lseek(2) - NetBSD Manual Pages

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LSEEK(2)                  NetBSD System Calls Manual                  LSEEK(2)


NAME

     lseek, seek -- reposition read/write file offset


LIBRARY

     Standard C Library (libc, -lc)


SYNOPSIS

     #include <unistd.h>

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


DESCRIPTION

     The lseek() function repositions the offset of the file descriptor fildes
     to the argument offset according to the directive whence.  The argument
     fildes must be an open file descriptor.  lseek() repositions the file
     pointer fildes as follows:

           If whence is SEEK_SET, the offset is set to offset bytes.

           If whence is SEEK_CUR, the offset is set to its current location
           plus offset bytes.

           If whence is 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).

     Some devices are incapable of seeking.  The value of the pointer associ-
     ated with such a device is undefined.


RETURN VALUES

     Upon successful completion, lseek() returns the resulting offset location
     as measured in bytes from the beginning of the file.  Otherwise, a value
     of -1 is returned and errno is set to indicate the error.


ERRORS

     lseek() will fail and the file pointer will remain unchanged if:

     [EBADF]            fildes is not an open file descriptor.

     [EINVAL]           whence is not a proper value, or the resulting file
                        offset would be invalid.

     [ESPIPE]           fildes is associated with a pipe, socket, or FIFO.


SEE ALSO

     dup(2), open(2)


STANDARDS

     The lseek() function conforms to ISO/IEC 9945-1:1990 (``POSIX.1'').


HISTORY

     A seek() function appeared in Version 1 AT&T UNIX, later renamed into
     lseek() for ``long seek'' due to a larger offset argument type.


BUGS

     This document's use of whence is incorrect English, but is maintained for
     historical reasons.

NetBSD 10.1                    September 2, 2019                   NetBSD 10.1