shm_unlink says “no such file or directory”

Question

I am trying to use shm_unlink(object_path) to clear up the shared memory I opened in signal handling function. However, the code doesn't work. What goes wrong? The code basically does this: A parent process is taking two integers from user input, and fork a child to compute the sum of the two. Once the sum is computed, the child informs the parent that it's done computing sum through a pipe. When receiving a SIGINT signal, the child should not terminate by itself, it should let the parent process handle the signal where the parent then send a SIGKILL signal to terminate the child process. Thank you!

#include <sys/mman.h>
#include <sys/types.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdlib.h>
#include <stdio.h>
#include <signal.h>


#define SHARED_OBJECT_PATH         "/shared_memory"
#define READ_END 0
#define WRITE_END 1
#define BUFFER_SIZE 1
//Global Variables
int pid; //Child Process ID

void signal_callback_handler(int signum){
    if (signum == SIGINT){
        printf("I am %d and I am handling SIGINT. \n", getpid()); /* process ID that's handling SIGINT */
        kill(pid,SIGKILL);
        if (shm_unlink(SHARED_OBJECT_PATH) != 0) {
            perror("In shm_unlink()");
            exit(1);
        }
    }
}
int main(int argc, char *argv[]){
    int fd,status;
    int pfd1[2],pfd2[2]; /*Two communication channels will be created. pfd1 for parent->child, pfd2 child->parent*/
    char msg[BUFFER_SIZE];
    int first_num, second_num, sum;

    int shared_seg_size = 3*(sizeof(int));
    int *shared_msg; /* We want a shared segment capable of storing 2 integers and 1 sum */

    if(pipe(pfd1) < 0 || pipe(pfd2) < 0){
        printf("Failed to create a pipe between parent and child \n");
        exit(-1);
    }


    /* Open the Shared Memory Segment */    
    fd = shm_open(SHARED_OBJECT_PATH, O_CREAT | O_EXCL | O_RDWR, S_IRWXU | S_IRWXG);
    if (fd < 0) {
        perror("In shm_open()");
        exit(1);
    }
    fprintf(stderr, "Created shared memory object %s\n", SHARED_OBJECT_PATH);   
    /* Adjust mapped file size (make room for the whole segment to map) using ftruncate() */
    ftruncate(fd, shared_seg_size);
    /* Request the shared segment using mmap() */    
    shared_msg = (int *) mmap(NULL, shared_seg_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
    if (shared_msg == NULL) {
        perror("In mmap()");
        exit(1);
    }
    fprintf(stderr, "Shared memory segment allocated correctly (%d bytes).\n", shared_seg_size);
    /*End of the opening of the Shared Memory Segment */

    if((pid = fork()) < 0){ /* Fork the process */
      printf("Fork error \n");
      exit(-1);
    }
    else if (pid > 0){/* Parent code */
        signal(SIGINT, signal_callback_handler); /* To handle the Ctrl+C signal and clean up shared memory */
        close(pfd1[READ_END]);
        close(pfd2[WRITE_END]);

        while(1){ /* Keep running the main process to get user input */
            printf("Enter the first number: ");
            scanf("%d",&first_num);
            printf("Enter the second number: ");
            scanf("%d",&second_num);

            /* Write the two integers to the shared memory segment */
            shared_msg[0] = first_num;
            shared_msg[1] = second_num;

            msg[0] = '1'; /*Write to the pipe and tell child to compute the sum */
            write(pfd1[WRITE_END], msg, BUFFER_SIZE);
            read(pfd2[READ_END],msg, BUFFER_SIZE);
            if (msg[0] == '1'){         
                sum = shared_msg[2];
                printf("The sum is %d\n",sum);
                msg[0] = '0'; /*Tell the child to get ready to compute new sum*/
                write(pfd1[WRITE_END],msg,BUFFER_SIZE);
            }
        }
    }
    else if (pid == 0){/* Child Code */
        close(pfd1[WRITE_END]);
        close(pfd2[READ_END]);

        while(1){
            read(pfd1[READ_END],msg,BUFFER_SIZE);
            if (msg[0] == '1'){ /*Child should compute sum*/
                sum = shared_msg[0]+shared_msg[1];
                shared_msg[2] = sum;    
                msg[0] = '1';   /*Tell the parent, the sum is computed */
                write(pfd2[WRITE_END],msg,BUFFER_SIZE);
            }
        }
    }
}

Answer 1

I tried your code. Change this:

    if (shm_unlink(SHARED_OBJECT_PATH) != 0) {
        perror("In shm_unlink()");
        exit(1);
    }

in

    if (shm_unlink(SHARED_OBJECT_PATH) != 0) {
        perror("In shm_unlink()");
        exit(1);
    }
    else {
        perror("All right!");
        exit(0);
    }

otherwise you need TWO Ctrl + C to exit... the first removes the shared memory, the second makes your program exit because shm_unlink() returned an error (cause shmem was previously removed by the first Ctrl + c)