Movatterモバイル変換

fseek(3) — Linux manual page

fseek(3)                 Library Functions Manualfseek(3)

NAME        top

       fgetpos, fseek, fsetpos, ftell, rewind - reposition a stream

LIBRARY        top

       Standard C library (libc,-lc)

SYNOPSIS        top

#include <stdio.h>int fseek(FILE *stream, longoffset, intwhence);long ftell(FILE *stream);void rewind(FILE *stream);int fgetpos(FILE *restrictstream, fpos_t *restrictpos);int fsetpos(FILE *stream, const fpos_t *pos);

DESCRIPTION        top

       Thefseek() function sets the file position indicator for the       stream pointed to bystream.  The new position, measured in bytes,       is obtained by addingoffset bytes to the position specified bywhence.  Ifwhence is set toSEEK_SET,SEEK_CUR, orSEEK_END, the       offset is relative to the start of the file, the current position       indicator, or end-of-file, respectively.  A successful call to thefseek() function clears the end-of-file indicator for the stream       and undoes any effects of theungetc(3) function on the same       stream.       Theftell() function obtains the current value of the file       position indicator for the stream pointed to bystream.       Therewind() function sets the file position indicator for the       stream pointed to bystream to the beginning of the file.  It is       equivalent to:              (void) fseek(stream, 0L, SEEK_SET)       except that the error indicator for the stream is also cleared       (seeclearerr(3)).       Thefgetpos() andfsetpos() functions are alternate interfaces       equivalent toftell() andfseek() (withwhence set toSEEK_SET),       setting and storing the current value of the file offset into or       from the object referenced bypos.  On some non-UNIX systems, anfpos_t object may be a complex object and these routines may be       the only way to portably reposition a text stream.       If the stream refers to a regular file and the resulting stream       offset is beyond the size of the file, subsequent writes will       extend the file with a hole, up to the offset, before committing       any data.  Seelseek(2) for details on file seeking semantics.

RETURN VALUE        top

       Therewind() function returns no value.  Upon successful       completion,fgetpos(),fseek(),fsetpos() return 0, andftell()       returns the current offset.  Otherwise, -1 is returned anderrno       is set to indicate the error.

ERRORS        top

EINVALThewhence argument tofseek() was notSEEK_SET,SEEK_END,              orSEEK_CUR.  Or: the resulting file offset would be              negative.ESPIPEThe file descriptor underlyingstream is not seekable              (e.g., it refers to a pipe, FIFO, or socket).       The functionsfgetpos(),fseek(),fsetpos(), andftell() may also       fail and seterrno for any of the errors specified for the       routinesfflush(3),fstat(2),lseek(2), andmalloc(3).

ATTRIBUTES        top

       For an explanation of the terms used in this section, seeattributes(7).       ┌──────────────────────────────────────┬───────────────┬─────────┐       │Interface│Attribute│Value│       ├──────────────────────────────────────┼───────────────┼─────────┤       │fseek(),ftell(),rewind(),          │ Thread safety │ MT-Safe │       │fgetpos(),fsetpos()                 │               │         │       └──────────────────────────────────────┴───────────────┴─────────┘

STANDARDS        top

       C11, POSIX.1-2008.

HISTORY        top

       POSIX.1-2001, C89.

SEE ALSO        top

lseek(2),fseeko(3)

COLOPHON        top

       This page is part of theman-pages (Linux kernel and C library       user-space interface documentation) project.  Information about       the project can be found at        ⟨https://www.kernel.org/doc/man-pages/⟩.  If you have a bug report       for this manual page, see       ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.       This page was obtained from the tarball man-pages-6.15.tar.gz       fetched from       ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on       2025-08-11.  If you discover any rendering problems in this HTML       version of the page, or you believe there is a better or more up-       to-date source for the page, or you have corrections or       improvements to the information in this COLOPHON (which isnot       part of the original manual page), send a mail to       man-pages@man7.orgLinux man-pages 6.15            2025-05-17fseek(3)

Pages that refer to this page:lseek(2), fgetc(3), fmemopen(3), fopen(3), fopencookie(3), fseeko(3), gets(3), open_memstream(3), puts(3), stdio(3), feature_test_macros(7)

HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH.