I'm trying to write my own shared library to link to an executable, but can't get the .so to link.
I'm using a very basic example to try and get it working. The shared library (test_lib.c):
#include "test_lib.h" //stdlib includes and function prototype
char *hello(void) {
char *c = malloc(100);
memcpy(c, "hello\n", 7);
return c;
}
The executable (test.c):
#include "test_lib.h"
int main() {
printf("%s", hello());
return 0;
}
Following all the guides I can find, I compile the .so with gcc -I . -fPIC -shared -o test_lib.so test_lib.c, and then the executable (in the same directory) with gcc -I . -L . test.c -ltest_lib
This gives the error:
/usr/bin/ld: cannot find -ltest_lib
collect2: error: ld returned 1 exit status
As I understand including the path through the -L flag should tell gcc where to find the .so, but this isn't working. What am I missing here?
When linking a library, the library usually has to be named libxxx.a|so for the linker to find it.
Compiling the library:
gcc -I . -fPIC -shared -o libtest.so test_lib.c
Then you can link with:
gcc -I . -L . test.c -ltest
Related
How to change the entry point of a C program compiled with gcc ?
Just like in the following code
#include<stdio.h>
int entry() //entry is the entry point instead of main
{
return 0;
}
It's a linker setting:
-Wl,-eentry
the -Wl,... thing passes arguments to the linker, and the linker takes a -e argument to set the entry function
You can modify your source code as:
#include<stdio.h>
const char my_interp[] __attribute__((section(".interp"))) = "/lib/ld-linux.so.2";
int entry() //entry is the entry point instead of main
{
exit(0);
}
The ".interp" section will let your program able to call external shared library.
The exit call will make your entry function to exit program instead of return.
Then build the program as a shared library which is executable:
$ gcc -shared -fPIC -e entry test_main.c -o test_main.so
$ ./test_main
If you are on a system that provides GNU Binutils (like Linux),
you can use the objcopy command
to make an arbitrary function the new entry point.
Suppose a file called program.c containing the entry function:
$ cat > program.c
#include <stdio.h>
int entry()
{
return 0;
}
^D
You first compile it using -c to generate a relocatable object file:
$ gcc -c program.c -o program.o
Then you redefine entry to be main:
$ objcopy --redefine-sym entry=main program.o
Now use gcc to compile the new object file:
$ gcc program.o -o program
NOTE: If your program already has a function called main, before step 2, you can perform a separate objcopy invocation:
objcopy --redefine-sym oldmain=main program.o
Minimal runnable example and notes on other answers
main.c
#include <stdio.h>
#include <stdlib.h>
int mymain(void) {
puts("hello");
exit(0);
}
compile and run:
gcc -nostartfiles -Wl,--entry=mymain -o main.out main.c
# or -Wl,-emymain
./main.out 1 2 3
The notes:
without -nostartfiles, the link fails with:
/usr/lib/gcc/x86_64-linux-gnu/7/../../../x86_64-linux-gnu/Scrt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status
presumably because the glibc setup code that runs before main in _start normally calls main.
command line arguments are not setup for you, presumably because they would be setup by the glibc code that runs before main, so trying to use them prints undefined values. I haven't found a method that works for them.
Tested in Ubuntu 20.10.
How to change the entry point of a C program compiled with gcc ?
Just like in the following code
#include<stdio.h>
int entry() //entry is the entry point instead of main
{
return 0;
}
It's a linker setting:
-Wl,-eentry
the -Wl,... thing passes arguments to the linker, and the linker takes a -e argument to set the entry function
You can modify your source code as:
#include<stdio.h>
const char my_interp[] __attribute__((section(".interp"))) = "/lib/ld-linux.so.2";
int entry() //entry is the entry point instead of main
{
exit(0);
}
The ".interp" section will let your program able to call external shared library.
The exit call will make your entry function to exit program instead of return.
Then build the program as a shared library which is executable:
$ gcc -shared -fPIC -e entry test_main.c -o test_main.so
$ ./test_main
If you are on a system that provides GNU Binutils (like Linux),
you can use the objcopy command
to make an arbitrary function the new entry point.
Suppose a file called program.c containing the entry function:
$ cat > program.c
#include <stdio.h>
int entry()
{
return 0;
}
^D
You first compile it using -c to generate a relocatable object file:
$ gcc -c program.c -o program.o
Then you redefine entry to be main:
$ objcopy --redefine-sym entry=main program.o
Now use gcc to compile the new object file:
$ gcc program.o -o program
NOTE: If your program already has a function called main, before step 2, you can perform a separate objcopy invocation:
objcopy --redefine-sym oldmain=main program.o
Minimal runnable example and notes on other answers
main.c
#include <stdio.h>
#include <stdlib.h>
int mymain(void) {
puts("hello");
exit(0);
}
compile and run:
gcc -nostartfiles -Wl,--entry=mymain -o main.out main.c
# or -Wl,-emymain
./main.out 1 2 3
The notes:
without -nostartfiles, the link fails with:
/usr/lib/gcc/x86_64-linux-gnu/7/../../../x86_64-linux-gnu/Scrt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status
presumably because the glibc setup code that runs before main in _start normally calls main.
command line arguments are not setup for you, presumably because they would be setup by the glibc code that runs before main, so trying to use them prints undefined values. I haven't found a method that works for them.
