nMaxFrames = sizeof(callstack) / sizeof(callstack[0]);
char buf[1024];
int nFrames = backtrace(callstack, nMaxFrames);
char **symbols = backtrace_symbols(callstack, nFrames);
std::ostringstream trace_buf;
for (int i = skip; i < nFrames; i++) {
Dl_info info;
if (dladdr(callstack[i], &info)) {
char *demangled = NULL;
int status;
demangled = abi::__cxa_demangle(info.dli_sname, NULL, 0, &status);
std::snprintf(
buf,
sizeof(buf),
"%-3d %*p %s + %zd\n",
i,
(int)(2 + sizeof(void*) * 2),
callstack[i],
status == 0 ? demangled : info.dli_sname,
(char *)callstack[i] - (char *)info.dli_saddr
);
free(demangled);
} else {
std::snprintf(buf, sizeof(buf), "%-3d %*p\n",
i, (int)(2 + sizeof(void*) * 2), callstack[i]);
}
trace_buf << buf;
std::snprintf(buf, sizeof(buf), "%s\n", symbols[i]);
trace_buf << buf;
}
free(symbols);
if (nFrames == nMaxFrames)
trace_buf << "[truncated]\n";
return trace_buf.str();
}
void my_func_2(void) {
std::cout << backtrace() << std::endl;
}
void my_func_1(double f) {
(void)f;
my_func_2();
}
void my_func_1(int i) {
(void)i;
my_func_2();
}
int main() {
my_func_1(1);
my_func_1(2.0);
}
Compile and run:
g++ -fno-pie -ggdb3 -O0 -no-pie -o glibc_ldl.out -std=c++11 -Wall -Wextra \
-pedantic-errors -fpic glibc_ldl.cpp -export-dynamic -ldl
./glibc_ldl.out
output:
1 0x40130a my_func_2() + 41
./glibc_ldl.out(_Z9my_func_2v+0x29) [0x40130a]
2 0x40139e my_func_1(int) + 16
./glibc_ldl.out(_Z9my_func_1i+0x10) [0x40139e]
3 0x4013b3 main + 18
./glibc_ldl.out(main+0x12) [0x4013b3]
4 0x7f7594552b97 __libc_start_main + 231
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xe7) [0x7f7594552b97]
5 0x400f3a _start + 42
./glibc_ldl.out(_start+0x2a) [0x400f3a]
1 0x40130a my_func_2() + 41
./glibc_ldl.out(_Z9my_func_2v+0x29) [0x40130a]
2 0x40138b my_func_1(double) + 18
./glibc_ldl.out(_Z9my_func_1d+0x12) [0x40138b]
3 0x4013c8 main + 39
./glibc_ldl.out(main+0x27) [0x4013c8]
4 0x7f7594552b97 __libc_start_main + 231
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xe7) [0x7f7594552b97]
5 0x400f3a _start + 42
./glibc_ldl.out(_start+0x2a) [0x400f3a]
Tested on Ubuntu 18.04.
glibc backtrace with C++ demangling hack 2: parse backtrace output
Shown at: https://panthema.net/2008/0901-stacktrace-demangled/
This is a hack because it requires parsing.
TODO get it to compile and show it here.
GDB scripting
We can also do this with GDB without recompiling by using: How to do an specific action when a certain breakpoint is hit in GDB?
We setup an empty backtrace function for our testing:
gdb/mystacktrace.h
void print_stacktrace(void) {}
and then with:
main.gdb
start
break print_stacktrace
commands
silent
backtrace
printf "\n"
continue
end
continue
we can run:
gdb -nh -batch -x main.gdb --args gdb.out
Sample output:
Temporary breakpoint 1 at 0x11a7: file main.cpp, line 21.
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Temporary breakpoint 1, main (argc=1, argv=0x7fffffffc3e8) at main.cpp:21
warning: Source file is more recent than executable.
21 if (argc > 1) {
Breakpoint 2 at 0x555555555151: file gdb/mystacktrace.h, line 1.
#0 print_stacktrace () at gdb/mystacktrace.h:1
#1 0x0000555555555161 in my_func_2 () at main.cpp:6
#2 0x0000555555555191 in my_func_1 (i=1) at main.cpp:16
#3 0x00005555555551ec in main (argc=1, argv=0x7fffffffc3e8) at main.cpp:27
[Inferior 1 (process 165453) exited normally]
The above can be made more usable with the following Bash function:
gdbbt() (
tmpfile=$(mktemp /tmp/gdbbt.XXXXXX)
fn="$1"
shift
printf '%s' "
start
break $fn
commands
silent
backtrace
printf \"\n\"
continue
end
continue
" > "$tmpfile"
gdb -nh -batch -x "$tmpfile" -args "$#"
rm -f "$tmpfile"
)
Usage:
gdbbt print_stacktrace gdb.out 2
I don't know how to make commands with -ex without the temporary file: Problems adding a breakpoint with commands from command line with ex command
Tested in Ubuntu 22.04, GDB 12.0.90.
