GNU/Linux man pages
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GNU/Linux |
CentOS 5.4 |
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lseek(2) |
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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 open
file associated with 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 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.
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.
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EBADF |
fildes is not an open file descriptor. | ||
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EINVAL |
whence is not one of SEEK_SET, SEEK_CUR, SEEK_END; or the resulting file offset would be negative, or beyond the end of a seekable device. |
EOVERFLOW
The resulting file offset cannot be represented in an off_t.
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ESPIPE |
fildes is associated with a pipe, socket, or FIFO. |
SVr4, 4.3BSD, POSIX.1-2001.
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.
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.
dup(2), fork(2), open(2), fseek(3), lseek64(3), posix_fallocate(3)
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lseek(2) | |