filelock/create_pid_file.c

+/* create_pid_file.c
+
+   Implement a function that can be used by a daemon (or indeed any program)
+   to ensure that only one instance of the program is running.
+*/
 #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. */
 
 int
 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 */
 
+        /* Instead of the following steps, we could (on Linux) have opened the
+           file with O_CLOEXEC flag. However, not all systems support open()
+           O_CLOEXEC (which was standardized only in SUSv4), so instead we use
+           fcntl() to set the close-on-exec flag after opening the file */
+
         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);
         else
             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;
 }