fseek(3p) — Linux manual page


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

       fseek, fseeko — reposition a file-position indicator in a stream

SYNOPSIS         top

       #include <stdio.h>

       int fseek(FILE *stream, long offset, int whence);
       int fseeko(FILE *stream, off_t offset, int whence);

DESCRIPTION         top

       The functionality described on this reference page is aligned
       with the ISO C standard. Any conflict between the requirements
       described here and the ISO C standard is unintentional. This
       volume of POSIX.1‐2017 defers to the ISO C standard.

       The fseek() function shall set the file-position indicator for
       the stream pointed to by stream.  If a read or write error
       occurs, the error indicator for the stream shall be set and
       fseek() fails.

       The new position, measured in bytes from the beginning of the
       file, shall be obtained by adding offset to the position
       specified by whence.  The specified point is the beginning of the
       file for SEEK_SET, the current value of the file-position
       indicator for SEEK_CUR, or end-of-file for SEEK_END.

       If the stream is to be used with wide-character input/output
       functions, the application shall ensure that offset is either 0
       or a value returned by an earlier call to ftell() on the same
       stream and whence is SEEK_SET.

       A successful call to fseek() shall clear the end-of-file
       indicator for the stream and undo any effects of ungetc() and
       ungetwc() on the same stream. After an fseek() call, the next
       operation on an update stream may be either input or output.

       If the most recent operation, other than ftell(), on a given
       stream is fflush(), the file offset in the underlying open file
       description shall be adjusted to reflect the location specified
       by fseek().

       The fseek() function shall allow the file-position indicator to
       be set beyond the end of 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 behavior of fseek() on devices which are incapable of seeking
       is implementation-defined.  The value of the file offset
       associated with such a device is undefined.

       If the stream is writable and buffered data had not been written
       to the underlying file, fseek() shall cause the unwritten data to
       be written to the file and shall mark the last data modification
       and last file status change timestamps of the file for update.

       In a locale with state-dependent encoding, whether fseek()
       restores the stream's shift state is implementation-defined.

       The fseeko() function shall be equivalent to the fseek() function
       except that the offset argument is of type off_t.

RETURN VALUE         top

       The fseek() and fseeko() functions shall return 0 if they

       Otherwise, they shall return -1 and set errno to indicate the

ERRORS         top

       The fseek() and fseeko() functions shall fail if, either the
       stream is unbuffered or the stream's buffer needed to be flushed,
       and the call to fseek() or fseeko() causes an underlying lseek()
       or write() to be invoked, and:

       EAGAIN The O_NONBLOCK flag is set for the file descriptor and the
              thread would be delayed in the write operation.

       EBADF  The file descriptor underlying the stream file is not open
              for writing or the stream's buffer needed to be flushed
              and the file is not open.

       EFBIG  An attempt was made to write a file that exceeds the
              maximum file size.

       EFBIG  An attempt was made to write a file that exceeds the file
              size limit of the process.

       EFBIG  The file is a regular file and an attempt was made to
              write at or beyond the offset maximum associated with the
              corresponding stream.

       EINTR  The write operation was terminated due to the receipt of a
              signal, and no data was transferred.

       EINVAL The whence argument is invalid. The resulting file-
              position indicator would be set to a negative value.

       EIO    A physical I/O error has occurred, or the process is a
              member of a background process group attempting to perform
              a write() to its controlling terminal, TOSTOP is set, the
              calling thread is not blocking SIGTTOU, the process is not
              ignoring SIGTTOU, and the process group of the process is
              orphaned.  This error may also be returned under
              implementation-defined conditions.

       ENOSPC There was no free space remaining on the device containing
              the file.

              For fseek(), the resulting file offset would be a value
              which cannot be represented correctly in an object of type

              For fseeko(), the resulting file offset would be a value
              which cannot be represented correctly in an object of type

       EPIPE  An attempt was made to write to a pipe or FIFO that is not
              open for reading by any process; a SIGPIPE signal shall
              also be sent to the thread.

       ESPIPE The file descriptor underlying stream is associated with a
              pipe, FIFO, or socket.

       The fseek() and fseeko() functions may fail if:

       ENXIO  A request was made of a nonexistent device, or the request
              was outside the capabilities of the device.

       The following sections are informative.

EXAMPLES         top




RATIONALE         top




SEE ALSO         top

       Section 2.5, Standard I/O Streams, fopen(3p), fsetpos(3p),
       ftell(3p), getrlimit(3p), lseek(3p), rewind(3p), ulimit(3p),
       ungetc(3p), write(3p)

       The Base Definitions volume of POSIX.1‐2017, stdio.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 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                         FSEEK(3P)

Pages that refer to this page: stdio.h(0p)fmemopen(3p)ftell(3p)rewind(3p)stdin(3p)ungetc(3p)ungetwc(3p)