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Linux man pages : lseek (2)
LSEEK(2)			 System calls			      LSEEK(2)


lseek - reposition read/write file offset


#include <sys/types.h> #include <unistd.h> off_t lseek(int fildes, off_t offset, int whence);


The lseek function repositions the offset of the file descriptor fildes to the argument offset according to the directive whence as follows: 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).


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.


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.


SVr4, POSIX, BSD 4.3


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.


This document's use of whence is incorrect English, but maintained for historical reasons. When converting old code, substitute values for whence with the follow- ing 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 cur- rent file position pointer, so seeking on such files may be subject to race conditions.


dup(2), fork(2), open(2), fseek(3) Linux 2001-09-24 LSEEK(2)