op_lockfile.c

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#include "op_lockfile.h"
#include "op_file.h"

#include <errno.h>

#include <sys/types.h>
#include <stdio.h>
#include <signal.h>
#include <unistd.h>

static pid_t op_read_lock_file(char const * file)
{
    FILE * fp;
    pid_t value;

    fp = fopen(file, "r");
    if (fp == NULL)
        return 0;

    if (fscanf(fp, "%d", &value) != 1) {
        fclose(fp);
        return 0;
    }

    fclose(fp);

    return value;
}


int op_write_lock_file(char const * file)
{
    FILE * fp;

    if (op_file_readable(file)) {
        pid_t pid = op_read_lock_file(file);

        /* FIXME: ESRCH vs. EPERM */
        if (kill(pid, 0)) {
            int err = unlink(file);
            fprintf(stderr, "Removing stale lock file %s\n",
                file);
            if (err)
                return err;
        } else {
            return EEXIST;
        }
    }

    fp = fopen(file, "w");
    if (!fp)
        return errno;

    fprintf(fp, "%d", getpid());
    fclose(fp);

    return 0;
}