This is filelock/create_pid_file.c (Listing 55-4, page 1143), an example from the book, The Linux Programming Interface.

The source code file is copyright 2010, Michael Kerrisk, and is licensed under the GNU Lesser General Public License, version 3.

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In the listing below, the names of Linux system calls and C library functions are hyperlinked to manual pages from the Linux man-pages project, and the names of functions implemented in the book are hyperlinked to the implementations of those functions.

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Function list (Bold in this list means a function is not static)

#include <sys/stat.h>
#include <fcntl.h>
#include "region_locking.h"             /* For lockRegion() */
#include "create_pid_file.h"            /* Declares createPidFile() and
                                           defines CPF_CLOEXEC */
#include "tlpi_hdr.h"

#define BUF_SIZE 100            /* Large enough to hold maximum PID as string */
/* Open/create the file named in 'pidFile', lock it, optionally set the
   close-on-exec flag for the file descriptor, write our PID into the file,
   and (in case the caller is interested) return the file descriptor
   referring to the locked file. The caller is responsible for deleting
   'pidFile' file (just) before process termination. 'progName' should be the
   name of the calling program (i.e., argv[0] or similar), and is used only for
   diagnostic messages. If we can't open 'pidFile', or we encounter some other
   error, then we print an appropriate diagnostic and terminate. */

createPidFile(const char *progName, const char *pidFile, int flags)
    int fd;
    char buf[BUF_SIZE];

    fd = open(pidFile, O_RDWR | O_CREAT, S_IRUSR | S_IWUSR);
    if (fd == -1)
        errExit("Could not open PID file %s", pidFile);

    if (flags & CPF_CLOEXEC) {

        /* Set the close-on-exec file descriptor flag */

        flags = fcntl(fd, F_GETFD);                     /* Fetch flags */
        if (flags == -1)
            errExit("Could not get flags for PID file %s", pidFile);

        flags |= FD_CLOEXEC;                            /* Turn on FD_CLOEXEC */

        if (fcntl(fd, F_SETFD, flags) == -1)            /* Update flags */
            errExit("Could not set flags for PID file %s", pidFile);

    if (lockRegion(fd, F_WRLCK, SEEK_SET, 0, 0) == -1) {
        if (errno  == EAGAIN || errno == EACCES)
            fatal("PID file '%s' is locked; probably "
                     "'%s' is already running", pidFile, progName);
            errExit("Unable to lock PID file '%s'", pidFile);

    if (ftruncate(fd, 0) == -1)
        errExit("Could not truncate PID file '%s'", pidFile);

    snprintf(buf, BUF_SIZE, "%ld\n", (long) getpid());
    if (write(fd, buf, strlen(buf)) != strlen(buf))
        fatal("Writing to PID file '%s'", pidFile);

    return fd;


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