536

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?

codeforester
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Ralph
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    Additionally: depending on the platform, you may need (a) a different compiler for threads, (b) a different libc for threads (i.e. `-lc_r`), (c) `-thread` or `-threads` or other, instead of or in addition to `-lpthread`. – ephemient Nov 03 '09 at 01:42
  • Just a little above that example, you'll see a table of the correct compiler commands, whether it be GCC, IBM, etc. 'Employed Russian' is correct. – Jonathon Reinhart Jun 26 '11 at 06:47
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    Can you please unmark my answer, so that I can delete it (and mark the one that is actually correct, which is the highest-voted one)? – Pavel Minaev Nov 22 '12 at 00:41
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    `-lpthread` is needed during compile – How Chen Jan 28 '14 at 08:21
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    solution `LDFLAGS= -pthread -lpthread` – dsnk Oct 29 '15 at 09:01
  • Is `pthread_exit(NULL);` in main required/good practice ? – Vikram Feb 05 '17 at 01:10

16 Answers16

913

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.

Employed Russian
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    +1 this solution worked... otheres did not. Also, suggestion that 'libraries should follow sources and objects' is great advice -- a citation or further explanation would be great. – sholsapp Jan 13 '12 at 03:02
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    @sholsapp Here is the explanation: http://webpages.charter.net/ppluzhnikov/linker.html – Employed Russian Jan 13 '12 at 04:33
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    This still errored for me till I put -lpthread at the very end of my command. `gcc term.c -lpthread` – CornSmith Apr 18 '13 at 23:49
  • I just ran into an issue compiling *snortsam* on Ubuntu 14.04 which actually has both libpthread.a and libpthread.so. I was getting ```undefined reference to 'pthread_cancel'``` and ```undefined reference to 'pthread_create'``` errors. I came across this SO post and thought I'd try out Employed Russian's answer. I opened up ```makesnortsam.sh``` in VI and ran the command ```:%s/lpthread/pthread/g``` to replace lpthread with pthread so that it would use ```-pthread``` instead of ```-lpthread``` when compiling. I was then able to compile snortsam. Thanks Employed Russian! – dcarrith Jul 05 '14 at 13:47
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    For anyone using CODEBLOCKS: Add `-pthread` to Project Build Options -> Linker Settings -> Other linker options. – FreelanceConsultant Oct 04 '15 at 13:23
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    *"On a system with only libpthread.a installed"*...... or `libpthread.so` , right ? – Accountant م Apr 13 '19 at 18:04
  • To make sure I remember to do this when compiling from the command line like this, I like to create a .sh file with the same name as the program I'm compiling to save the command. Then I just run it when I want to compile. Thanks for the great answer. It's still helping people. – David Peterson Harvey Feb 26 '20 at 08:12
  • Re: "In general, libraries should follow sources and objects on command line": I understand it in case of e.g. `accept`, which is _not_ part of the C standard library, but placed in libc.a. So, in case if user defines its own `accept` and then manually links with libc.a, then the libc.a has to follow the objects (otherwise, `accept` from libc.a may be used). However: why using `-lpthread` _before_ the objects leads to "undefined reference to 'pthread_create'"? Why linker cannot find `pthread_create`? – pmor Dec 07 '22 at 08:07
  • @pmor I've edited the answer with links to the explanation of "why linker cannot ...". – Employed Russian Dec 08 '22 at 04:19
  • Thanks. So, per [this](https://eli.thegreenplace.net/2013/07/09/library-order-in-static-linking) "since it doesn't need `pthread_create`, this object file is not included in the link" The question is _why_ linker is implemented in this way? Why linker does not assume that later some object file may need `pthread_create`? – pmor Dec 09 '22 at 10:00
  • @pmor This is a question answering site, and asking questions is free :-) Please ask a new question as a real question, instead of continuing discussion in comments. – Employed Russian Dec 09 '22 at 16:47
55

For Linux the correct command is:

gcc -o term term.c -lpthread
  1. you have to put -lpthread just after the compile command,this command will tell to the compiler to execute program with pthread.h library.
  2. gcc -l links with a library file.Link -l with library name without the lib prefix.
Gopika BG
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44

in eclipse

properties->c/c++Build->setting->GCC C++ linker->libraries in top part add "pthread"

sam
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  • Same tip applyes in code::project (and I think others IDE too) – Fil Jan 29 '17 at 17:00
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    I know this might be a little late, but. If you cannot find the C/C++ Build setting in the properties (I couldn't, maybe it's by installation or a bug), then there is a direct lower level workaround using the CMakeLists.txt file. You need to insert `SET(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -pthread")` before the add_executable command. This will instruct the linker to do the same (see [CMAKE_EXE_LINKER_FLAGS](https://cmake.org/cmake/help/latest/variable/CMAKE_EXE_LINKER_FLAGS.html) and [SET](https://cmake.org/cmake/help/latest/command/set.html) documentation for more help). – Avneesh Mishra Sep 10 '20 at 07:13
34

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!

Alexander Nenartovich
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32

If you are using cmake, you can use:

add_compile_options(-pthread)

Or 

SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pthread")
Jayhello
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28

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}
)
Rufus
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    This has the same effect but used the Threads::Threads target rather than the CMAKE_THREAD_LIBS_INIT variable. ```target_link_libraries(helloworld PUBLIC Threads::Threads) ``` – Aidan Gallagher Aug 09 '20 at 16:34
20

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

enter image description here

dylanninin
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11

Compile it like this : gcc demo.c -o demo -pthread

leesagacious
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8

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.

WBuck
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  • When I do that, I get the error "pthread: No such file or directory" – Niko O Oct 26 '21 at 12:40
  • My link was failing in a Release build but succeeding in the Debug build. It turns out that the -pthread was missing from the Release build. Make sure both build environments match. – J. Martin Sep 22 '22 at 14:43
4

you need only Add "pthread" in proprieties=>C/C++ build=>GCC C++ Linker=>Libraries=> top part "Libraries(-l)". thats it

sam
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4

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.
       ....
kalu Wang
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4

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"],

Add this line to your c_cpp_properties.json file

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.

Sold Out
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4

You need to use the option -lpthread with gcc.

Waynn Lue
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xiaolong
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2

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.

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.

artless noise
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  • _REENTRANT is an example that was used with some GCC versions. Newer standards, etc may change the structure that is used to alter they system/compiler headers depending on what language version, OS and C library is used. – artless noise Apr 03 '22 at 22:01
0

In Anjuta, go to the Build menu, then Configure Project. In the Configure Options box, add:

LDFLAGS='-lpthread'

Hope it'll help somebody too...

naXa stands with Ukraine
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0

Sometimes, if you use multiple library, check the library dependency. (e.g. -lpthread -lSDL... <==> ... -lSDL -lpthread)

user1793997
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