I have a C program which compiles and runs fine under Linux without any warnings, but when trying to compile it on SunOS, I get the following warning:
test.c: In function `my_function':
test.c:412: warning: implicit declaration of function `strerror_r'
Undefined first referenced
symbol in file
strerror_r /var/tmp/ccRiPoGl.o
ld: fatal: Symbol referencing errors. No output written to test
collect2: ld returned 1 exit status
make: *** [test] Error 1
Any ideas?
The "implicit declaration" warning is telling you that none of the headers you have #included have defined that function, and the "undefined symbol" warning is telling you that the function itself isn't defined in any of the libraries that you're linking in.
Taken together, this implies that C library you're compiling against doesn't provide the strerror_r function. You'll have to roll your own alternative.
For example, if you're using pthreads, you could do this:
int my_strerror_r(int errnum, char *buf, size_t buflen)
{
static pthread_mutex_t strerror_lock = PTHREAD_MUTEX_INITIALIZER;
if (!buflen)
{
errno = ENOSPC;
return -1;
}
buf[0] = 0;
pthread_mutex_lock(&strerror_lock);
strncat(buf, strerror(errnum), buflen - 1);
pthread_mutex_unlock(&strerror_lock);
return 0;
}
It looks like strerror_r may not be available on that platform. I just checked on an old Solaris 2.8 box and strerror_r is not available.
It looks like you'll have to use strerror and deal with the occasional incorrect results due to the lack of thread safety.
Sounds like you need to give an extra hint to the linker to specify the library, it may not be in a standard path as is the case with Linux, if you know the name of the library, explicitly specify the library path as part of the CLI compiler when invoking gcc or equivalent under SunOS.
Here's an example of using a LIBS variable in a makefile.
LIBS = -L/usr/lib -L/usr/sys/lib -lsome_lib1 -lsome_lib2
Here's a line that is used to invoke the compiler in the makefile - notice the reference to the LIBS variable as I have shown in the above.
$(CC) -o $# $(FILES) $(LIBS) $(CFLAGS)
Hope this helps,
Best regards,
Tom.
Linux and SunOS are different operating systems and possibly have different implementations of the function you are using.
You might find the following article on POSIX compatibility by Dr. Bernstein at the University of Chicago at Illinois helpful:
http://cr.yp.to/docs/unixport.html
Good luck,
-bn
Related
I'm a longtime python hobbyist porting a script over to c. I believe there is something wrong in the environment preventing the code from compiling. Research elsewhere leads me to believe it has something to do with posix header files? Maybe something with macros? I'm insufficiently experienced in c to figure it out.
The relevant snippet is here:
pthread_t id;
thread_create(&id, NULL, refreshqb,NULL);
void *status;
pthread_start(id, (void**)&status);
The error I receive is this.
t.c:91:4: warning: implicit declaration of function 'thread_create' is invalid in C99
[-Wimplicit-function-declaration]
thread_create(&id, NULL, refreshqb,NULL);
^
t.c:93:4: warning: implicit declaration of function 'pthread_start' is invalid in C99
[-Wimplicit-function-declaration]
pthread_start(id, (void**)&status);
^
2 warnings generated.
Undefined symbols for architecture x86_64:
"_pthread_start", referenced from:
_main in t-0d3a02.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
Support for pthreads must be explicitly enabled when compiling your code. It looks like you're using clang, so just add the -pthread flag when you compile using clang.
Please try using pthread_create, along with including correct posix library header or "#include <pthread.h>".
I picked up the following demo off the web from https://computing.llnl.gov/tutorials/pthreads/
#include <pthread.h>
#include <stdio.h>
#define NUM_THREADS 5
void *PrintHello(void *threadid)
{
long tid;
tid = (long)threadid;
printf("Hello World! It's me, thread #%ld!\n", tid);
pthread_exit(NULL);
}
int main (int argc, char *argv[])
{
pthread_t threads[NUM_THREADS];
int rc;
long t;
for(t=0; t<NUM_THREADS; t++){
printf("In main: creating thread %ld\n", t);
rc = pthread_create(&threads[t], NULL, PrintHello, (void *)t);
if (rc){
printf("ERROR; return code from pthread_create() is %d\n", rc);
exit(-1);
}
}
pthread_exit(NULL);
}
But when I compile it on my machine (running Ubuntu Linux 9.04) I get the following error:
corey#ubuntu:~/demo$ gcc -o term term.c
term.c: In function ‘main’:
term.c:23: warning: incompatible implicit declaration of built-in function ‘exit’
/tmp/cc8BMzwx.o: In function `main':
term.c:(.text+0x82): undefined reference to `pthread_create'
collect2: ld returned 1 exit status
This doesn't make any sense to me, because the header includes pthread.h, which should have the pthread_create function. Any ideas what's going wrong?
