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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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LSEEK(3P) POSIX Programmer's Manual LSEEK(3P)
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.
lseek — move the read/write file offset
#include <unistd.h>
off_t lseek(int fildes, off_t offset, int whence);
The lseek() function shall set the file offset for the open file
description associated with the file descriptor fildes, as
follows:
* If whence is SEEK_SET, the file offset shall be set to offset
bytes.
* 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
file.
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.
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.
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.
EOVERFLOW
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
socket.
The following sections are informative.
None.
None.
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.
None.
open(3p)
The Base Definitions volume of POSIX.1‐2017, sys_types.h(0p),
unistd.h(0p)
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 http://www.opengroup.org/unix/online.html .
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
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
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)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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