This is fileio/bad_exclusive_open.c (Listing 5-1, page 90), an example from the book, The Linux Programming Interface.

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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/* bad_exclusive_open.c

   The following code shows why we need the open() O_EXCL flag.

   This program to tries ensure that it is the one that creates the file
   named in its command-line argument. It does this by trying to open()
   the filename once without the O_CREAT flag (if this open() succeeds
   then the program know it is not the creator of the file), and if
   that open() fails, it calls open() a second time, with the O_CREAT flag.

   If the first open() fails, the program assumes that it is the creator
   of the file. However this may not be true: some other process may have
   created the file between the two calls to open().
#include <sys/stat.h>
#include <fcntl.h>
#include "tlpi_hdr.h"
main(int argc, char *argv[])
    int fd;

    if (argc < 2 || strcmp(argv[1], "--help") == 0)
        usageErr("%s file\n", argv[0]);

    fd = open(argv[1], O_WRONLY);       /* Open 1: check if file exists */
    if (fd != -1) {                     /* Open succeeded */
        printf("[PID %ld] File \"%s\" already exists\n",
                (long) getpid(), argv[1]);
    } else {
        if (errno != ENOENT) {          /* Failed for unexpected reason */
        } else {
            printf("[PID %ld] File \"%s\" doesn't exist yet\n",
                    (long) getpid(), argv[1]);
            if (argc > 2) {             /* Delay between check and create */
                sleep(5);               /* Suspend execution for 5 seconds */
                printf("[PID %ld] Done sleeping\n", (long) getpid());
            fd = open(argv[1], O_WRONLY | O_CREAT, S_IRUSR | S_IWUSR);
            if (fd == -1)

            printf("[PID %ld] Created file \"%s\" exclusively\n",
                    (long) getpid(), argv[1]);          /* MAY NOT BE TRUE! */



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