lseek(2) -- Linux man page
NAMElseek - reposition read/write file offset
DESCRIPTIONThe lseek function repositions the offset of the file descriptor fildes to the argument offset according to the directive whence as follows:
- The offset is set to offset bytes.
- The offset is set to its current location plus offset bytes.
- 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 (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 return bytes of zeros (until data is actually written into the gap).
RETURN VALUEUpon 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.
- fildes is not an open file descriptor.
- fildes is associated with a pipe, socket, or FIFO.
- whence is not one of SEEK_SET, SEEK_CUR, SEEK_END, or the resulting file offset would be negative.
- The resulting file offset cannot be represented in an off_t.
CONFORMING TOSVr4, POSIX, BSD 4.3
RESTRICTIONSSome devices are incapable of seeking and POSIX does not specify which devices must support it.
NOTESThis 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.
SEE ALSOdup(2), fork(2), open(2), fseek(3)