fseek(3)

💖 fseek(3) — the "okay bestie let's rewind" page 💖

Hiii so this is the manual page for fseek(3) (that's the man page name, the libc crew lives in section 3, iykyk ✨). It's about literally moving the little cursor thingy inside a stream. Five functions, one vibe: getting your file position exactly where you want it. 🌸

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✨ 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 💅

Okay so fseek() 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. And when 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. Slay. 🎯

When an fseek() call succeeds it also clears the end-of-file indicator for the stream and undoes any effects of the ungetc(3) function on that same stream. like it never happened bestie 💀

The ftell() function obtains the current value of the file position indicator for the stream pointed to by stream. It's literally just "where am I rn". 📍

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)). so rewind() is the "forget everything and start over" queen 👑

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. so like, cross-platform icon behavior 🧚

⚠️ heads up: 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. spicy but true 🌶️

🔥 RETURN VALUE 🔥

• rewind() returns no value. it's a void girlie, she gives nothing back 💅
• Upon successful completion, fgetpos(), fseek(), and 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. (negative offset?? not today bestie)
• ESPIPE — The file descriptor underlying stream is not seekable (e.g., it refers to a pipe, FIFO, or socket). you genuinely cannot rewind a pipe, that's not a skill issue, it's a physics issue 🚫

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). so many ways to get humbled fr fr 🙈

🌈 ATTRIBUTES 🌈

For an explanation of the terms used in this section, see attributes(7).

• Interface: fseek(), ftell(), rewind(), fgetpos(), fsetpos()
  • Attribute: Thread safety
  • Value: MT-Safe

(yes all five of them, one big happy thread-safe family 👑✨)

⭐ STANDARDS ⭐

C11, POSIX.1-2008.

🎀 HISTORY 🎀

POSIX.1-2001, C89. vintage but still serving 🫶

👀 SEE ALSO 👀

lseek(2), fseeko(3)

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and that's the whole page bestie, go move some file pointers, you got this 💜

this fine piece of documentation came to you from Linux man-pages 6.20, dated 2025-05-17, and it's still fseek(3) at heart. mwah 💋