check if another instance of this program is already running

linux_check_instance.c

/* Linux: check if another instance of this program is already running
 * by looking through the /proc/<PID>/ entries */

/**
 * MIT License
 *
 * Copyright (C) 2024 Carsten Janssen
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */

#include <ctype.h>
#include <dirent.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>


/**
 * return values:
 * -1  error
 *  0  this is the first instance
 * >0  PID of the other running instance
 */
long simple_instance_check()
{
    DIR *d;
    struct dirent *dir;
    char *self;
    char buf[64];
    long pid_self, rv = -1;

    memset(buf, 0, sizeof(buf));

    pid_self = (long)getpid();

    if ((self = realpath("/proc/self/exe", NULL)) == NULL) {
        return -1;
    }

    if ((d = opendir("/proc")) == NULL) {
        return -1;
    }

    while ((dir = readdir(d)) != NULL) {
        char *rp, *endptr;
        long pid;

        /* looking for a directory */
        if (dir->d_type != DT_DIR && dir->d_type != DT_UNKNOWN) {
            continue;
        }

        pid = strtol(dir->d_name, &endptr, 10);

        /* name should be numbers only, ignore our own PID */
        if (*endptr != 0 || pid == pid_self) {
            continue;
        }

        /* get executable path */
        snprintf(buf, sizeof(buf)-1, "/proc/%ld/exe", pid);

        if ((rp = realpath(buf, NULL)) == NULL) {
            continue;
        }

        /* compare paths */
        if (strcmp(rp, self) == 0) {
            rv = pid;
            free(rp);
            goto JMP_EXIT;
        }

        free(rp);
    }

    rv = 0;

JMP_EXIT:

    closedir(d);
    free(self);

    return rv;
}


int main()
{
    long l = simple_instance_check();

    if (l == -1) {
        perror("simple_instance_check()");
        return 1;
    } else if (l > 0) {
        fprintf(stderr, "another instance is already running: PID %ld\n", l);
        return 1;
    }

    printf("PID %ld\n", (long)getpid());
    puts("press ENTER");
    (void)fgetc(stdin);

    return 0;
}