FILENO(3P)

💖 FILENO(3P) 💖

okay bestie, welcome to the POSIX Programmer's Manual — this one's the fileno page, and it's short, sweet, and lowkey iconic. 🎀

🫶 PROLOG

This manual page is part of the POSIX Programmer's Manual. Heads up though: the Linux implementation of this interface may differ (so pls consult the corresponding Linux manual page for the deets on Linux behavior), or — plot twist — the interface may not be implemented on Linux at all. 🙈

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✨ NAME ✨

fileno — map a stream pointer to a file descriptor

Like. That's the whole job. It takes a stream and tells you which file descriptor is behind it. 💅

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🌸 SYNOPSIS 🌸

#include <stdio.h>

int fileno(FILE *stream);

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🦋 DESCRIPTION 🦋

The fileno() function shall return the integer file descriptor associated with the stream pointed to by stream.

That's it, that's the function. You hand it a stream, it hands you back the fd number. no cap. ✨

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🔥 RETURN VALUE 🔥

Upon successful completion, fileno() shall return the integer value of the file descriptor associated with stream.

Otherwise — and this is the part where we do a little 😭 — the value -1 shall be returned and errno set to indicate the error.

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💀 ERRORS 💀

The fileno() function shall fail if:

• EBADF — the stream is not associated with a file. periodt.

The fileno() function may fail if:

• EBADF — the file descriptor underlying stream is not a valid file descriptor. 👀

The following sections are informative.

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🌈 EXAMPLES 🌈

None.

(not me putting a whole heading here for absolutely nothing 😭 but it's in the source so it stays, bestie)

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🍓 APPLICATION USAGE 🍓

None.

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🧚 RATIONALE 🧚

Without some specification of which file descriptors are associated with these streams, it is impossible for an application to set up the streams for another application it starts with fork() and exec.

In particular, it would not be possible to write a portable version of the sh command interpreter — although, ngl, there may be other constraints that would prevent that portability anyway. so like. it's giving "necessary but not sufficient". 💜

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⭐ FUTURE DIRECTIONS ⭐

None.

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👑 SEE ALSO 👑

• Section 2.5.1, Interaction of File Descriptors and Standard I/O Streams
• dirfd()
• fdopen()
• fopen()
• stdin
• The Base Definitions volume of POSIX.1‐2017, <stdio.h>

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💖 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 .

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and that's a wrap, bestie 🫶 this page came to you courtesy of IEEE/The Open Group, 2017 (yes, the 2017 copyright date and the source line, both present and accounted for, no cap) — go forth and fileno() responsibly. mwah. 💅✨