IPC through shared memory and pipe gives segmentation fault: 11 in C

Question

I am trying to share a file between parent and child processes. Parent sends the file via a pipe and child write that lines into shared memory so that parent can read and print out the file via shared memory. However, I am getting segmentation fault: 11. Besides, I did something similar like the code below, but that time I could not get the correct content and even I was getting different results at each call.

I am not sure about increasing the pointer part. But, it is better to have an extra eye on the code.

Edit: I corrected char* to char[] and segmentation fault is now gone. However, I get different results at each run, some extra characters are seen in output.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <sys/shm.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <unistd.h>

#define SHM_NAME_1 "Child 1"

int main(){

    pid_t pid;
    FILE *file;
    char *infile = "in.txt";

    pid = fork();

    if(pid < 0){
        fprintf(stderr, "Fork failed\n");
        return 1;
    }

    if(pid > 0){ // parent

        file = fopen(infile, "r");

        if(file == 0){
            fprintf(stderr, "File failed\n");
            return 1;
        }
        // close read end of pipe
        mknod("FIFO", S_IFIFO | 0666, 0);
        int fd = open("FIFO", O_WRONLY);

        char str[300];
        while(fgets(str, sizeof(str), file) > 0)
        {
            // write all lines of file
            write(fd, str, strlen(str));
        }
        // close file and pipe
        close(fd);
        fclose(file);
        // wait for child to write to shared memory
        wait(NULL);

        // open shared segment
        int shm_first = shm_open(SHM_NAME_1, O_RDONLY, 0666);

        if (shm_first == -1) {
            fprintf(stderr, "Failed: Shared Memory 1");
            exit(-1);
        }

        // create memory pointer
        void *ptr = mmap(0,4096, PROT_READ, MAP_SHARED, shm_first, 0);

        if (ptr == MAP_FAILED) {
            printf("Map failed 1\n");
            return -1;
        }
        // print out result and unlibk shared segment
        fprintf(stdout, "Normal input: \n%s\n", ptr);
        shm_unlink(SHM_NAME_1);

    } else { // child

        // create the shared segment for the first time
        int shm_child_1 = shm_open(SHM_NAME_1, O_CREAT | O_RDWR, 0666);

        // configure the size of the shared memory segment 
        ftruncate(shm_child_1,4096);
        // map the pointer to the segment
        void *ptr_child_1 = mmap(0,4096, PROT_READ | PROT_WRITE, MAP_SHARED, shm_child_1, 0);

        if (ptr_child_1 == MAP_FAILED) 
        {
            printf("Map failed in first child\n");
            return -1;
        }


        mknod("FIFO", S_IFIFO | 0666, 0);
        int fd = open("FIFO", O_RDONLY);

        int num;
        char s[300];
        while((num = read(fd, s, sizeof(s)))> 0)
        {     
            sprintf(ptr_child_1, "%s", s);
            ptr_child_1 += num;
        }

        close(fd);
        exit(0);

    }

    return 0;
}

Answer 1

One quick observation.

In the following code, you have a char pointer that is not initialized to point to anything. Which causes fgets to copy what it reads from file to some arbitrary place in memory.

    char *str;
    while(fgets(str, 100, file) > 0)

Now that the buffer issues are resolved, there is also an issue with the braces in the following expression

    while((num = read(fd, s, sizeof(s)) > 0))

num is going to be 1 or 0 rather than the number of bytes read or 0 for eof. This should be

    while((num = read(fd, s, sizeof(s))) > 0)

Once you have the number of bytes read, you need to zero terminate the buffer. Because you are using sprintf which expects the argument for %s to be a zero terminated string.

    while((num = read(fd, s, sizeof(s)))> 0)
    {
        s[num] = '\0';  // Terminate the string to the number of bytes read
        sprintf(ptr_child_1, "%s", s);
        ptr_child_1 += num;
    }