GDB code injection
TODO this is the dream! It might allow for both compiled-liked speeds, but without the need to recompile! Either:
with compile code + one of the other options, ideally C++23 <stacktrace>: How to call assembly in gdb? Might already be possible. But compile code is mega-quirky so I'm lazy to even try
a built-in dbt command analogous to dprintf dynamic printf: How to do an specific action when a certain breakpoint is hit in GDB?
libunwind
TODO does this have any advantage over glibc backtrace? Very similar output, also requires modifying the build command, but not part of glibc so requires an extra package installation.
Code adapted from: https://eli.thegreenplace.net/2015/programmatic-access-to-the-call-stack-in-c/
main.c
/* This must be on top. */
#define _XOPEN_SOURCE 700
#include <stdio.h>
#include <stdlib.h>
/* Paste this on the file you want to debug. */
#define UNW_LOCAL_ONLY
#include <libunwind.h>
#include <stdio.h>
void print_trace() {
char sym[256];
unw_context_t context;
unw_cursor_t cursor;
unw_getcontext(&context);
unw_init_local(&cursor, &context);
while (unw_step(&cursor) > 0) {
unw_word_t offset, pc;
unw_get_reg(&cursor, UNW_REG_IP, &pc);
if (pc == 0) {
break;
}
printf("0x%lx:", pc);
if (unw_get_proc_name(&cursor, sym, sizeof(sym), &offset) == 0) {
printf(" (%s+0x%lx)\n", sym, offset);
} else {
printf(" -- error: unable to obtain symbol name for this frame\n");
}
}
puts("");
}
void my_func_3(void) {
print_trace();
}
void my_func_2(void) {
my_func_3();
}
void my_func_1(void) {
my_func_3();
}
int main(void) {
my_func_1(); /* line 46 */
my_func_2(); /* line 47 */
return 0;
}
Compile and run:
sudo apt-get install libunwind-dev
gcc -fno-pie -ggdb3 -O3 -no-pie -o main.out -std=c99 \
-Wall -Wextra -pedantic-errors main.c -lunwind
Either #define _XOPEN_SOURCE 700 must be on top, or we must use -std=gnu99:
Is the type `stack_t` no longer defined on linux?
Glibc - error in ucontext.h, but only with -std=c11
Run:
./main.out
Output:
0x4007db: (main+0xb)
0x7f4ff50aa830: (__libc_start_main+0xf0)
0x400819: (_start+0x29)
0x4007e2: (main+0x12)
0x7f4ff50aa830: (__libc_start_main+0xf0)
0x400819: (_start+0x29)
and:
addr2line -e main.out 0x4007db 0x4007e2
gives:
/home/ciro/main.c:34
/home/ciro/main.c:49
With -O0:
0x4009cf: (my_func_3+0xe)
0x4009e7: (my_func_1+0x9)
0x4009f3: (main+0x9)
0x7f7b84ad7830: (__libc_start_main+0xf0)
0x4007d9: (_start+0x29)
0x4009cf: (my_func_3+0xe)
0x4009db: (my_func_2+0x9)
0x4009f8: (main+0xe)
0x7f7b84ad7830: (__libc_start_main+0xf0)
0x4007d9: (_start+0x29)
and:
addr2line -e main.out 0x4009f3 0x4009f8
gives:
/home/ciro/main.c:47
/home/ciro/main.c:48
Tested on Ubuntu 16.04, GCC 6.4.0, libunwind 1.1.
libunwind with C++ name demangling
Code adapted from: https://eli.thegreenplace.net/2015/programmatic-access-to-the-call-stack-in-c/
unwind.cpp
#define UNW_LOCAL_ONLY
#include <cxxabi.h>
#include <libunwind.h>
#include <cstdio>
#include <cstdlib>
#include <iostream>
void backtrace() {
unw_cursor_t cursor;
unw_context_t context;
// Initialize cursor to current frame for local unwinding.
unw_getcontext(&context);
unw_init_local(&cursor, &context);
// Unwind frames one by one, going up the frame stack.
