fseek(3) — Linux manual page

fseek(3)                Library Functions Manual                fseek(3)

NAME

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

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <stdio.h>

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

       void rewind(FILE *stream);

       int fgetpos(FILE *restrict stream, fpos_t *restrict pos);
       int fsetpos(FILE *stream, const fpos_t *pos);

DESCRIPTION

       The fseek() function sets the file position indicator for the
       stream pointed to by stream.  The new position, measured in
       bytes, is obtained by adding offset bytes to the position
       specified by whence.  If whence is set to SEEK_SET, SEEK_CUR, or
       SEEK_END, the offset is relative to the start of the file, the
       current position indicator, or end-of-file, respectively.  A
       successful call to the fseek() function clears the end-of-file
       indicator for the stream and undoes any effects of the ungetc(3)
       function on the same stream.

       The ftell() function obtains the current value of the file
       position indicator for the stream pointed to by stream.

       The rewind() function sets the file position indicator for the
       stream pointed to by stream 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
       (see clearerr(3)).

       The fgetpos() and fsetpos() functions are alternate interfaces
       equivalent to ftell() and fseek() (with whence set to SEEK_SET),
       setting and storing the current value of the file offset into or
       from the object referenced by pos.  On some non-UNIX systems, an
       fpos_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.  See lseek(2) for details on file seeking semantics.

RETURN VALUE

       The rewind() function returns no value.  Upon successful
       completion, fgetpos(), fseek(), fsetpos() return 0, and ftell()
       returns the current offset.  Otherwise, -1 is returned and errno
       is set to indicate the error.

ERRORS

       EINVAL The whence argument to fseek() was not SEEK_SET, SEEK_END,
              or SEEK_CUR.  Or: the resulting file offset would be
              negative.

       ESPIPE The file descriptor underlying stream is not seekable
              (e.g., it refers to a pipe, FIFO, or socket).

       The functions fgetpos(), fseek(), fsetpos(), and ftell() may also
       fail and set errno for any of the errors specified for the
       routines fflush(3), fstat(2), lseek(2), and malloc(3).

ATTRIBUTES

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

STANDARDS

       C11, POSIX.1-2008.

HISTORY

       POSIX.1-2001, C89.

SEE ALSO

       lseek(2), fseeko(3)

COLOPHON

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Linux man-pages 6.9.1          2024-05-02                       fseek(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)

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