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lseek(2) [redhat 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)

Check Out this Related Man Page

LSEEK(2)						      BSD System Calls Manual							  LSEEK(2)

NAME
lseek, seek -- reposition read/write file offset LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#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. The argument fildes must be an open file descriptor. lseek() repositions the file pointer fildes as follows: If whence is SEEK_SET, the offset is set to offset bytes. If whence is SEEK_CUR, the offset is set to its current location plus offset bytes. If whence is 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). Some devices are incapable of seeking. The value of the pointer associated with such a device is undefined. RETURN VALUES
Upon successful completion, lseek() returns the resulting offset location as measured in bytes from the beginning of the file. 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. [EINVAL] whence is not a proper value, or the resulting file offset would be invalid. [ESPIPE] fildes is associated with a pipe, socket, or FIFO. SEE ALSO
dup(2), open(2) STANDARDS
The lseek() function conforms to ISO/IEC 9945-1:1990 (``POSIX.1''). HISTORY
A seek() function appeared in Version 2 AT&T UNIX, later renamed into lseek() for ``long seek'' due to a larger offset argument type. BUGS
This document's use of whence is incorrect English, but is maintained for historical reasons. BSD
April 3, 2010 BSD

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