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Full Discussion: sem_t variable in a C structure????
Top Forums Programming sem_t variable in a C structure???? Post 302595445 by gabam on Friday 3rd of February 2012 06:51:18 AM
Old 02-03-2012
sem_t variable in a C structure????
Hi friends,
I hope you guys are doing well. I am facing this problem, hope you can help me with it. I am trying to create a semaphore inside a structure. The idea is that I am creating a shared memory space between two processes. The reason for a semaphore is that I want some kind of synchronization between the two processes. I think you should see my code, it will become clear to you. I am also including the error that the compiler is giving me.



Code:
 
$ cat producer1.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/stat.h>
#include <semaphore.h>
#include <sys/sem.h>
#define MEM_SZ 4096
struct shared_use_st
{
        sem_t p;
        sem_t c;
        char some_text[MEM_SZ];
};
int main()
{
int running = 1;
void *shared_memory = (void *)0;
struct shared_use_st *shared_stuff;
char buffer[MEM_SZ];
int shmid;
shmid = shmget((key_t)1234, MEM_SZ, 0666 | IPC_CREAT);
if(shmid == -1)
{
printf("shmget failed\n");
exit(EXIT_FAILURE);
}
shared_memory = shmat(shmid, NULL, 0);
if(shared_memory == (void *)-1)
{
printf("\nshmat failed\n");
exit(EXIT_FAILURE);
}
printf("Memory attached at %x\n", (int)shared_memory);
shared_stuff = (struct shared_use_st *)shared_memory;
sem_init(&(shared_stuff->p), 1, 1);
sem_init(&(shared_stuff->c), 1, 0);
while(running)
{
        printf("\nwaiting for client...\n");
        sem_wait(&(shared_stuff->p));
        printf("Enter some text: ");
        fgets(buffer, MEM_SZ, stdin);
        strcpy(shared_stuff->some_text, buffer);
        if(strncmp(buffer, "end", 3) == 0)
        {
                running = 0;
        }
        sem_post(&(shared_stuff->c));
}
if(shmdt(shared_memory) == -1)
{
printf("shmdt failed\n");
exit(EXIT_FAILURE);
}
exit(EXIT_SUCCESS);
}

And here is what the compiler is telling me.



Code:
 
$ gcc producer1.c -o producer1
Undefined                       first referenced
 symbol                             in file
sem_init                            /var/tmp//ccjymEo2.o
sem_post                            /var/tmp//ccjymEo2.o
sem_wait                            /var/tmp//ccjymEo2.o
ld: fatal: Symbol referencing errors. No output written to producer1
collect2: ld returned 1 exit status

 

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LEARN ABOUT DEBIAN

sem_init

SEM_INIT(3)						     Linux Programmer's Manual						       SEM_INIT(3)

NAME

       sem_init - initialize an unnamed semaphore

SYNOPSIS

       #include <semaphore.h>

       int sem_init(sem_t *sem, int pshared, unsigned int value);

       Link with -pthread.

DESCRIPTION

       sem_init() initializes the unnamed semaphore at the address pointed to by sem.  The value argument specifies the initial value for the sem-
       aphore.

       The pshared argument indicates whether this semaphore is to be shared between the threads of a process, or between processes.

       If pshared has the value 0, then the semaphore is shared between the threads of a process, and should be located at some  address  that	is
       visible to all threads (e.g., a global variable, or a variable allocated dynamically on the heap).

       If pshared is nonzero, then the semaphore is shared between processes, and should be located in a region of shared memory (see shm_open(3),
       mmap(2), and shmget(2)).  (Since a child created by fork(2) inherits its parent's memory mappings, it can also access the semaphore.)   Any
       process that can access the shared memory region can operate on the semaphore using sem_post(3), sem_wait(3), etc.

       Initializing a semaphore that has already been initialized results in undefined behavior.

RETURN VALUE

       sem_init() returns 0 on success; on error, -1 is returned, and errno is set to indicate the error.

ERRORS

       EINVAL value exceeds SEM_VALUE_MAX.

       ENOSYS pshared is nonzero, but the system does not support process-shared semaphores (see sem_overview(7)).

CONFORMING TO

       POSIX.1-2001.

NOTES

       Bizarrely,  POSIX.1-2001  does  not  specify  the value that should be returned by a successful call to sem_init().  POSIX.1-2008 rectifies
       this, specifying the zero return on success.

SEE ALSO

       sem_destroy(3), sem_post(3), sem_wait(3), sem_overview(7)

COLOPHON

       This page is part of release 3.44 of the Linux man-pages project.  A description of the project, and information about reporting bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux								    2012-05-13							       SEM_INIT(3)
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