Today's Question:  What are you most afraid of as a programmer?        GIVE A SHOUT

Technical Article => Programming =>  C

A trick of building multithreaded application on Solaris

  NanXiao      2014-10-14 02:59:40      3,739    0    0

Firstly, Let’s see a simple multithreaded application:

#include <stdio.h>
#include <pthread.h>
#include <errno.h>

void *thread1_func(void *p_arg)
           errno = 0;
           errno = 1;
           printf("%s exit, errno is %d\n", (char*)p_arg, errno);

void *thread2_func(void *p_arg)
           errno = 0;
           printf("%s exit, errno is %d\n", (char*)p_arg, errno);

int main(void)
        pthread_t t1, t2;

        pthread_create(&t1, NULL, thread1_func, "Thread 1");
        pthread_create(&t2, NULL, thread2_func, "Thread 2");


What output do you expect from this program? Per my understanding, the errnoshould be a thread-safe variable. Though The thread1_func function changes the errno, it should not affect errno in thread2_func function.

Let’s check it on Solaris 10:

bash-3.2# gcc -g -o a a.c -lpthread
bash-3.2# ./a
Thread 1 exit, errno is 1
Thread 2 exit, errno is 1

Oh! The errno in thread2_func function is also changed to 1. Why does it happen? Let’s find the root cause from the errno.h file:

 * Error codes

#include <sys/errno.h>

#ifdef  __cplusplus
extern "C" {

#if defined(_LP64)
 * The symbols _sys_errlist and _sys_nerr are not visible in the
 * LP64 libc.  Use strerror(3C) instead.
#endif /* _LP64 */

#if defined(_REENTRANT) || defined(_TS_ERRNO) || _POSIX_C_SOURCE - 0 >= 199506L
extern int *___errno();
#define errno (*(___errno()))
extern int errno;
/* ANSI C++ requires that errno be a macro */
#if __cplusplus >= 199711L
#define errno errno
#endif  /* defined(_REENTRANT) || defined(_TS_ERRNO) */

#ifdef  __cplusplus

#endif  /* _ERRNO_H */

We can find the errno can be a thread-safe variable(#define errno (*(___errno()))) only when the following macros defined:

defined(_REENTRANT) || defined(_TS_ERRNO) || _POSIX_C_SOURCE - 0 >= 199506L

Let’s try it:

bash-3.2# gcc -D_POSIX_C_SOURCE=199506L -g -o a a.c -lpthread
bash-3.2# ./a
Thread 1 exit, errno is 1
Thread 2 exit, errno is 0

Yes, the output is right!

From Compiling a Multithreaded Application, we can see:

For POSIX behavior, compile applications with the -D_POSIX_C_SOURCE flag set >= 199506L. For Solaris behavior, compile multithreaded programs with the -D_REENTRANT flag.

So we should pay more attentions when building multithreaded application on Solaris.

(1) Compiling a Multithreaded Application
(2) What is the correct way to build a thread-safe, multiplatform C library?



Share on Facebook  Share on Twitter  Share on Google+  Share on Weibo  Share on Reddit  Share on Digg  Share on Tumblr    Delicious



No comment for this article.


Be serious, Google

By sonic0002
This is the Google doodle in Hong Kong for 2014 Mid-Autumn festival. Are you serious, Google? Do Chinese need to climb the ladder to see the moon? Are you meaning that the Chinese need to bypass the Great Firewall to use Google?