Writing in System V shared memory segment

Question

I'm studying shared memory and now I'm writing a program that use system v shared memory. This is my code:

#include <stdlib.h> #include <stdio.h> #include <unistd.h> #include <string.h> #include <sys/wait.h> #include <sys/shm.h> #include <sys/ipc.h> #include <fcntl.h> #define PG_SIZE sysconf(_SC_PAGESIZE) #define PROJ_ID 1 #define SHM_FILE "/dev/shm/myshm" void executeChild( void ); void executeParent( void ); int main ( int argc, char *argv[] ) { unlink(SHM_FILE); int fd = creat(SHM_FILE, S_IRWXU ); if( fd == -1 ){ perror("Segment memory file creation failed"); exit(1); } int pid = fork(); if( pid < 0){ perror("Fork failed\n"); return EXIT_FAILURE; } if( pid ){ executeParent(); printf("Parent waiting...\n"); int status = 0; wait(&status); //wait for child process printf("Parent done\n"); }else{ executeChild(); } close( fd ); return EXIT_SUCCESS; } void executeChild( void ) { printf("Child running\n"); sleep(15); } void executeParent( void ) { printf("Parent running\n"); key_t token = ftok(SHM_FILE, PROJ_ID); if( token == -1 ){ perror("Token creation failed"); exit(1); } int segment = shmget( token, PG_SIZE, IPC_CREAT | IPC_EXCL | S_IRWXU); if ( segment == -1 ){ perror("Segment creation failed"); exit(1); } void * shm_ptr = shmat(segment, NULL, 0); if( shm_ptr == (void *)(-1) ){ perror("Segment attachament failed"); exit(1); } printf("Shared memory ( %d ) attached\n", segment); struct shmid_ds shm_info; if( shmctl(segment, IPC_STAT, &shm_info) == -1 ){ perror("shmctl failed"); exit(1); } printf("Segment size = %zu\n", shm_info.shm_segsz); printf("Writing...\n"); const char * test = "teste"; memcpy(shm_ptr, test, strlen(teste)); }

The file of the shared memory is created. I can see it on /dev/shm and icps commando also show it. But the size of the file of my shared memory segment is not increasing. So I presume that the memcpy is not working properly like I was expected. Why?

PG_SIZE is the size of shared memory segment. It's not going to change by itself. And you should be careful not to exceed it. — oakad
– oakad, Commented Apr 11, 2014 at 3:31
Yeap, thank you for your advice. But I'm not exceed the PG_SIZE. I tested write on a 40mg shared memory segment and does not work too. — vanz
– vanz, Commented Apr 11, 2014 at 10:04
By the way, have you tried other shared memory approaches? /dev/shm is normally the place used by "shm_open()" to put its objects there and not supposed to be modified directly by user, so you should try using that first (shm_open/mmap). Alternatively, try to create your underlying file elsewhere, but even then you may not need SYSV shmem: you can just mmap that file with MAP_SHARED flag. — oakad
– oakad, Commented Apr 14, 2014 at 1:26

vanz · Accepted Answer · 2014-04-16 04:19:43Z

Finally, I got it how shared memory really works... special thanks for Thiago Klock. The file passed to ftok does not change its size. It exists just for generate the unique key used in the Interprocess Communication... :)

This is my source code where my parent process write a simple message to the child process:

https://github.com/guilhermesft/vanzstuff/blob/master/ipc/shmsysv.c

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