Consider the following test project:
test.h:
#ifndef TEST_H
#define TEST_H
void test1(int);
void test2(int);
#endif /* TEST_H */
text.c:
#include "test.h"
void test1(int x) { (void) x; }
Oops, I forgot to define test2()! I would like some kind of feedback when I do this, preferably refusal to compile although a warning at least would be nice. However GCC 10.2 (on Ubuntu 20.10) compiles it fine with no warnings:
gcc -Wall -Wextra -Wpedantic -std=c11 -o libtest.o -c test.c
I think I understand why: what if test2() is actually meant to come from another library, maybe a system library? Make it the problem of whichever program ends up linking everything into an executable! But I want to know about it before then. In this case, it's not declared in any included header file. It's not called anywhere. Can that be detected?
I've tried:
--no-undefined which resulted in gcc: error: unrecognized command-line option ‘--no-undefined’; did you mean ‘-Wno-undef’?
-Wno-undef - accepted but no warning
-z,defs - accepted but no warning
-Wimplicit-function-declaration - accepted but no warning
-Werror=missing-declarations - I know this is for the opposite situation but I was getting desperate.
This isn't possible, as no linking is performed at the stage of assembling a static library.
I'd suggest having a "test container" for your library. Set up your build system to build the test executable any time you are building the library. It could even just be a single .c file in the same directory as the library sources, but obviously not in the list of objects that are part of the library.
The test executable calls all of the functions that you wish to be entry points for the library.
Probably that is something you should be doing anyway in order to test the library's functionality before doing a release.
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 new programming in C, now I'm trying to implement threads in a motor control program I am developing but my problem is that I can not run the pthread.h library in eclipse. despite haver installed POSIX threading library for Win32
Info: Internal Builder is used for build
gcc -O3 -g3 -Wall -pthread -c -fmessage-length=0 -o src\main.o ..\src\main.c
gcc: error: unrecognized option '-pthread'
Info: Parallel threads used: 1
Even though I'm just stating nothing but the library
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
int main(void) {
return EXIT_SUCCESS;
}
check forum several times and did what they wanted to establish in the -pthread miscellaneous and link phtread but I can not solve the problem, could someone please help me?
I have Windows 10, 64 bits.
If your code is not compiling, you need to add -pthread argument in your gcc compilation as follows.
Navigate to: Project -> Properties
On the left: c/c++ build -> GCC C Compiler -> Miscellaneous
Add -pthread argument at the beginning Other Flags section
Also pthread library add c/c++ build -> Settings -> GCC C Linker -> Libraries
And include pthread library into other libraries.
Click apply.
Clean and build the project.
If you have only Unresolved inclusion problem alone then specify the file system path for the pthread.h as your compiler does not know this.
Check here how to add pthread.h in your filesystem.
I gave up trying, resort to the use of counters card that I use (KL25Z) and events programming.
I am programming a c project which must use the pow function defined in math.h.
And when I tried to make the project, gcc gave the following link error:
undefined reference to `pow'.
I know the -lm option must be added into my link instruction, but there are still several questions puzzling me.
Q1:When I pass two constants into pow function, link is successful without -lm. Why?
Q2:-lm being at the end or at the start of link instruction makes different results. gcc -lm $(OBJS) -o exbin is wrong, but gcc $(OBJS) -o exbin -lm is correct. Why?
I use ubuntu 11.10 and gcc 4.4.4.
Thanks! Please excuse my pool English.
The compiler knows about pow(3, 4) or whatever, and optimizes the program by computing the result at compile time, so it doesn't need the library at link and run time.
A linker doesn't add stuff from a library unless it knows it needs it. With static libraries, that was strictly true. With shared libraries, some versions of the compilers would keep a note of all the symbols in all the shared libraries that were read, even if they weren't needed at the time when the library was scanned. More recent versions only take in shared libraries if at least one of the symbols is needed at the time when it is scanned. When the library comes first, the only symbol needed is main() (that's why main() is designated as the start point for a hosted environment), and therefore the maths library was ignored because there were no symbols in it that were needed.
I cannot answer question 1 (that seems odd), but in regards to question 2 the reason gcc -lm $(OBJS) -o exbin does not work is because you must link things in order of useage. This is best explained by example:
/* File func_a.h */
/* Declare func_a */
void func_a();
/* File func_a.c */
#include "func_a.h"
void func_a()
{
/* do stuff */
}
/* File func_b.c */
#include "func_a.h"
void func_b()
{
/* Call func_a */
func_a();
}
To properly link func_a and func_b into an executable, you must link them as gcc func_b func_a -o exec because func_b uses func_a. In short, you always want to link your library functions last.
I'm trying to start developing a program using ncurses on Linux. I can't even get the Hello World example to compile. Here's the code:
#include <curses.h>
int main()
{
initscr();
printw("Hello, world.");
refresh();
getch();
endwin();
return 0;
}
When I attempt to compile, I get:
hello.c:(.text+0x12): undefined reference to `initscr'
For every one of those called functions.
I installed ncurses via apt-get, and also by downloading the sources and compiling, installing, etc.
I have tried #include both curses.h and ncurses.h.
What is going on?
Have you used the -lcurses option when linking?
Including the header files let the code compile (because the compiler knows what the function call looks like from the .h file), but the linker needs the library file to find the actual code to link into your program.
As Greg Hewgill said, you need to pass in -lcurses or -lncurses to link to the curses library.
gcc -o hello hello.c -lncurses
You also probably mean to use initscr() and getch(). Once I make those substitutions, the above compiles for me.
I was having a similar issue and found a solution which helped me, but was slightly different from the other answers posted here. I was trying to use the panels library with curses and my compile command was:
$ gcc -o hello hello.c -lncurses -lpanel
when I read the other answers, I was baffled because I was including the -lncurses flag, but it still was not compiling, and with similar errors to what you were getting:
$ gcc -o hello hello.c -lncurses -lpanel
/usr/lib/gcc/i686-linux-gnu/4.7/../../../../lib/libpanel.a(p_new.o): In function `new_panel':
p_new.c:(.text+0x18): undefined reference to `_nc_panelhook'
I finally found my answer in the tldp:
"To use panels library functions, you have to include panel.h and to link the program with panels library the flag -lpanel should be added along with -lncurses in that order."
So, it appears that order is important when using the compile flags! I tried switching the order:
gcc -o hello hello.c -lpanel -lncurses
This allowed it to compile successfully. I know you already have your answer, so I hope this helps someone.
For anyone having similar problems: -lx arguments, where x is your library, should always follow the source and object files.