lseek() System Call in Linux

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NAME

lseek - reposition read/write file offset

SYNOPSIS

#include <sys/types.h>
#include <unistd.h>

off_t lseek(int fildes, off_t offset, int whence);

DESCRIPTION

The lseek() function repositions the offset of the open file associated with the file descriptor fildes to the argument offset according to the directive whence as follows: The lseek() function allows the file offset to be set beyond the end of the file (but this does not change the size of the file). If data is later written at this point, subsequent reads of the data in the gap (a "hole") return null bytes (’\0’) until data is actually written into the gap.

RETURN VALUE

Upon successful completion, lseek() returns the resulting offset location as measured in bytes from the beginning of the file. Otherwise, a value of (off_t)-1 is returned and errno is set to indicate the error.

ERRORS

CONFORMING TO

SVr4, 4.3BSD, POSIX.1-2001.

RESTRICTIONS

Some devices are incapable of seeking and POSIX does not specify which devices must support lseek().

Linux specific restrictions: using lseek() on a tty device returns ESPIPE.

NOTES

This document’s use of whence is incorrect English, but maintained for historical reasons.

When converting old code, substitute values for whence with the following macros:

SVr1-3 returns long instead of off_t, BSD returns int.

Note that file descriptors created by dup(2) or fork(2) share the current file position pointer, so seeking on such files may be subject to race conditions.

SEE ALSO

• dup (2)

• fork (2)

• open (2)