Writing in System V shared memory segment - c

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?

Answer 1

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