For Linux the correct command is:
gcc -pthread -o term term.c
In general, libraries should follow sources and objects on command line, and -lpthread is not an "option", it's a library specification. On a system with only libpthread.a installed,
gcc -lpthread ...
will fail to link.
Read this or this detailed explanation.
For Linux the correct command is:
gcc -o term term.c -lpthread
you have to put -lpthread just after the compile command,this command will tell to the compiler to execute program with pthread.h library.
gcc -l links with a library file.Link -l with library name without the lib prefix.
in eclipse
properties->c/c++Build->setting->GCC C++ linker->libraries in top part add "pthread"
Running from the Linux terminal, what worked for me was compiling using the following command (suppose the c file I want to compile is called test.c):
gcc -o test test.c -pthread
Hope it helps somebody!
If you are using cmake, you can use:
add_compile_options(-pthread)
Or
SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pthread")
I believe the proper way of adding pthread in CMake is with the following
find_package (Threads REQUIRED)
target_link_libraries(helloworld
${CMAKE_THREAD_LIBS_INIT}
)
Acutally, it gives several examples of compile commands used for pthreads codes are listed in the table below, if you continue reading the following tutorial:
https://computing.llnl.gov/tutorials/pthreads/#Compiling
Compile it like this : gcc demo.c -o demo -pthread
In Visual Studio 2019 specify -pthread in the property pages for the project under:
Linker -> Command Line -> Additional Options
Type in -pthread in the textbox.
You need to use the option -lpthread with gcc.
you need only Add "pthread" in proprieties=>C/C++ build=>GCC C++ Linker=>Libraries=> top part "Libraries(-l)".
thats it
check man page and you will get.
Compile and link with -pthread.
SYNOPSIS
#include <pthread.h>
int pthread_create(pthread_t *thread, const pthread_attr_t *attr,
void *(*start_routine) (void *), void *arg);
Compile and link with -pthread.
....
Since none of the answers exactly covered my need (using MSVS Code), I add here my experience with this IDE and CMAKE build tools too.
Step 1: Make sure in your .cpp, (or .hpp if needed) you have included:
#include <functional>
Step 2 For MSVSCode IDE users:
Add this line to your c_cpp_properties.json file:
"compilerArgs": ["-pthread"],
Step 2 For CMAKE build tools users:
Add this line to your CMakeLists.txt
set(CMAKE_CXX_FLAGS "-pthread")
Note: Adding flag -lpthread (instead of -pthread) results in failed linking.
From man gcc,
-pthread
Define additional macros required for using the POSIX threads library.
You should use this option consistently for both compilation and linking.
This option is supported on GNU/Linux targets,
most other Unix derivatives,
and also on x86 Cygwin and MinGW targets.
It is correct that -pthread is an option and the best way to handle this.
There are statements in some answers that it generates different compiled code. This is misleading.
If you wish to duplicate -pthread, you could use -lpthread -D_REENTRANT=1. So there are two things going on with the -pthread option.
Indeed it links with the pthread library as many answers express. Also, the order of the pthread library is important because it may override some weak symbols. So a correct version using -lpthread may need to have it multiple times on the command line.
The other important part is the _REENTRANT define. Note, that this is in the implementation namespace. Some people may care for portability and other not. However, it is very important that it is defined as the first thing in the compilation unit. This symbol will alter the way that many system headers files are parsed.
You can include #define _REENTRANT 1 at the top of every source file, but it is much easier to have it on the command line. Again, the -pthread is the best way to achieve this. Also, gcc may change the way this is implemented in the future. However, I think it is important for programmers to understand what is going on.
term.c: In function ‘main’: term.c:23: warning: incompatible implicit
declaration of built-in function ‘exit’
You never included <stdlib.h>, where exit() is declared. Also, I think newer versions of gcc have removed the need for _REENTRANT.
Why features.h?
Example on godbolt, without -pthread.
So, it is NOT generating different code. Ie, the backend of the compiler is NOT different. It is only conditional compilation and linking to different libraries. It does not generate 'lock free' code or add appropriate machine barriers because you have used this option.
In Anjuta, go to the Build menu, then Configure Project.