while (unw_step(&cursor) > 0) {
unw_word_t offset, pc;
unw_get_reg(&cursor, UNW_REG_IP, &pc);
if (pc == 0) {
break;
}
std::printf("0x%lx:", pc);
char sym[256];
if (unw_get_proc_name(&cursor, sym, sizeof(sym), &offset) == 0) {
char* nameptr = sym;
int status;
char* demangled = abi::__cxa_demangle(sym, nullptr, nullptr, &status);
if (status == 0) {
nameptr = demangled;
}
std::printf(" (%s+0x%lx)\n", nameptr, offset);
std::free(demangled);
} else {
std::printf(" -- error: unable to obtain symbol name for this frame\n");
}
}
}
void my_func_2(void) {
backtrace();
std::cout << std::endl; // line 43
}
void my_func_1(double f) {
(void)f;
my_func_2();
}
void my_func_1(int i) {
(void)i;
my_func_2();
} // line 54
int main() {
my_func_1(1);
my_func_1(2.0);
}
Compile and run:
sudo apt-get install libunwind-dev
g++ -fno-pie -ggdb3 -O0 -no-pie -o unwind.out -std=c++11 \
-Wall -Wextra -pedantic-errors unwind.cpp -lunwind -pthread
./unwind.out
Output:
0x400c80: (my_func_2()+0x9)
0x400cb7: (my_func_1(int)+0x10)
0x400ccc: (main+0x12)
0x7f4c68926b97: (__libc_start_main+0xe7)
0x400a3a: (_start+0x2a)
0x400c80: (my_func_2()+0x9)
0x400ca4: (my_func_1(double)+0x12)
0x400ce1: (main+0x27)
0x7f4c68926b97: (__libc_start_main+0xe7)
0x400a3a: (_start+0x2a)
and then we can find the lines of my_func_2 and my_func_1(int) with:
addr2line -e unwind.out 0x400c80 0x400cb7
which gives:
/home/ciro/test/unwind.cpp:43
/home/ciro/test/unwind.cpp:54
TODO: why are the lines off by one?
Tested on Ubuntu 18.04, GCC 7.4.0, libunwind 1.2.1.
Linux kernel
How to print the current thread stack trace inside the Linux kernel?
libdwfl
This was originally mentioned at: https://stackoverflow.com/a/60713161/895245 and it might be the best method, but I have to benchmark a bit more, but please go upvote that answer.
TODO: I tried to minimize the code in that answer, which was working, to a single function, but it is segfaulting, let me know if anyone can find why.
dwfl.cpp: answer reached 30k chars and this was the easiest cut: https://gist.github.com/cirosantilli/f1dd3ee5d324b9d24e40f855723544ac
Compile and run:
sudo apt install libdw-dev libunwind-dev
g++ -fno-pie -ggdb3 -O0 -no-pie -o dwfl.out -std=c++11 -Wall -Wextra -pedantic-errors dwfl.cpp -ldw -lunwind
./dwfl.out
We also need libunwind as that makes results more correct. If you do without it, it runs, but you will see that some of the lines are a bit wrong.
Output:
0: 0x402b72 stacktrace[abi:cxx11]() at /home/ciro/test/dwfl.cpp:65
1: 0x402cda my_func_2() at /home/ciro/test/dwfl.cpp:100
2: 0x402d76 my_func_1(int) at /home/ciro/test/dwfl.cpp:111
3: 0x402dd1 main at /home/ciro/test/dwfl.cpp:122
4: 0x7ff227ea0d8f __libc_start_call_main at ../sysdeps/nptl/libc_start_call_main.h:58
5: 0x7ff227ea0e3f __libc_start_main##GLIBC_2.34 at ../csu/libc-start.c:392
6: 0x402534 _start at ../csu/libc-start.c:-1
0: 0x402b72 stacktrace[abi:cxx11]() at /home/ciro/test/dwfl.cpp:65
1: 0x402cda my_func_2() at /home/ciro/test/dwfl.cpp:100
2: 0x402d5f my_func_1(double) at /home/ciro/test/dwfl.cpp:106
3: 0x402de2 main at /home/ciro/test/dwfl.cpp:123
4: 0x7ff227ea0d8f __libc_start_call_main at ../sysdeps/nptl/libc_start_call_main.h:58
5: 0x7ff227ea0e3f __libc_start_main##GLIBC_2.34 at ../csu/libc-start.c:392
6: 0x402534 _start at ../csu/libc-start.c:-1
Benchmark run:
g++ -fno-pie -ggdb3 -O3 -no-pie -o dwfl.out -std=c++11 -Wall -Wextra -pedantic-errors dwfl.cpp -ldw
time ./dwfl.out 1000 >/dev/null
Output:
real 0m3.751s
user 0m2.822s
sys 0m0.928s
So we see that this method is 10x faster than Boost's stacktrace, and might therefore be applicable to more use cases.
Tested in Ubuntu 22.04 amd64, libdw-dev 0.186, libunwind 1.3.2.
libbacktrace
https://github.com/ianlancetaylor/libbacktrace
Considering the harcore library author, it is worth trying this out, maybe it is The One. TODO check it out.
A C library that may be linked into a C/C++ program to produce symbolic backtraces
As of October 2020, libbacktrace supports ELF, PE/COFF, Mach-O, and XCOFF executables with DWARF debugging information. In other words, it supports GNU/Linux, *BSD, macOS, Windows, and AIX. The library is written to make it straightforward to add support for other object file and debugging formats.
The library relies on the C++ unwind API defined at https://itanium-cxx-abi.github.io/cxx-abi/abi-eh.html This API is provided by GCC and clang.
