This is a short example to show you how to read and write a process id to a file in linux.

First the write.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>

int main(int argc, char **argv) {
    pid_t pid = getpid();

    if (argc < 2) {
        printf("Usage: %s <filename>\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    FILE *fp = fopen(argv[1], "w");
    if (!fp) {
        perror("fopen");
        exit(EXIT_FAILURE);
    }

    fprintf(fp, "%d\n", pid);

    fclose(fp);

    return EXIT_SUCCESS;
}

Then the read

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>

int main(int argc, char **argv) {
    pid_t pid = 0;

    if (argc < 2) {
        printf("Usage: %s <filename>\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    FILE *fp = fopen(argv[1], "r");
    if (!fp) {
        perror("fopen");
        exit(EXIT_FAILURE);
    }

    if (fscanf(fp, "%d\n", &pid) == 1) {
        printf("PID: %d\n", pid);
    } else {
        printf("Failed to Read PID\n");
    }

    fclose(fp);

    return EXIT_SUCCESS;
}