total lines of file using fork in c

Question

My code :

char *filename = "/home/septian/Documents/Repository.cs";
FILE *file = fopen(filename, "r");

if (file == NULL) {
    perror("File open failed");
    return 1;
}

int num_processes = 4; // Number of child processes to create
pid_t pid[num_processes];

for (int i = 0; i < num_processes; i++) {
    pid_t child_pid = fork();

    if (child_pid < 0) {
        perror("Fork failed");
        return 1;
    } else if (child_pid == 0) {
        // This code runs in the child process
        int lines = 0;
        char ch;
        fseek(file, 0, SEEK_SET); // Rewind the file to the beginning

        // Read the file and count lines
        while ((ch = fgetc(file)) != EOF) {
            if (ch == '\n') {
                lines++;
            }
        }

        **printf("Child process %d counted %d lines\n", i, lines);
        exit(lines); // Terminate the child process with the line count**
    } else {
        pid[i] = child_pid; // Store child process IDs in an array
    }
}

// This code runs in the parent process
printf("parent process\n");
int total_lines = 0;
for (int i = 0; i < num_processes; i++) {
    int status;
    waitpid(pid[i], &status, 0); // Wait for child processes to finish
    total_lines += WEXITSTATUS(status);
}

printf("Total lines in the file: %d\n", total_lines);
fclose(file);

when run, console show :

parent process

Child process 1 counted 15340 lines

Child process 3 counted 8866 lines

Child process 2 counted 5361 lines

Child process 0 counted 7089 lines

Total lines in the file: 816

Total lines not show real total lines in the file. How to show real total lines in the file for this case use fork?

Answer 1

When you call exit(lines) the parent process will receive lines & 0xFF which is at most 255 on Linux. When the client needs to communicate lines back to the parent process use a different IPC mechanism like pipe, message queue, shared memory segment (shmget, shmget), memory mapped file (mmap), socket etc.

A particular easy way to understand is for parent and child to agree on a path that the client is going to write the data, say, lines.<pid>. Client can get it's own pid with getpid() and the server already has it from the return value of fork().