fsync() — Write changes to direct-access storage

Standards

Standards / Extensions	C or C++	Dependencies
POSIX.1 XPG4 XPG4.2 Single UNIX Specification, Version 3	both

Format

#define _POSIX1_SOURCE 2
#include <unistd.h>

int fsync(int fildes);

General description

Transfers all data for the file indicated by the open file descriptor fildes to the storage device associated with fildes. fsync() does not return until the transfer has completed, or until an error is detected.

Returned value

If successful, fsync() returns 0.

If unsuccessful, fsync() returns -1 and sets errno to one of the following values:

Error Code: Description
EBADF: fildes is not a valid open file descriptor.
EINVAL: The file is not a regular file.

Example

CELEBF48

⁄* CELEBF48 *⁄
#define _POSIX_SOURCE
#include <fcntl.h>
#include <sys⁄stat.h>
#include <sys⁄types.h>
#include <unistd.h>
#undef _POSIX_SOURCE
#include <stdio.h>
#include <stdlib.h>

#define mega_string_len 250000

main() {
  char *mega_string;
  int  fd, ret;
  char fn[]="fsync.file";

  if ((mega_string = (char*) malloc(mega_string_len)) == NULL)
    perror("malloc() error");
  else if ((fd = creat(fn, S_IWUSR)) < 0)
    perror("creat() error");
  else {
    memset(mega_string, 's', mega_string_len);
    if ((ret = write(fd, mega_string, mega_string_len)) == -1)
      perror("write() error");
    else {
      printf("write() wrote %d bytes\n", ret);
      if (fsync(fd) != 0)
        perror("fsync() error");
      else if ((ret = write(fd, mega_string, mega_string_len)) == -1)
        perror("write() error");
      else
        printf("write() wrote %d bytes\n", ret);
    }
    close(fd);
    unlink(fn);
  }
}

Output

write() wrote 250000 bytes
write() wrote 250000 bytes