unix - why vi can modify a file while this file is write locked? -

i compile file , run in 1 console.

#include <stdio.h> #include <stdlib.h> #include <errno.h> #include <fcntl.h> #include <unistd.h>  int main(int argc, char *argv[]) {                 /* l_type   l_whence  l_start  l_len  l_pid   */     struct flock fl = {f_wrlck, seek_set,   0,      0,     0 };     int fd;      fl.l_pid = getpid();      if (argc > 1)          fl.l_type = f_rdlck;      if ((fd = open("lockdemo.c", o_rdwr)) == -1) {         perror("open");         exit(1);     }      printf("press <return> try lock: ");     getchar();     printf("trying lock...");      if (fcntl(fd, f_setlkw, &fl) == -1) {         perror("fcntl");         exit(1);     }      printf("got lock\n");     printf("press <return> release lock: ");     getchar();      fl.l_type = f_unlck;  /* set unlock same region */      if (fcntl(fd, f_setlk, &fl) == -1) {         perror("fcntl");         exit(1);     }      printf("unlocked.\n");      close(fd);      return 0; } 

it outputs:

zj:~/documents/c$ ./a.out press <return> try lock:  trying lock...got lock press <return> release lock: 

i open console , vi lockdemo.c , had modified lockdemo.c successfully. why? isn't file locked? while open console

zj:~/documents/c$ ./a.out press <return> try lock:  

the a.out running getchar(), , cannot execute printf("trying lock..."); totally confused.

you applying advisory lock on lockdemo.c file. vi free ignore design. should have used mandatory lock afaik isn't standardized under unix enforce vi not it.