I'm new with semaphores and want to add multithreading to my program, but I cannot get around the following problem: sem_wait() should be able to receive a EINTR and unblock, as long as I didn't set the SA_RESTART flag. I am sending a SIGUSR1 to the worker thread that is blocking in sem_wait(), it does receive the signal and get interrupted, but it will then continue to block and so it will never give me a -1 return code together with errno = EINTR. However, if I do a sem_post from the main thread, it will unblock, give me an errno of EINTR but a RC of 0. I am totally puzzled with this behavior. Is it some weird NetBSD implementation or am I doing something wrong here? According to the man page, sem_wait is conform POSIX.1 (ISO/IEC 9945-1:1996). A simple code:
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <signal.h>
#include <pthread.h>
#include <semaphore.h>
typedef struct workQueue_s
{
int full;
int empty;
sem_t work;
int sock_c[10];
} workQueue_t;
void signal_handler( int sig )
{
switch( sig )
{
case SIGUSR1:
printf( "Signal: I am pthread %p\n", pthread_self() );
break;
}
}
extern int errno;
workQueue_t queue;
pthread_t workerbees[8];
void *BeeWork( void *t )
{
int RC;
pthread_t tid;
struct sigaction sa;
sa.sa_handler = signal_handler;
sigaction( SIGUSR1, &sa, NULL );
printf( "Bee: I am pthread %p\n", pthread_self() );
RC = sem_wait( &queue.work );
printf( "Bee: got RC = %d and errno = %d\n", RC, errno );
RC = sem_wait( &queue.work );
printf( "Bee: got RC = %d and errno = %d\n", RC, errno );
pthread_exit( ( void * ) t );
}
int main()
{
int RC;
long tid = 0;
pthread_attr_t attr;
pthread_attr_init( &attr );
pthread_attr_setdetachstate( &attr, PTHREAD_CREATE_JOINABLE );
queue.full = 0;
queue.empty = 0;
sem_init( &queue.work, 0, 0 );
printf( "I am pthread %p\n", pthread_self() );
pthread_create( &workerbees[tid], &attr, BeeWork, ( void * ) tid );
pthread_attr_destroy( &attr );
sleep( 2 );
sem_post( &queue.work );
sleep( 2 );
pthread_kill( workerbees[tid], SIGUSR1 );
sleep( 2 );
// Remove this and sem_wait will stay blocked
sem_post( &queue.work );
sleep( 2 );
return( 0 );
}
I know the printf is not aloud in the signal handler, but just for the heck of it, if I remove it I get the same results.
These are the results without sem_post:
I am pthread 0x7f7fffc00000
Bee: I am pthread 0x7f7ff6c00000
Bee: got RC = 0 and errno = 0
Signal: I am pthread 0x7f7ff6c00000
And with the sem_post:
I am pthread 0x7f7fffc00000
Bee: I am pthread 0x7f7ff6c00000
Bee: got RC = 0 and errno = 0
Signal: I am pthread 0x7f7ff6c00000
Bee: got RC = 0 and errno = 4
I know I don't really need to unblock and can simply do an exit from main, but I want to see it working anyway. The reason I'm using sem_wait is because I want to keep the worker threads alive and wake the one up waiting the longest from the main thread with sem_post, as soon as there is a new client connection from Postfix. I don't want to do pthread_create all the time, since I will receive calls multiple times per second and I don't want to lose speed and make Postfix unresponsive to new smtpd clients. It is a policydaemon for Postfix and the server is quite busy.
Am I missing something here? Is NetBSD just messed up with this?
struct sigaction sa;, either dostruct sigaction sa = {0};ormemset(&sa, 0, sizeof sa);– LimoniteBee: got RC = -1 and errno = 4(note that you should removeextern int errno, as declaring errno like that is the wrong thing to do in a multithreaded program) – Limonite