In the Configure Options box, add:
LDFLAGS='-lpthread'
Hope it'll help somebody too...
Sometimes, if you use multiple library, check the library dependency.
(e.g. -lpthread -lSDL... <==> ... -lSDL -lpthread)
I'm getting the following error and can't for the life of me figure out what I'm doing wrong.
$ gcc main.c -o main
Undefined symbols:
"_wtf", referenced from:
_main in ccu2Qr2V.o
ld: symbol(s) not found
collect2: ld returned 1 exit status
main.c:
#include <stdio.h>
#include "wtf.h"
main(){
wtf();
}
wtf.h:
void wtf();
wtf.c:
void wtf(){
printf("I never see the light of day.");
}
Now, if I include the entire function in the header file instead of just the signature, it complies fine so I know wtf.h is being included. Why doesn't the compiler see wtf.c? Or am I missing something?
Regards.
You need to link wtf with your main. Easiest way to compile it together - gcc will link 'em for you, like this:
gcc main.c wtf.c -o main
Longer way (separate compilation of wtf):
gcc -c wtf.c
gcc main.c wtf.o -o main
Even longer (separate compilation and linking)
gcc -c wtf.c
gcc -c main.c
gcc main.o wtf.o -o main
Instead of last gcc call you can run ld directly with the same effect.
You are missing the fact that merely including a header doesn't tell the compiler anything about where the actual implementation (the definitions) of the things declared in the header are.
They could be in a C file next to the one doing the include, they could come from a pre-compiled static link library, or a dynamic library loaded by the system linker when reading your executable, or they could come at run-time user programmer-controlled explicit dynamic loading (the dlopen() family of function in Linux, for instance).
C is not like Java, there is no implicit rule that just because a C file includes a certain header, the compiler should also do something to "magically" find the implementation of the things declared in the header. You need to tell it.
I am using Kubuntu 14.04, and installed the FreeImage library with
sudo apt-get install libfreeimage-dev
As far as I can tell, it is correctly installed, with FreeImage.h in /usr/include and libfreeimage.a in /usr/lib. However, this trivial C program
#include <FreeImage.h>
int main(int argc, char **argv) {
FreeImage_Initialise(FALSE);
FreeImage_DeInitialise();
return 0;
}
fails to compile. Running
gcc -lfreeimage fitest.c -o fitest
outputs
/tmp/ccFbb0HQ.o: In function `main':
fitest.c:(.text+0x15): undefined reference to `FreeImage_Initialise'
fitest.c:(.text+0x1a): undefined reference to `FreeImage_DeInitialise'
collect2: error: ld returned 1 exit status
What am I doing wrong?
This wouldn't normally be the case with shared libraries but only static ones, but I'm going to give it a shot anyway since it matches your symptoms, and you also mention libfreeimage.a instead of libfreeimage.so, indicating that you're trying to use the static library.
When linking against static libraries, you need to give the library arguments after the explcit source/object arguments to the compiler, because the compiler will only process yet unresolved symbols from the library:
gcc -o fitest fitest.c -lfreeimage
If you give a static library argument before any source/object arguments, then, no symbols will yet be unresolved, nothing will be picked from the library, and the symbols will instead be seen as unresolved at the end.
I picked up the following demo off the web from https://computing.llnl.gov/tutorials/pthreads/
#include <pthread.h>
#include <stdio.h>
#define NUM_THREADS 5
void *PrintHello(void *threadid)
{
long tid;
tid = (long)threadid;
printf("Hello World! It's me, thread #%ld!\n", tid);
pthread_exit(NULL);
}
int main (int argc, char *argv[])
{
pthread_t threads[NUM_THREADS];
int rc;
long t;
for(t=0; t<NUM_THREADS; t++){
printf("In main: creating thread %ld\n", t);
rc = pthread_create(&threads[t], NULL, PrintHello, (void *)t);
if (rc){
printf("ERROR; return code from pthread_create() is %d\n", rc);
exit(-1);
}
}
pthread_exit(NULL);
}
But when I compile it on my machine (running Ubuntu Linux 9.04) I get the following error:
corey#ubuntu:~/demo$ gcc -o term term.c
term.c: In function ‘main’:
term.c:23: warning: incompatible implicit declaration of built-in function ‘exit’
/tmp/cc8BMzwx.o: In function `main':
term.c:(.text+0x82): undefined reference to `pthread_create'
collect2: ld returned 1 exit status
This doesn't make any sense to me, because the header includes pthread.h, which should have the pthread_create function. Any ideas what's going wrong?
