lseek(3p) — Linux manual page


LSEEK(3P)               POSIX Programmer's Manual              LSEEK(3P)

PROLOG         top

       This manual page is part of the POSIX Programmer's Manual.  The
       Linux implementation of this interface may differ (consult the
       corresponding Linux manual page for details of Linux behavior),
       or the interface may not be implemented on Linux.

NAME         top

       lseek — move the read/write file offset

SYNOPSIS         top

       #include <unistd.h>

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

DESCRIPTION         top

       The lseek() function shall set the file offset for the open file
       description associated with the file descriptor fildes, as

        *  If whence is SEEK_SET, the file offset shall be set to offset

        *  If whence is SEEK_CUR, the file offset shall be set to its
           current location plus offset.

        *  If whence is SEEK_END, the file offset shall be set to the
           size of the file plus offset.

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

       The behavior of lseek() on devices which are incapable of seeking
       is implementation-defined.  The value of the file offset
       associated with such a device is undefined.

       The lseek() function shall allow the file offset to be set beyond
       the end of the existing data in the file. If data is later
       written at this point, subsequent reads of data in the gap shall
       return bytes with the value 0 until data is actually written into
       the gap.

       The lseek() function shall not, by itself, extend the size of a

       If fildes refers to a shared memory object, the result of the
       lseek() function is unspecified.

       If fildes refers to a typed memory object, the result of the
       lseek() function is unspecified.

RETURN VALUE         top

       Upon successful completion, the resulting offset, as measured in
       bytes from the beginning of the file, shall be returned.
       Otherwise, -1 shall be returned, errno shall be set to indicate
       the error, and the file offset shall remain unchanged.

ERRORS         top

       The lseek() function shall fail if:

       EBADF  The fildes argument is not an open file descriptor.

       EINVAL The whence argument is not a proper value, or the
              resulting file offset would be negative for a regular
              file, block special file, or directory.

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

       ESPIPE The fildes argument is associated with a pipe, FIFO, or

       The following sections are informative.

EXAMPLES         top




RATIONALE         top

       The ISO C standard includes the functions fgetpos() and
       fsetpos(), which work on very large files by use of a special
       positioning type.

       Although lseek() may position the file offset beyond the end of
       the file, this function does not itself extend the size of the
       file. While the only function in POSIX.1‐2008 that may directly
       extend the size of the file is write(), truncate(), and
       ftruncate(), several functions originally derived from the ISO C
       standard, such as fwrite(), fprintf(), and so on, may do so (by
       causing calls on write()).

       An invalid file offset that would cause [EINVAL] to be returned
       may be both implementation-defined and device-dependent (for
       example, memory may have few invalid values). A negative file
       offset may be valid for some devices in some implementations.

       The POSIX.1‐1990 standard did not specifically prohibit lseek()
       from returning a negative offset. Therefore, an application was
       required to clear errno prior to the call and check errno upon
       return to determine whether a return value of (off_t)-1 is a
       negative offset or an indication of an error condition. The
       standard developers did not wish to require this action on the
       part of a conforming application, and chose to require that errno
       be set to [EINVAL] when the resulting file offset would be
       negative for a regular file, block special file, or directory.



SEE ALSO         top


       The Base Definitions volume of POSIX.1‐2017, sys_types.h(0p),

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic
       form from IEEE Std 1003.1-2017, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The
       Open Group Base Specifications Issue 7, 2018 Edition, Copyright
       (C) 2018 by the Institute of Electrical and Electronics
       Engineers, Inc and The Open Group.  In the event of any
       discrepancy between this version and the original IEEE and The
       Open Group Standard, the original IEEE and The Open Group
       Standard is the referee document. The original Standard can be
       obtained online at .

       Any typographical or formatting errors that appear in this page
       are most likely to have been introduced during the conversion of
       the source files to man page format. To report such errors, see .

IEEE/The Open Group               2017                         LSEEK(3P)

Pages that refer to this page: aio.h(0p)unistd.h(0p)tee(1p)aio_error(3p)aio_read(3p)aio_return(3p)aio_write(3p)fseek(3p)fsetpos(3p)ftell(3p)lio_listio(3p)open(3p)read(3p)write(3p)