exclusive.c

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <fcntl.h>

#define CMD_OFFSET 2
#define MAX_CMD_LENGTH 2048


int strlen_total(char **str, int length) {
    int i, total = 0;
    for (i = 0; i < length; i++) {
        total += strlen(str[i]);
    }
    return total;
}

void join(char *res, char **str, int length, char *sep) {
    int i;
    for (i = 0; i < length; i++) {
        if (i > 0) {
            strcat(res, sep);
        }
        strcat(res, str[i]);
    }
}

int main(int argc, char **argv) {
    if (argc <= CMD_OFFSET) {
        fprintf(stderr, "%s ensures that only one instance of an application is running by creating an exclusive lockfile.\n\n", argv[0]);
        fprintf(stderr, "Usage: %s <lockfile> <command> [arg ...]\n", argv[0]);
        return 1;
    }

    // Grab name of lock file
    char *lock_fname = argv[1];

    // Grab a pointer to where the command starts - and the length
    char **cmd = argv + CMD_OFFSET;
    int length = argc - CMD_OFFSET;

    // Calculate total length of the command, assuming there is a space after each argument (and \0 after the last argument)
    int length_total = strlen_total(cmd, length) + length;

    // Make sure that our buffer is large enough to hold the command
    if (length_total > MAX_CMD_LENGTH) {
        fprintf(stderr, "ERROR: Max command length is %d bytes, given command is %d bytes\n", MAX_CMD_LENGTH, length_total);
        return 1;
    }

    // Build command string - separated by spaces
    char cmd_str[MAX_CMD_LENGTH] = "";
    join(cmd_str, cmd, length, " ");

    // Try to create lockfile in exclusive mode (this is an atomic operation)
    int lock_fd = open(lock_fname, O_CREAT|O_RDONLY|O_EXCL, 0600);
    if (lock_fd == -1) {
        fprintf(stderr, "ERROR: Failed to create exclusive lockfile: %s\n", lock_fname);
        return 1;
    }

    // Execute command
    int status = system(cmd_str);
    if (status == -1) {
        fprintf(stderr, "ERROR: Failed to execute command: '%s'\n", cmd_str);
    }

    // Delete lockfile
    remove(lock_fname);
}