Random Thoughts about Learning the Unix Philosophy

Why O_APPEND exists

Why use O_APPEND when you can just use lseek(fd, 0, SEEK_END) to move the file offset to the end of the file and call write from there?

The O_APPEND flag used in the Unix open(const char *path, int oflag, ...) system call exists because it guarantees an atomic appending write.

Consider the following code prone to a race condition:

char buf[1024];
if (lseek(fd, 0, SEEK_END) == -1) {
  fprintf(stderr, "Error with lseek");
  perror("lseek");
  exit(EXIT_FAILURE);
}

/* HAZARDOUS!!! WINDOW FOR FAILURE! RACE CONDITION POSSIBLE! */

if (write(fd, buf, 1024) != 1024) {
  fprintf(stderr, "Error with write");
  perror("write");
  exit(EXIT_FAILURE); 
}

Between the lseek and write system calls (the /* HAZARDOUS!!! WINDOW FOR FAILURE */ section), another process, P2, running the same program could run. Thus, when the CPU returns to this process, P1, the file offset set by P1’s lseek will be incorrect and P1’s write system call will overwrite whatever P2 wrote starting at the file offset set by P1’s lseek.

To address this, an atomic operation is needed, a need that O_APPEND fulfills.