For Linux the correct command is:
gcc -pthread -o term term.c
In general, libraries should follow sources and objects on command line, and -lpthread is not an "option", it's a library specification. On a system with only libpthread.a installed,
gcc -lpthread ...
will fail to link.
Read this or this detailed explanation.
For Linux the correct command is:
gcc -o term term.c -lpthread
you have to put -lpthread just after the compile command,this command will tell to the compiler to execute program with pthread.h library.
gcc -l links with a library file.Link -l with library name without the lib prefix.
in eclipse
properties->c/c++Build->setting->GCC C++ linker->libraries in top part add "pthread"
Running from the Linux terminal, what worked for me was compiling using the following command (suppose the c file I want to compile is called test.c):
gcc -o test test.c -pthread
Hope it helps somebody!
If you are using cmake, you can use:
add_compile_options(-pthread)
Or
SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pthread")
I believe the proper way of adding pthread in CMake is with the following
find_package (Threads REQUIRED)
target_link_libraries(helloworld
${CMAKE_THREAD_LIBS_INIT}
)
Acutally, it gives several examples of compile commands used for pthreads codes are listed in the table below, if you continue reading the following tutorial:
https://computing.llnl.gov/tutorials/pthreads/#Compiling
Compile it like this : gcc demo.c -o demo -pthread
In Visual Studio 2019 specify -pthread in the property pages for the project under:
Linker -> Command Line -> Additional Options
Type in -pthread in the textbox.
You need to use the option -lpthread with gcc.
you need only Add "pthread" in proprieties=>C/C++ build=>GCC C++ Linker=>Libraries=> top part "Libraries(-l)".
thats it
check man page and you will get.
Compile and link with -pthread.
SYNOPSIS
#include <pthread.h>
int pthread_create(pthread_t *thread, const pthread_attr_t *attr,
void *(*start_routine) (void *), void *arg);
Compile and link with -pthread.
....
Since none of the answers exactly covered my need (using MSVS Code), I add here my experience with this IDE and CMAKE build tools too.
Step 1: Make sure in your .cpp, (or .hpp if needed) you have included:
#include <functional>
Step 2 For MSVSCode IDE users:
Add this line to your c_cpp_properties.json file:
"compilerArgs": ["-pthread"],
Step 2 For CMAKE build tools users:
Add this line to your CMakeLists.txt
set(CMAKE_CXX_FLAGS "-pthread")
Note: Adding flag -lpthread (instead of -pthread) results in failed linking.
From man gcc,
-pthread
Define additional macros required for using the POSIX threads library.
You should use this option consistently for both compilation and linking.
This option is supported on GNU/Linux targets,
most other Unix derivatives,
and also on x86 Cygwin and MinGW targets.
It is correct that -pthread is an option and the best way to handle this.
There are statements in some answers that it generates different compiled code. This is misleading.
If you wish to duplicate -pthread, you could use -lpthread -D_REENTRANT=1. So there are two things going on with the -pthread option.
Indeed it links with the pthread library as many answers express. Also, the order of the pthread library is important because it may override some weak symbols. So a correct version using -lpthread may need to have it multiple times on the command line.
The other important part is the _REENTRANT define. Note, that this is in the implementation namespace. Some people may care for portability and other not. However, it is very important that it is defined as the first thing in the compilation unit. This symbol will alter the way that many system headers files are parsed.
You can include #define _REENTRANT 1 at the top of every source file, but it is much easier to have it on the command line. Again, the -pthread is the best way to achieve this. Also, gcc may change the way this is implemented in the future. However, I think it is important for programmers to understand what is going on.
term.c: In function ‘main’: term.c:23: warning: incompatible implicit
declaration of built-in function ‘exit’
You never included <stdlib.h>, where exit() is declared. Also, I think newer versions of gcc have removed the need for _REENTRANT.
Why features.h?
Example on godbolt, without -pthread.
So, it is NOT generating different code. Ie, the backend of the compiler is NOT different. It is only conditional compilation and linking to different libraries. It does not generate 'lock free' code or add appropriate machine barriers because you have used this option.
In Anjuta, go to the Build menu, then Configure Project.
In the Configure Options box, add:
LDFLAGS='-lpthread'
Hope it'll help somebody too...
Sometimes, if you use multiple library, check the library dependency.
(e.g. -lpthread -lSDL... <==> ... -lSDL -lpthread)