fstat

Synopsis

#include <sys/stat.h>

int fstat(int fildes, struct stat *buf);

Status

Implemented

Conformance

IEEE Std 1003.1-2017

Description

The fstat() function shall obtain information about an open file associated with the file descriptor fildes, and shall write it to the area pointed to by buf.

If fildes references a shared memory object, the implementation shall update in the stat structure pointed to by the buf argument the st_uid, st_gid, st_size, and st_mode fields, and only the S_IRUSR, S_IWUSR, S_IRGRP, S_IWGRP, S_IROTH, and S_IWOTH file permission bits need be valid. The implementation may update other fields and flags.

If fildes references a typed memory object, the implementation shall update in the stat structure pointed to by the buf argument the st_uid, st_gid, st_size, and st_mode fields, and only the S_IRUSR, S_IWUSR, S_IRGRP, S_IWGRP, S_IROTH, and S_IWOTH file permission bits need be valid. The implementation may update other fields and flags.

The buf argument is a pointer to a stat structure, as defined in <sys/stat.h>, into which information is placed concerning the file.

For all other file types defined in this volume of POSIX.1-2017, the structure members st_mode, st_ino, st_dev, st_uid, st_gid, st_atim, st_ctim, and st_mtim shall have meaningful values and the value of the st_nlink member shall be set to the number of links to the file.

An implementation that provides additional or alternative file access control mechanisms may, under implementation-defined conditions, cause fstat() to fail.

The fstat() function shall update any time-related fields, before writing into the stat structure. For a pipe, a FIFO, a socket, the kernel console, and the /dev/null, /dev/zero, /dev/full and /dev/urandom special files the st_atim, st_mtim and st_ctim fields are set to zero (see POSIX deviations).

Return value

Upon successful completion, 0 shall be returned. Otherwise, -1 shall be returned and errno set to indicate the error.

Errors

The fstat() function shall fail if:

  • [EBADF] - The fildes argument is not a valid file descriptor.

  • [EIO] - An I/O error occurred while reading from the file system.

  • [EOVERFLOW] - The file size in bytes or the number of blocks allocated to the file or the file serial number cannot be represented correctly in the structure pointed to by buf.

The fstat() function may fail if:

  • [EOVERFLOW]- One of the values is too large to store into the structure pointed to by the buf argument.

POSIX deviations

For an open file descriptor that refers to a pipe, a FIFO, a socket or the kernel console (the terminal set up by the kernel as the standard streams of the first process), fstat() does not query the server handling the file, and the st_atim, st_mtim and st_ctim fields are set to zero. The kernel also treats the /dev/null, /dev/zero, /dev/full and /dev/urandom special files as pipes, so fstat() reports zero timestamps and the S_IFIFO file type for them.

For other files the timestamps are reported by the server handling the file, and whether they are updated by I/O operations depends on that server. Servers without a built-in filesystem (posixsrv and most device drivers) do not update them, as this would require an additional message to the filesystem server on every read() and write(), which adds latency to I/O operations that is not acceptable on a real-time system. dummyfs does not update st_mtim and st_ctim of a regular file on write().