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Shared Memory Creator
Creating a shared memory segment with shmget, attaching it with shmat, and writing initial data.
02-basic-example/creator.c131 lines
/*
* creator.c — Shared memory creator — shmget + shmat basics
*
* Key concepts: shmget, shmat, IPC_CREAT, writing to shared memory
* Compile: gcc -o creator creator.c
* Run: ./prog
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/shm.h>
#include <sys/ipc.h>
#include <sys/types.h>
const int MAX_STRING_SIZE = 100;
const int SHM_SIZE = MAX_STRING_SIZE + 1;
const char EMPTY = '-';
const char FULL = '+';
int main(int argc, char const *argv[])
{
printf("\n");
printf("══════════════════════════════════════\n");
printf(" Shared memory creator — shmget + s\n");
printf("══════════════════════════════════════\n\n");
key_t key;
int shmid;
char *shared_mem_ptr;
char str[MAX_STRING_SIZE];
//first create key using ftok
key = ftok("/tmp", 65);
//check if key is created successfully
if (key == -1) {
perror("ftok");
exit(EXIT_FAILURE);
}
//this program will create a shared memory segment
//https://man7.org/linux/man-pages/man2/shmget.2.html
// we can see all flags and what they do
if((shmid = shmget(key, SHM_SIZE, IPC_CREAT | IPC_EXCL | 0666)) < 0) {
perror("shmget");
exit(EXIT_FAILURE);
}
//now we created the shared memeory
// but we need to create a ptr to access it
shared_mem_ptr = (char *) shmat(shmid, NULL, 0);
//we know that each cell represents a char
//that why we did the casting
if (shared_mem_ptr == (char *) -1) {
perror("shmat");
exit(EXIT_FAILURE);
}
//now, we will use the [0] cell as an indicator
//to know if the shared memory is empty or not
shared_mem_ptr[0] = EMPTY;
printf("[Creator PID %d] Shared memory created (shmid=%d). Ready for input.\n", getpid(), shmid);
while (1) {
printf("Enter a string to write to shared memory (or 'exit' to quit): ");
fflush(stdout);
fgets(str, MAX_STRING_SIZE, stdin);
//remove the newline character
str[strcspn(str, "\n")] = 0;
//check if the shared memory is full
while (shared_mem_ptr[0] == FULL) {
printf("[Creator] Waiting — consumer hasn't read the last message yet...\n");
sleep(1);
}
//copy the string to the shared memory
strcpy(shared_mem_ptr + 1, str);
shared_mem_ptr[0] = FULL;
printf("[Creator] Wrote to shared memory: \"%s\"\n", str);
//check if the user wants to exit
if (strcmp(str, "exit") == 0) {
break;
}
}
//we send the other process an exit message
//so we want to sleep a little bit
//to make sure the other process reads the message
// and detaches from the shared memory
sleep(2);
//now we need to clean up
//detach the ptr
if(shmdt(shared_mem_ptr) < 0) {
perror("shmdt");
exit(EXIT_FAILURE);
}
//now we need to remove the shared memory
if(shmctl(shmid, IPC_RMID, NULL) < 0) {
perror("shmctl");
exit(EXIT_FAILURE);
}
//https://man7.org/linux/man-pages/man2/shmctl.2.html
/*
IPC_RMID
Mark the segment to be destroyed. The segment will
actually be destroyed only after the last process detaches
it (i.e., when the shm_nattch member of the associated
structure shmid_ds is zero). The caller must be the owner
or creator of the segment, or be privileged. The buf
argument is ignored.
If a segment has been marked for destruction, then the
(nonstandard) SHM_DEST flag of the shm_perm.mode field in
the associated data structure retrieved by IPC_STAT will
be set.
The caller must ensure that a segment is eventually
destroyed; otherwise its pages that were faulted in will
remain in memory or swap.
*/
return 0;
}