filelock/i_fcntl_locking.c

#include <sys/stat.h>
#include <fcntl.h>
#include "tlpi_hdr.h"

#define MAX_LINE 100

static void
displayCmdFmt(void)
{
    printf("\n    Format: cmd lock start length [whence]\n\n");
    printf("    'cmd' is 'g' (GETLK), 's' (SETLK), or 'w' (SETLKW)\n");
    printf("    'lock' is 'r' (READ), 'w' (WRITE), or 'u' (UNLOCK)\n");
    printf("    'start' and 'length' specify byte range to lock\n");
    printf("    'whence' is 's' (SEEK_SET, default), 'c' (SEEK_CUR), "
           "or 'e' (SEEK_END)\n\n");
}

int
main(int argc, char *argv[])
{
    int fd, numRead, cmd, status;
    char lock, cmdCh, whence, line[MAX_LINE];
    struct flock fl;
    long long len, st;

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

    fd = open(argv[1], O_RDWR);
    if (fd == -1)
        errExit("open (%s)", argv[1]);

    printf("Enter ? for help\n");

    for (;;) {          /* Prompt for locking command and carry it out */
        printf("PID=%ld> ", (long) getpid());
        fflush(stdout);

        if (fgets(line, MAX_LINE, stdin) == NULL)       /* EOF */
            exit(EXIT_SUCCESS);
        line[strlen(line) - 1] = '\0';          /* Remove trailing '\n' */

        if (*line == '\0')
            continue;                           /* Skip blank lines */

        if (line[0] == '?') {
            displayCmdFmt();
            continue;
        }

        whence = 's';                   /* In case not otherwise filled in */

        numRead = sscanf(line, "%c %c %lld %lld %c", &cmdCh, &lock,
                        &st, &len, &whence);
        fl.l_start = st;
        fl.l_len = len;

        if (numRead < 4 || strchr("gsw", cmdCh) == NULL ||
                strchr("rwu", lock) == NULL || strchr("sce", whence) == NULL) {
            printf("Invalid command!\n");
            continue;
        }

        cmd = (cmdCh == 'g') ? F_GETLK : (cmdCh == 's') ? F_SETLK : F_SETLKW;
        fl.l_type = (lock == 'r') ? F_RDLCK : (lock == 'w') ? F_WRLCK : F_UNLCK;
        fl.l_whence = (whence == 'c') ? SEEK_CUR :
                      (whence == 'e') ? SEEK_END : SEEK_SET;

        status = fcntl(fd, cmd, &fl);           /* Perform request... */

        if (cmd == F_GETLK) {                   /* ... and see what happened */
            if (status == -1) {
                errMsg("fcntl - F_GETLK");
            } else {
                if (fl.l_type == F_UNLCK)
                    printf("[PID=%ld] Lock can be placed\n", (long) getpid());
                else                            /* Locked out by someone else */
                    printf("[PID=%ld] Denied by %s lock on %lld:%lld "
                            "(held by PID %ld)\n", (long) getpid(),
                            (fl.l_type == F_RDLCK) ? "READ" : "WRITE",
                            (long long) fl.l_start,
                            (long long) fl.l_len, (long) fl.l_pid);
            }
        } else {                /* F_SETLK, F_SETLKW */
            if (status == 0)
                printf("[PID=%ld] %s\n", (long) getpid(),
                        (lock == 'u') ? "unlocked" : "got lock");
            else if (errno == EAGAIN || errno == EACCES)        /* F_SETLK */
                printf("[PID=%ld] failed (incompatible lock)\n",
                        (long) getpid());
            else if (errno == EDEADLK)                          /* F_SETLKW */
                printf("[PID=%ld] failed (deadlock)\n", (long) getpid());
            else
                errMsg("fcntl - F_SETLK(W)");
        }
    }
}