See also
How can one grab a stack trace in C?
How to make backtrace()/backtrace_symbols() print the function names?
Is there a portable/standard-compliant way to get filenames and linenumbers in a stack trace?
Best way to invoke gdb from inside program to print its stacktrace?
automatic stack trace on failure:
on C++ exception: C++ display stack trace on exception
generic: How to automatically generate a stacktrace when my program crashes
For a linux-only solution you can use backtrace(3) that simply returns an array of void * (in fact each of these point to the return address from the corresponding stack frame). To translate these to something of use, there's backtrace_symbols(3).
Pay attention to the notes section in backtrace(3):
The symbol names may be unavailable
without the use of special linker
options.
For systems using the GNU linker, it is necessary to use the
-rdynamic linker
option. Note that names of "static" functions are not exposed,
and won't be
available in the backtrace.
In C++23, there will be <stacktrace>, and then you can do:
#include <stacktrace>
/* ... */
std::cout << std::stacktrace::current();
Further details:
• https://en.cppreference.com/w/cpp/header/stacktrace
• https://en.cppreference.com/w/cpp/utility/basic_stacktrace/operator_ltlt
Is there any way to dump the call stack in a running process in C or C++ every time a certain function is called?
You can use a macro function instead of return statement in the specific function.
For example, instead of using return,
int foo(...)
{
if (error happened)
return -1;
... do something ...
return 0
}
You can use a macro function.
#include "c-callstack.h"
int foo(...)
{
if (error happened)
NL_RETURN(-1);
... do something ...
NL_RETURN(0);
}
Whenever an error happens in a function, you will see Java-style call stack as shown below.
Error(code:-1) at : so_topless_ranking_server (sample.c:23)
Error(code:-1) at : nanolat_database (sample.c:31)
Error(code:-1) at : nanolat_message_queue (sample.c:39)
Error(code:-1) at : main (sample.c:47)
Full source code is available here.
c-callstack at https://github.com/Nanolat
Linux specific, TLDR:
backtrace in glibc produces accurate stacktraces only when -lunwind is linked (undocumented platform-specific feature).
To output function name, source file and line number use #include <elfutils/libdwfl.h> (this library is documented only in its header file). backtrace_symbols and backtrace_symbolsd_fd are least informative.
On modern Linux your can get the stacktrace addresses using function backtrace. The undocumented way to make backtrace produce more accurate addresses on popular platforms is to link with -lunwind (libunwind-dev on Ubuntu 18.04) (see the example output below). backtrace uses function _Unwind_Backtrace and by default the latter comes from libgcc_s.so.1 and that implementation is most portable. When -lunwind is linked it provides a more accurate version of _Unwind_Backtrace but this library is less portable (see supported architectures in libunwind/src).
Unfortunately, the companion backtrace_symbolsd and backtrace_symbols_fd functions have not been able to resolve the stacktrace addresses to function names with source file name and line number for probably a decade now (see the example output below).
However, there is another method to resolve addresses to symbols and it produces the most useful traces with function name, source file and line number. The method is to #include <elfutils/libdwfl.h>and link with -ldw (libdw-dev on Ubuntu 18.04).
Working C++ example (test.cc):
#include <stdexcept>
#include <iostream>
#include <cassert>
#include <cstdlib>
#include <string>
#include <boost/core/demangle.hpp>
#include <execinfo.h>
#include <elfutils/libdwfl.h>
struct DebugInfoSession {
Dwfl_Callbacks callbacks = {};
char* debuginfo_path = nullptr;
Dwfl* dwfl = nullptr;
DebugInfoSession() {
callbacks.find_elf = dwfl_linux_proc_find_elf;
callbacks.find_debuginfo = dwfl_standard_find_debuginfo;
callbacks.debuginfo_path = &debuginfo_path;
dwfl = dwfl_begin(&callbacks);
assert(dwfl);
int r;
r = dwfl_linux_proc_report(dwfl, getpid());
assert(!r);
r = dwfl_report_end(dwfl, nullptr, nullptr);
assert(!r);
static_cast<void>(r);
}
~DebugInfoSession() {
dwfl_end(dwfl);
}
DebugInfoSession(DebugInfoSession const&) = delete;
DebugInfoSession& operator=(DebugInfoSession const&) = delete;
};
struct DebugInfo {
void* ip;
std::string function;
char const* file;
int line;
DebugInfo(DebugInfoSession const& dis, void* ip)
: ip(ip)
, file()
, line(-1)
{
// Get function name.
uintptr_t ip2 = reinterpret_cast<uintptr_t>(ip);
Dwfl_Module* module = dwfl_addrmodule(dis.dwfl, ip2);
char const* name = dwfl_module_addrname(module, ip2);
function = name ? boost::core::demangle(name) : "<unknown>";
// Get source filename and line number.
