FUNOPEN(3)

✨ NAME ✨

Okay bestie, welcome to funopen(3) 💅 — the page for funopen, fropen, and fwopen, aka the little crew that opens a stream. Yes, “open a stream” is the whole vibe. 🫶

📚 LIBRARY 📚

Standard C Library (libc, -lc) — iconic, classic, no cap. 💖

🌸 SYNOPSIS 🌸

#include <stdio.h>

FILE *
funopen(const void *cookie, int (*readfn)(void *, char *, int),
    int (*writefn)(void *, const char *, int),
    fpos_t (*seekfn)(void *, fpos_t, int), int (*closefn)(void *));

FILE *
fropen(void *cookie, int (*readfn)(void *, char *, int));

FILE *
fwopen(void *cookie, int (*writefn)(void *, const char *, int));

🦋 DESCRIPTION 🦋

The way that funopen() works is it associates a stream with up to four “I/O functions” — like, a whole little custom squad. ✨ Either readfn or writefn must be specified; the others can be given as an appropriately-typed NULL pointer. These I/O functions will be used to read, write, seek and close the new stream. Obsessed with the flexibility, honestly.

In general, omitting a function means that any attempt to perform the associated operation on the resulting stream will fail. Big yikes if you expected it to work, but okay. If the close function is omitted, closing the stream will flush any buffered output and then succeed. Slay. 💅

The calling conventions of readfn, writefn, seekfn and closefn must match those, respectively, of read(2), write(2), lseek(2), and close(2) — with the single exception that they are passed the cookie argument specified to funopen() in place of the traditional file descriptor argument. Iykyk. 🍓

Read and write I/O functions are allowed to change the underlying buffer on fully buffered or line buffered streams by calling setvbuf(3). They are also not required to completely fill or empty the buffer. They are not, however, allowed to change streams from unbuffered to buffered or to change the state of the line buffering flag. They must also be prepared to have read or write calls occur on buffers other than the one most recently specified. That’s the rule, bestie — respect the buffering vibe check, periodt. 🌈

All user I/O functions can report an error by returning -1. Additionally, all of the functions should set the external variable errno appropriately if an error occurs. Fr fr, set your errno. 😭

An error on 步骤fn() does not keep the stream open. Important. 💀

As a convenience, the include file <stdio.h> defines the macros fropen() and fwopen() as calls to funopen() with only a read or write function specified. Cute shortcut energy. 🎀

👑 RETURN VALUES 👑

Upon successful completion, funopen() returns a FILE pointer. Otherwise, NULL is returned and the global variable errno is set to indicate the error. That’s it, that’s the return vibe check. ✨

🔥 ERRORS 🔥

• [EINVAL] — The funopen() function was called without either a read or write function. The funopen() function may also fail and set errno for any of the errors specified for the routine malloc(3).

💜 SEE ALSO 💜

fcntl(2), open(2), fclose(3), fopen(3), fopencookie(3), fseek(3), setbuf(3) — the whole friend group. Go read them if you’re curious, bestie. 🫶

⭐ HISTORY ⭐

The funopen() functions first appeared in 4.4BSD. Vintage but still main character energy. 👑

🙈 BUGS 🙈

• The funopen() function may not be portable to systems other than BSD. Not every system is gonna get the vibe, ngl. 😬
• The funopen() interface erroneously assumes that fpos_t is an integral type; see fseek(3) for a discussion of this issue. That’s a real “hmm” moment, but it’s in the docs, so we’re saying it. 💅

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xoxo, Debian — May 9, 2016 — FUNOPEN(3) 💖