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;
sleep(3);
errno = 1;
printf("%s exit, errno is %d\n", (char*)p_arg, errno);
}
void *thread2_func(void *p_arg)
{
errno = 0;
sleep(5);
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");
sleep(10);
return;
}
What output do you expect from this program? Per my understanding, the errno
should be a thread-safe variable. Though The thread1_func
function changes theerrno
, 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" {
#endif
#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()))
#else
extern int errno;
/* ANSI C++ requires that errno be a macro */
#if __cplusplus >= 199711L
#define errno errno
#endif
#endif /* defined(_REENTRANT) || defined(_TS_ERRNO) */
#ifdef __cplusplus
}
#endif
#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.
P.S., the full code is here.
Reference:
(1) Compiling a Multithreaded Application;
(2) What is the correct way to build a thread-safe, multiplatform C library?