Programming

Hi,

I am trying to write stuff to a shared memory using a writer, and reading the corresponding stuff using a reader. I am facing problems while releasing the lock, as a result of which I am having segmentation faults. The code is as follows...


/********** writer.c ***********/

#include <unistd.h>
#include <semaphore.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <sys/fcntl.h>

char shm_fn[] = "my_shm";
char sem_fn[] = "my_sem";

/**** WRITER ****/

main(){
caddr_t shmptr;
unsigned int mode;
int shmdes, index;
sem_t *semdes;
int SHM_SIZE;

mode = S_IRWXU|S_IRWXG;

/* Open the shared memory object */

if ( (shmdes = shm_open(shm_fn,O_CREAT|O_RDWR|O_TRUNC, mode)) == -1 ) {
perror("shm_open failure");
exit(-1);
}

/* Preallocate a shared memory area */

SHM_SIZE = sysconf(_SC_PAGE_SIZE);

if(ftruncate(shmdes, SHM_SIZE) == -1){
perror("ftruncate failure");
exit(-1);
}

if((shmptr = mmap(0, SHM_SIZE, PROT_WRITE|PROT_READ, MAP_SHARED,
shmdes,0)) == (caddr_t) -1){
perror("mmap failure");
exit(-1);
}

/* Create a semaphore in locked state */

semdes = sem_open(sem_fn, O_CREAT, 0644, 0);

if(semdes == (void*)-1){
perror("sem_open failure");
exit(-1);
}

/* Access to the shared memory area */

for(index = 0; index < 100; index++){
printf("write %d into the shared memory shmptr[%d]\n", index*2, index);
shmptr[index]=index*2;
}

/* Release the semaphore lock */
printf("\nRelease lock\n");

sem_post(semdes);
printf("\nBefore shmptr\n");
munmap(shmptr, SHM_SIZE);

/* Close the shared memory object */

close(shmdes);
printf("\nShmdes closed\n");
/* Close the Semaphore */

sem_close(semdes);
printf("\nSemdes closed\n");

/* Delete the shared memory object */

shm_unlink(shm_fn);
}

Code tags for code please. Like {code} stuff {/code} except with [ ] instead of { } . It will make it a lot more readable.

I might suggest using fprintf(stderr,"stuff") instead of printf("stuff). stdout is buffered -- you might not see all printed messages depending on where it crashes.

If sem_post is crashing, either you're giving it an invalid fd or you corrupted memory somewhere earlier on.