if(Dwfl_Line* dwfl_line = dwfl_module_getsrc(module, ip2)) {
Dwarf_Addr addr;
file = dwfl_lineinfo(dwfl_line, &addr, &line, nullptr, nullptr, nullptr);
}
}
};
std::ostream& operator<<(std::ostream& s, DebugInfo const& di) {
s << di.ip << ' ' << di.function;
if(di.file)
s << " at " << di.file << ':' << di.line;
return s;
}
void terminate_with_stacktrace() {
void* stack[512];
int stack_size = ::backtrace(stack, sizeof stack / sizeof *stack);
// Print the exception info, if any.
if(auto ex = std::current_exception()) {
try {
std::rethrow_exception(ex);
}
catch(std::exception& e) {
std::cerr << "Fatal exception " << boost::core::demangle(typeid(e).name()) << ": " << e.what() << ".\n";
}
catch(...) {
std::cerr << "Fatal unknown exception.\n";
}
}
DebugInfoSession dis;
std::cerr << "Stacktrace of " << stack_size << " frames:\n";
for(int i = 0; i < stack_size; ++i) {
std::cerr << i << ": " << DebugInfo(dis, stack[i]) << '\n';
}
std::cerr.flush();
std::_Exit(EXIT_FAILURE);
}
int main() {
std::set_terminate(terminate_with_stacktrace);
throw std::runtime_error("test exception");
}
Compiled on Ubuntu 18.04.4 LTS with gcc-8.3:
g++ -o test.o -c -m{arch,tune}=native -std=gnu++17 -W{all,extra,error} -g -Og -fstack-protector-all test.cc
g++ -o test -g test.o -ldw -lunwind
Outputs:
Fatal exception std::runtime_error: test exception.
Stacktrace of 7 frames:
0: 0x55f3837c1a8c terminate_with_stacktrace() at /home/max/src/test/test.cc:76
1: 0x7fbc1c845ae5 <unknown>
2: 0x7fbc1c845b20 std::terminate()
3: 0x7fbc1c845d53 __cxa_throw
4: 0x55f3837c1a43 main at /home/max/src/test/test.cc:103
5: 0x7fbc1c3e3b96 __libc_start_main at ../csu/libc-start.c:310
6: 0x55f3837c17e9 _start
When no -lunwind is linked, it produces a less accurate stacktrace:
0: 0x5591dd9d1a4d terminate_with_stacktrace() at /home/max/src/test/test.cc:76
1: 0x7f3c18ad6ae6 <unknown>
2: 0x7f3c18ad6b21 <unknown>
3: 0x7f3c18ad6d54 <unknown>
4: 0x5591dd9d1a04 main at /home/max/src/test/test.cc:103
5: 0x7f3c1845cb97 __libc_start_main at ../csu/libc-start.c:344
6: 0x5591dd9d17aa _start
For comparison, backtrace_symbols_fd output for the same stacktrace is least informative:
/home/max/src/test/debug/gcc/test(+0x192f)[0x5601c5a2092f]
/usr/lib/x86_64-linux-gnu/libstdc++.so.6(+0x92ae5)[0x7f95184f5ae5]
/usr/lib/x86_64-linux-gnu/libstdc++.so.6(_ZSt9terminatev+0x10)[0x7f95184f5b20]
/usr/lib/x86_64-linux-gnu/libstdc++.so.6(__cxa_throw+0x43)[0x7f95184f5d53]
/home/max/src/test/debug/gcc/test(+0x1ae7)[0x5601c5a20ae7]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xe6)[0x7f9518093b96]
/home/max/src/test/debug/gcc/test(+0x1849)[0x5601c5a20849]
In a production version (as well as C language version) you may like to make this code extra robust by replacing boost::core::demangle, std::string and std::cout with their underlying calls.
You can also override __cxa_throw to capture the stacktrace when an exception is thrown and print it when the exception is caught. By the time it enters catch block the stack has been unwound, so it is too late to call backtrace, and this is why the stack must be captured on throw which is implemented by function __cxa_throw. Note that in a multi-threaded program __cxa_throw can be called simultaneously by multiple threads, so that if it captures the stacktrace into a global array that must be thread_local.
You can also make the stack trace printing function async-signal safe, so that you can invoke it directly from your SIGSEGV, SIGBUS signal handlers (which should use their own stacks for robustness). Obtaining function name, source file and line number using libdwfl from a signal handler may fail because it is not async-signal safe or if the address space of the process has been substantially corrupted, but in practice it succeeds 99% of the time (I haven't seen it fail).
To summarize, a complete production-ready library for automatic stacktrace output should:
Capture the stacktrace on throw into thread-specific storage.
Automatically print the stacktrace on unhandled exceptions.
Print the stacktrace in async-signal-safe manner.
Provide a robust signal handler function which uses its own stack that prints the stacktrace in a async-signal-safe manner. The user can install this function as a signal handler for SIGSEGV, SIGBUS, SIGFPE, etc..
