lseek(2) [bsd man page]
LSEEK(2) System Calls Manual LSEEK(2) NAME
lseek - move read/write pointer SYNOPSIS
#include <sys/file.h> #define L_SET 0 /* set the seek pointer */ #define L_INCR 1 /* increment the seek pointer */ #define L_XTND 2 /* extend the file size */ pos = lseek(d, offset, whence) off_t pos; int d; off_t offset; int whence; DESCRIPTION
The descriptor d refers to a file or device open for reading and/or writing. Lseek sets the file pointer of d as follows: If whence is L_SET, the pointer is set to offset bytes. If whence is L_INCR, the pointer is set to its current location plus offset. If whence is L_XTND, the pointer is set to the size of the file plus offset. Upon successful completion, the resulting pointer location as measured in bytes from beginning of the file is returned. Some devices are incapable of seeking. The value of the pointer associated with such a device is undefined. NOTES
Seeking far beyond the end of a file, then writing, creates a gap or "hole", which occupies no physical space and reads as zeros. RETURN VALUE
Upon successful completion, the current file pointer value is returned. Otherwise, a value of -1 is returned and errno is set to indicate the error. ERRORS
Lseek will fail and the file pointer will remain unchanged if: [EBADF] Fildes is not an open file descriptor. [ESPIPE] Fildes is associated with a pipe or a socket. [EINVAL] Whence is not a proper value. SEE ALSO
dup(2), open(2) BUGS
This document's use of whence is incorrect English, but maintained for historical reasons. 4th Berkeley Distribution February 24, 1986 LSEEK(2)
Check Out this Related Man Page
LSEEK(2) System calls LSEEK(2) 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 file descriptor fildes to the argument offset according to the directive whence as fol- lows: SEEK_SET The offset is set to offset bytes. SEEK_CUR The offset is set to its current location plus offset bytes. SEEK_END The offset is set to the size of the file plus offset bytes. The lseek function allows the file offset to be set beyond the end of the existing end-of-file of the file. If data is later written at this point, subsequent reads of the data in the gap return bytes of zeros (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
EBADF Fildes is not an open file descriptor. ESPIPE Fildes is associated with a pipe, socket, or FIFO. EINVAL Whence is not a proper value. CONFORMING TO
SVr4, POSIX, BSD 4.3 RESTRICTIONS
Some devices are incapable of seeking and POSIX does not specify which devices must support it. Linux specific restrictions: using lseek on a tty device returns ESPIPE. Other systems return the number of written characters, using SEEK_SET to set the counter. Some devices, e.g. /dev/null do not cause the error ESPIPE, but return a pointer which value is undefined. 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: old new 0 SEEK_SET 1 SEEK_CUR 2 SEEK_END L_SET SEEK_SET L_INCR SEEK_CUR L_XTND SEEK_END 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), fseek(3) Linux 2001-09-24 LSEEK(2)