bus error on sem_wait() - c

Question

I am writing a multi-process program using named semaphores, in the master process I am opening the semaphore with the following code
semaphore = sem_open("/msema",O_RDWR|O_CREAT|O_TRUNC,00777,1);
if (semaphore == SEM_FAILED)
perror("SEMAPHORE");
and in the child program
count_sem=sem_open("/msema",O_RDWR);
if(count_sem==SEM_FAILED)
{
perror("sem_open");
return 1;
}
on sem_wait()
do {
errno=0;
printf("BeforeSemWait\n");
rtn=sem_wait(count_sem);
printf("afterSemWait\n");
} while(errno==EINTR);
if(rtn < 0) {
printf("Error\n");
perror("sem_wait()");
sem_close(count_sem);
return 1;
}
I am getting a bus error from sem_wait()
BeforeSemWait
Program received signal SIGBUS, Bus error.
0x00a206c9 in sem_wait##GLIBC_2.1 () from /lib/libpthread.so.0`
What am I doing wrong?
edit: entire code:
master.c: http://pastebin.com/3MnMjUUM
worker.c http://pastebin.com/rW5qYFqg

Answer 1

You must have somewhere else a bug in your program. The following works here (O_TRUNC is not needed):
semproducer.c:
#include <fcntl.h>
#include <stdio.h>
#include <semaphore.h>
int main () {
sem_t *sem=sem_open("/msema",O_RDWR|O_CREAT /* |O_TRUNC*/ ,00777,1);
if (sem==SEM_FAILED) {
perror("sem_open");
}
else {
while (1) {
sem_post (sem);
printf ("sem_post done\n");
sleep (5);
}
}
}
semconsumer.c:
#include <fcntl.h>
#include <stdio.h>
#include <semaphore.h>
#include <errno.h>
int main () {
sem_t *count_sem=sem_open("/msema",O_RDWR);
if(count_sem==SEM_FAILED) {
perror("sem_open");
return 1;
}
do {
int rtn;
do {
errno=0;
rtn=sem_wait(count_sem);
} while(errno==EINTR);
if(rtn < 0) {
perror("sem_wait()");
sem_close(count_sem);
return 1;
}
printf ("sema signalled\n");
} while (1);
}
compile with gcc semproducer.c -o semproducer -lrt and gcc semconsumer.c -o semconsumer -lrt

Answer 2

I encountered this exact error.
It was because I hadn't linked in the Real-Time library that provides POSIX real-time functionality. The commenter above briefly mentioned the "-lrt" parameter in his solution, but did not emphasize it as the reason that you were getting your exception. You can verify this with the following code:
int main(void)
{
sem_t* my_sem = sem_open("/test_sem", O_CREAT, S_IRUSR | S_IWUSR, 0);
sem_wait(my_sem);
}
Assuming this is in the file "test.c" and I run the following:
gcc ./test.c -o test.out
./test.out
I get the Bus Error Output:
Bus error
But with the following command:
gcc ./test.c -o test.out -lrt
./test.out
The program does not except, and instead waits on the semaphore forever which is expected with a value of zero.
You should NOT pass -lrt, however. The linux documentation for sem_wait(3) indicates that the correct command is "-pthread". Substituting "-lrt" with "-pthread" in my testing indicates that it has the same effect, so the list of flags passed by -pthread on Linux likely includes "-lrt".
As such, the correct compiler invocation would be
gcc ./test.c -o test.out -pthread