The signal handler may as well print the values of all CPU registers at the point of the fault from ucontext_t signal function argument (may be excluding vector registers), a-la Linux kernel oops log messages.
Another answer to an old thread.
When I need to do this, I usually just use system() and pstack
So something like this:
#include <sys/types.h>
#include <unistd.h>
#include <string>
#include <sstream>
#include <cstdlib>
void f()
{
pid_t myPid = getpid();
std::string pstackCommand = "pstack ";
std::stringstream ss;
ss << myPid;
pstackCommand += ss.str();
system(pstackCommand.c_str());
}
void g()
{
f();
}
void h()
{
g();
}
int main()
{
h();
}
This outputs
#0 0x00002aaaab62d61e in waitpid () from /lib64/libc.so.6
#1 0x00002aaaab5bf609 in do_system () from /lib64/libc.so.6
#2 0x0000000000400c3c in f() ()
#3 0x0000000000400cc5 in g() ()
#4 0x0000000000400cd1 in h() ()
#5 0x0000000000400cdd in main ()
This should work on Linux, FreeBSD and Solaris. I don't think that macOS has pstack or a simple equivalent, but this thread seems to have an alternative.
If you are using C, then you will need to use C string functions.
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
void f()
{
pid_t myPid = getpid();
/*
length of command 7 for 'pstack ', 7 for the PID, 1 for nul
*/
char pstackCommand[7+7+1];
sprintf(pstackCommand, "pstack %d", (int)myPid);
system(pstackCommand);
}
I've used 7 for the max number of digits in the PID, based on this post.
There is no standardized way to do that. For windows the functionality is provided in the DbgHelp library
You can use the Boost libraries to print the current callstack.
#include <boost/stacktrace.hpp>
// ... somewhere inside the `bar(int)` function that is called recursively:
std::cout << boost::stacktrace::stacktrace();
Man here: https://www.boost.org/doc/libs/1_65_1/doc/html/stacktrace.html
I know this thread is old, but I think it can be useful for other people. If you are using gcc, you can use its instrument features (-finstrument-functions option) to log any function call (entry and exit). Have a look at this for more information: http://hacktalks.blogspot.fr/2013/08/gcc-instrument-functions.html
You can thus for instance push and pop every calls into a stack, and when you want to print it, you just look at what you have in your stack.
I've tested it, it works perfectly and is very handy
UPDATE: you can also find information about the -finstrument-functions compile option in the GCC doc concerning the Instrumentation options: https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html
You can implement the functionality yourself:
Use a global (string)stack and at start of each function push the function name and such other values (eg parameters) onto this stack; at exit of function pop it again.
Write a function that will printout the stack content when it is called, and use this in the function where you want to see the callstack.
This may sound like a lot of work but is quite useful.
Of course the next question is: will this be enough ?
The main disadvantage of stack-traces is that why you have the precise function being called you do not have anything else, like the value of its arguments, which is very useful for debugging.
If you have access to gcc and gdb, I would suggest using assert to check for a specific condition, and produce a memory dump if it is not met. Of course this means the process will stop, but you'll have a full fledged report instead of a mere stack-trace.
If you wish for a less obtrusive way, you can always use logging. There are very efficient logging facilities out there, like Pantheios for example. Which once again could give you a much more accurate image of what is going on.
You can use Poppy for this. It is normally used to gather the stack trace during a crash but it can also output it for a running program as well.
Now here's the good part: it can output the actual parameter values for each function on the stack, and even local variables, loop counters, etc.
You can use the GNU profiler. It shows the call-graph as well! the command is gprof and you need to compile your code with some option.
Is there any way to dump the call stack in a running process in C or C++ every time a certain function is called?
No there is not, although platform-dependent solutions might exist.
Let's say I have a file called library.c
#include <stdio.h>
void someFunc(int n)
{
printf("%s: %d\n", LIBNAME, n);
}
I compile it into two shared object files, using different macros (so resulting code is different). In this example I provide different LIBNAME:
gcc -DLIBNAME=\"lib1\" -fPIC -shared -g -Og library.c -o library1.so
gcc -DLIBNAME=\"lib2\" -fPIC -shared -g -Og library.c -o library2.so
Then I load both libraries from executable:
#include <dlfcn.h>
#include <stdio.h>
typedef void (*functype)(int);
int callFunc(const char* libname, int n)
{
void* lib = dlopen(libname, RTLD_NOW|RTLD_LOCAL);
if (!lib) {
fprintf(stderr, "%s\n", dlerror());
return -1;
}
functype func = dlsym(lib, "someFunc");
if (!func) {
fprintf(stderr, "%s\n", dlerror());
dlclose(lib);
return -1;
}
func(n);
dlclose(lib);
return 0;
}
int main()
{
int res = callFunc("./library1.so", 42);
if (res == 0)
return callFunc("./library2.so", 13);
else
return res;
}
In gdb I want to debug function from a certain library (e.g. library1.so), while not having to worry about another one (library2.so). I can't set breakpoint by function name, because it's the same in both libraries. I can't do it with sourcefile:linenumber either, since source file is the same for both libraries. How can I tell gdb to set breakpoint only in one library?
In gdb I want to debug function from a certain library (e.g.
library1.so), while not having to worry about another one (library2.so)
You can set pending breakpoint on someFunc and ignore it manually if it was called from library2.so. You can know this from info sharedlibrary output: if someFunc was called from library2.so, library2.so will be loaded and you will see it in info sharedlibrary output.
(gdb) info sharedlibrary
From To Syms Read Shared Object Library
0x00007ffff7dd6f60 0x00007ffff7df5030 Yes (*) /lib64/ld-linux-x86-64.so.2
0x00007ffff7bd2ee0 0x00007ffff7bd3bbe Yes (*) /lib64/libdl.so.2
0x00007ffff7834340 0x00007ffff797b27f Yes (*) /lib64/libc.so.6
0x00007ffff7611550 0x00007ffff761162c Yes ./library2.so
(*): Shared library is missing debugging information.
(gdb)
When you see it loaded, you can ignore this breakpoint and continue execution. Though it is not fully automated way of debugging someFunc, this is probably the best you can do in this case.
Why don't you use "break if"? Something like
break library.c:<line_no> if libname == "./library1.so"
In case libname does not work, you can also use the second parameter n.
I am trying to execute a PintOS command pintos -f (do not worry if you are not familiar with PintOS). Internally, init.c program is called, whose parse_options() function handles the command line arguments passed. Below is the relevant snippet of init.c
static char **
parse_options (char **argv)
{
for (; *argv != NULL && **argv == '-'; argv++)
{
char *save_ptr;
char *name = strtok_r (*argv, "=", &save_ptr); //fn to tokenise the string
char *value = strtok_r (NULL, "", &save_ptr);
if (!strcmp (name, "-h"))
usage ();
else if (!strcmp (name, "-q"))
power_off_when_done = true;
else if (!strcmp (name, "-r"))
reboot_when_done = true;
/*$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$*/
#ifdef FILESYS
else if (!strcmp (name, "-f"))
format_filesys = true;
#endif
/*$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$*/
else if (!strcmp (name, "-rs"))
random_init (atoi (value));
else if (!strcmp (name, "-mlfqs"))
thread_mlfqs = true;
#ifdef USERPROG
else if (!strcmp (name, "-ul"))
user_page_limit = atoi (value);
#endif
else
PANIC ("unknown option `%s' (use -h for help)", name);
}
return argv;
}
As per the code inside the $$$$ section, the -f option passed will be processed only if FILESYS is defined.
On executing make, the following command is executed
gcc -m32 -c ../../threads/init.c -o threads/init.o -g -msoft-float -O -fno-stack- protector -nostdinc -I../.. -I../../lib -I../../lib/kernel -Wall -W -Wstrict-prototypes -Wmissing-prototypes -Wsystem-headers -DUSERPROG -DFILESYS -MMD -MF threads/init.d
Here, -DUSERPROG and -DFILESYS options are provided to define FILESYS and USERPROG, so that the relevant sections of the code can be executed. However, somehow, FILESYS is not getting defined, as evident from the following output of pintos -f command
Kernel command line: -f
Kernel PANIC at ../../threads/init.c:261 in parse_options(): unknown option `-f' (use -h for help)
Several other tests confirm that FILESYS not getting defined IS the issue. I checked the gcc syntax and even wrote the following Dummy program to check the -DNAME option with gcc.
DummyProg.c
#include "stdio.h"
int main()
{
#ifdef CHECK
printf("WORKING\n");
#endif
return 0;
}
With gcc -DCHECK DummyProg.c and ./a.out, WORKING was displayed on the screen, conforming the validity of syntax etc. The gcc version I am using is gcc (Ubuntu/Linaro 4.7.3-1ubuntu1) 4.7.3
Could someone please point how to resolve this issue.
The compiler can help you understand this; the trick is to capture the proprocessed code with the #define statements left in:
gcc -E -dD ...blah...
You have to remove the -c and -o <outfile> options from the compile command, but otherwise leave it unchanged, apart from adding the two new options.
-E tells it to do the preprocess step only, and output it to screen.
-dD tells it to leave in the #define and #undef directives.
My guess is that your code has a #undef FILESYS directive somewhere, and it's overriding your -DFILESYS option.
