The lseek() function 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.

The symbolic constants SEEK_SET, SEEK_CUR, and SEEK_END are defined in the header <unistd.h>.

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

The lseek() function 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.

If fildes is a remote file descriptor and offset is negative, lseek() returns the file pointer even if it is negative. The lseek() function will not, by itself, extend the size of a file.

Upon successful completion, the resulting offset, as measured in bytes from the beginning of the file, is returned. Otherwise, (off_t)-1 is returned, the file offset remains unchanged, and errno is set to indicate the error.

The lseek() function will fail if:

EBADF

The fildes argument is not an open file descriptor.

EINVAL

The whence argument is not SEEK_SET, SEEK_CUR, or SEEK_END; or the fildes argument is not a remote file descriptor and the resulting file pointer would be negative.

EOVERFLOW

The resulting file offset would be a value which cannot be represented correctly in an object of type off_t for regular files.

ESPIPE

The fildes argument is associated with a pipe, a FIFO, or a socket.

The lseek() function has a transitional interface for 64-bit file offsets. See lf64(5).

In multithreaded applications, using lseek() in conjunction with a read(2) or write(2) call on a file descriptor shared by more than one thread is not an atomic operation. To ensure atomicity, use pread() or pwrite().

See attributes(5) for descriptions of the following attributes:

creat(2), dup(2), fcntl(2), open(2), read(2), write(2), attributes(5), lf64(5)