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Manual page for lseek(2)

lseek - move read/write file pointer

SYNOPSIS

#include <sys/types.h>
#include <unistd.h>

off_t lseek(int fildes, off_t offset, int whence);

MT-LEVEL

Async-Signal-Safe

DESCRIPTION

lseek() sets the file pointer associated with the open file descriptor specified by fildes as follows:

If whence is SEEK_SET, the pointer is set to offset bytes.
If whence is SEEK_CUR, the pointer is set to its current location plus offset.
If whence is SEEK_END, the pointer is set to the size of the file plus offset.

On success, lseek() returns the resulting pointer location, as measured in bytes from the beginning of the file. Note that if fildes is a remote file descriptor and offset is negative, lseek() returns the file pointer even if it is negative.

lseek() allows the file pointer to be set beyond the existing data in the file. If data are later written at this point, subsequent reads in the gap between the previous end of data and the newly written data will return bytes of value 0 until data are written into the gap.

RETURN VALUES

Upon successful completion, the resulting file pointer is returned. Remote file descriptors are the only ones that allow negative file pointers. Otherwise, a value of -1 is returned and errno is set to indicate the error.

ERRORS

lseek() fails and the file pointer remains unchanged if one or more of the following are true:

EBADF: fildes is not an open file descriptor.
EINVAL: whence is not SEEK_SET, SEEK_CUR, or SEEK_END.
EINVAL: fildes is not a remote file descriptor, and the resulting file pointer would be negative.
ESPIPE: fildes is associated with a pipe or fifo.

Some devices are incapable of seeking. The value of the file pointer associated with such a device is undefined.

NOTES

In multithreaded programs, using lseek() in conjunction with a read() or write() on a file descriptor shared amongst more than one thread is not an atomic operation. To ensure atomicity, use pread() or pwrite().

Last modified 21/April/97