Tested in Ubuntu 20.10.
I am trying to compile following test files to create a shared library:
answer.c
#include <stdio.h>
#include "add.h"
extern int myvar();
int answer()
{
printf("\r\n myvar:%d \r\n", myvar());
setSummand(20);
return add(22); // Will return 42 (=20+22)
}
add.c
#include <stdio.h>
int gSummand;
void setSummand(int summand)
{
printf("1Library is initialized\n");
gSummand = summand;
}
int add(int summand)
{
return gSummand + summand;
}
I want to create a shared library from the 2 files "answer.c" "add.c", I am using following commands:
gcc -c answer.c -o answer.o
gcc -c add.c -o add.o
gcc -shared add.o answer.o -o libtest.so
However third command gives following error:
answer.o:answer.c:(.text+0x9): undefined reference to `myvar'
answer.o:answer.c:(.text+0x9): relocation truncated to fit: R_X86_64_PC32 against undefined symbol `myvar'
collect2: error: ld returned 1 exit status
Same setup links successfully if I use the following command to create static library instead of dynamic. Therefore, I don't see any error if I try following command:
ar rcs libtest.a add.o answer.o
Want to know if I am missing here something. Also want to know how to use external symbols in shared library.
ELF shared libraries need to be position independent. They will be mapped into the executable's address space at an address that isn't known until run time. This means no absolute address call instructions, such as might be used to call myvar().
You need to specify -fpic when you compile the source into object files, when those object files will be placed into a shared library. This tells the compiler to generate code that does not use absolute addresses, etc. so it can be position independent.
Example main.c file to use this library:
extern int answer(void);
int myvar() { return 1; }
int main(void) { return answer(); }
Example without -fpic:
[test]$ gcc -c add.c
[test]$ gcc -c answer.c
[test]$ gcc -shared add.o answer.o -o libtest.so
/usr/bin/ld: add.o: relocation R_X86_64_32 against `.rodata' can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld: answer.o: relocation R_X86_64_32 against `.rodata' can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld: final link failed: Nonrepresentable section on output
collect2: error: ld returned 1 exit status
Example with -fpic:
[test]$ gcc -fpic -c add.c
[test]$ gcc -fpic -c answer.c
[test]$ gcc -shared add.o answer.o -o libtest.so
[test]$ gcc main.c libtest.so
[test]$ LD_LIBRARY_PATH=. ./a.out
myvar:1
1Library is initialized
I try to write simple mongo c client. Source file (a.c):
#include <stdio.h>
#define MONGO_HAVE_STDINT
#include <mongo.h>
void mongo_init_c(mongo *con)
{
mongo_init(con);
}
int main() {
return 0;
}
And i try to compile it with:
gcc -I/usr/local/include -L/usr/local/lib -lmongoc a.c
But get an error:
a.c:(.text+0xd): undefined reference to `mongo_init'
Files /usr/local/include/mongo.h and /usr/local/lib/libmongoc.so exists
How can I correctly compile a.c?
p.s. mongo-2.0.4, gcc-4.6, mongo-c-driver - pulled from github
update
$ nm /usr/local/lib/libmongoc.so | grep init
000034e0 T _init
0000dd10 T bson_init
0000c740 T bson_init_data
0000c7b0 T bson_init_finished_data
0000dc10 T bson_init_size
0000d060 T bson_iterator_init
0000a5e0 T gridfile_init
00009af0 T gridfile_writer_init
000095e0 T gridfs_init
00010a18 R initialBufferSize
00005f40 T mongo_cursor_init
00008da0 T mongo_env_sock_init
00005d90 T mongo_init
000057b0 T mongo_init_sockets
00004800 T mongo_md5_init
00005e40 T mongo_replica_set_init
00005f00 T mongo_replset_init
00005b80 T mongo_write_concern_init
$ gcc -I/usr/local/include -L/usr/local/lib -Wall -Werror -lmongoc a.c
/tmp/cccuNEp1.o: In function `mongo_init_c':
a.c:(.text+0xd): undefined reference to `mongo_init'
Try linking the library after the source file, like gcc a.c -lmongoc. This is because you're using a traditional single-pass linker, which expects to satisfy dependencies with subsequent, not previous, objects specified on the command line.
I create a .so file with the code below, but when I compile a file that invokes functions in the .so file with GCC, I get an "undefined reference to 'outlib1'" error.
What's wrong with my code or my command? Thanks.
OS Ubuntu 11.10
gcc 4.6.1
//file name outscreen.c
#include <stdio.h>
void outlib1(void)
{
printf("out screen func1\n");
}
//file name main.c
int main(int argc, char* argv[])
{
outlib1();
}
gcc outscreen.c -fPIC -shared -o outscreen.so
gcc main.c -L. -loutscreen -o call
./call
Try:
$ gcc outscreen.c -fPIC -shared -o liboutscreen.so
$ gcc main.c -L. -loutscreen -o call
(note the change to the first line - the second line is unchanged)
What is the output of this?
nm outscreen.so | grep outlib1
Perhaps it is exporting with an underscore.