FSETPOS(3P)

💖 FSETPOS(3P) — the POSIX Programmer's Manual era 💖

okay bestie, quick vibe check before we dive in: this page is literally part of the POSIX Programmer's Manual. BUT — and this is important — the Linux implementation of this interface may differ (so go consult the corresponding Linux manual page for the deets on Linux behavior), or honestly the interface may just not be implemented on Linux at all. 🙈 iykyk.

---

✨ NAME ✨

fsetpos — set current file position. that's it, that's the whole job, and she does it flawlessly 💅

---

🎀 SYNOPSIS 🎀

#include <stdio.h>

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

---

🦋 DESCRIPTION 🦋

so here's the tea. the functionality on this reference page is aligned with the ISO C standard. any conflict between what's described here and the ISO C standard is unintentional (oops, not on purpose fr fr). and this volume of POSIX.1‐2017 defers to the ISO C standard. periodt.

The fsetpos() function shall set the file position and state indicators for the stream pointed to by stream according to the value of the object pointed to by pos. and the application shall ensure that pos is a value obtained from an earlier call to fgetpos() on the same stream — no swapping positions between different streams, bestie, that's not the move. If a read or write error occurs, the error indicator for the stream shall be set and fsetpos() fails. 😭

A successful call to fsetpos() shall clear the end-of-file indicator for the stream and undo any effects of ungetc() on the same stream. the way that it just wipes the slate clean?? iconic. After an fsetpos() call, the next operation on an update stream may be either input or output. freedom. 🌈

The behavior of fsetpos() on devices which are incapable of seeking is implementation-defined. The value of the file offset associated with such a device is undefined. so like, don't come for her when a non-seekable device acts weird, that's not her fault. 💀

Also!! The fsetpos() function shall not change the setting of errno if successful. she's polite like that. 🫶

---

⭐ RETURN VALUE ⭐

• 0 if it succeeds 💖
• otherwise, a non-zero value, and errno gets set to indicate the error. not the vibe but okay.

---

🔥 ERRORS 🔥

The fsetpos() function shall fail if — either the stream is unbuffered, or the stream's buffer needed to be flushed — and the call to fsetpos() 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. she said "not right now." ⏳
• 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. bestie the door is literally locked. 🚪
• 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. interrupted, mid-sentence, rude. 😤
• 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. the device is full, no room, sorry. 🍽️
• 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. she's screaming into the void AND she's getting signaled about it. 💀
• ESPIPE — The file descriptor underlying stream is associated with a pipe, FIFO, or socket.

The fsetpos() function may fail if:

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

heads up: the following sections are informative. (translation: they're context, not hard requirements. slay.)

---

🌸 EXAMPLES 🌸

None. yeah. nothing. we love a minimalist queen. 💅

---

💜 APPLICATION USAGE 💜

None. again!! nothing to add, she's self-explanatory fr fr.

---

🧚 RATIONALE 🧚

None. the reasoning is simply that she's correct. no notes. 👑

---

🌈 FUTURE DIRECTIONS 🌈

None. she's staying exactly as she is. no glow-up needed. periodt.

---

🍓 SEE ALSO 🍓

• Section 2.5, Standard I/O Streams
• fopen()
• ftell()
• lseek()
• rewind()
• ungetc()
• write()
• The Base Definitions volume of POSIX.1‐2017, <stdio.h>

---

👑 COPYRIGHT 👑

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 .

---

okay that's the whole page bestie!! this absolute banger of a spec was served up by IEEE/The Open Group back in 2017, and yeah it's officially FSETPOS(3P) in the manual. go set some file positions, you got this. 🫶✨💖