To debug issues like this, run gcc with -E. This will just run the code through the preprocessor cpp. Look into the output file to see what the input of the C compiler would be after the post-processing step.
are you sure you are building the entire executable/library/...?
looking at the command that gets triggered when calling make, only a single file is compiled into an intermediate object file. no linking into the final binary happens.
so it guess that you are still using an outdated build. try going to the toplevel source directory and execute:
make clean && make
I have a linux C program that handles request sent to a TCP socket (bound to a particular port). I want to be able to query the internal state of the C program via a request to that port, but I dont want to hard code what global variables can be queried. Thus I want the query to contain the string name of a global and the C code to look that string up in the symbol table to find its address and then send its value back over the TCP socket. Of course the symbol table must not have been stripped. So can the C program even locate its own symbol table, and is there a library interface for looking up symbols given their name? This is an ELF executable C program built with gcc.
This is actually fairly easy. You use dlopen / dlsym to access symbols. In order for this to work, the symbols have to be present in the dynamic symbol table. There are multiple symbol tables!
#include <dlfcn.h>
#include <stdio.h>
__attribute__((visibility("default")))
const char A[] = "Value of A";
__attribute__((visibility("hidden")))
const char B[] = "Value of B";
const char C[] = "Value of C";
int main(int argc, char *argv[])
{
void *hdl;
const char *ptr;
int i;
hdl = dlopen(NULL, 0);
for (i = 1; i < argc; ++i) {
ptr = dlsym(hdl, argv[i]);
printf("%s = %s\n", argv[i], ptr);
}
return 0;
}
In order to add all symbols to the dynamic symbol table, use -Wl,--export-dynamic. If you want to remove most symbols from the symbol table (recommended), set -fvisibility=hidden and then explicitly add the symbols you want with __attribute__((visibility("default"))) or one of the other methods.
~ $ gcc dlopentest.c -Wall -Wextra -ldl
~ $ ./a.out A B C
A = (null)
B = (null)
C = (null)
~ $ gcc dlopentest.c -Wall -Wextra -ldl -Wl,--export-dynamic
~ $ ./a.out A B C
A = Value of A
B = (null)
C = Value of C
~ $ gcc dlopentest.c -Wall -Wextra -ldl -Wl,--export-dynamic -fvisibility=hidden
~ $ ./a.out A B C
A = Value of A
B = (null)
C = (null)
Safety
Notice that there is a lot of room for bad behavior.
$ ./a.out printf
printf = ▯▯▯▯ (garbage)
If you want this to be safe, you should create a whitelist of permissible symbols.
file: reflect.c
#include <stdio.h>
#include "reflect.h"
struct sym_table_t gbl_sym_table[1] __attribute__((weak)) = {{NULL, NULL}};
void * reflect_query_symbol(const char *name)
{
struct sym_table_t *p = &gbl_sym_table[0];
for(; p->name; p++) {
if(strcmp(p->name, name) == 0) {
return p->addr;
}
}
return NULL;
}
file: reflect.h
#include <stdio.h>
struct sym_table_t {
char *name;
void *addr;
};
void * reflect_query_symbol(const char *name);
file: main.c
just #include "reflect.h" and call reflect_query_symbol
example:
#include <stdio.h>
#include "reflect.h"
void foo(void)
{
printf("bar test\n");
}
int uninited_data;
int inited_data = 3;
int main(int argc, char *argv[])
{
int i;
void *addr;
for(i=1; i<argc; i++) {
addr = reflect_query_symbol(argv[i]);
if(addr) {
printf("%s lay at: %p\n", argv[i], addr);
} else {
printf("%s NOT found\n", argv[i], addr);
}
}
return 0;
}
file:Makefile
objs = main.o reflect.o
main: $(objs)
gcc -o $# $^
nm $# | awk 'BEGIN{ print "#include <stdio.h>"; print "#include \"reflect.h\""; print "struct sym_table_t gbl_sym_table[]={" } { if(NF==3){print "{\"" $$3 "\", (void*)0x" $$1 "},"}} END{print "{NULL,NULL} };"}' > .reflect.real.c
gcc -c .reflect.real.c -o .reflect.real.o
gcc -o $# $^ .reflect.real.o
nm $# | awk 'BEGIN{ print "#include <stdio.h>"; print "#include \"reflect.h\""; print "struct sym_table_t gbl_sym_table[]={" } { if(NF==3){print "{\"" $$3 "\", (void*)0x" $$1 "},"}} END{print "{NULL,NULL} };"}' > .reflect.real.c
gcc -c .reflect.real.c -o .reflect.real.o
gcc -o $# $^ .reflect.real.o
The general term for this sort of feature is "reflection", and it is not part of C.
If this is for debugging purposes, and you want to be able to inspect the entire state of a C program remotely, examine any variable, start and stop its execution, and so on, you might consider GDB remote debugging:
GDB offers a 'remote' mode often used when debugging embedded systems.
Remote operation is when GDB runs on one machine and the program being
debugged runs on another. GDB can communicate to the remote 'stub'
which understands GDB protocol via Serial or TCP/IP. A stub program
can be created by linking to the appropriate stub files provided with
GDB, which implement the target side of the communication
protocol. Alternatively, gdbserver can be used to remotely debug
the program without needing to change it in any way.