FFLUSH(3POSIX)

🦋 FFLUSH(3POSIX) 🦋

okay bestie, gather round 💖 this one lives in the POSIX Programmer's Manual, and it's literally the "hey, go ahead and actually write that out" function. Iconic. Essential. The moment your buffered output stops being shy.

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🌸 PROLOG 🌸

real talk before we start: this manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (so go check the corresponding Linux manual page for the deets on Linux behavior), or the interface may legitimately not be implemented on Linux at all. not me being dramatic, that's just what it says 🫶

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✨ NAME ✨

fflush — flush a stream

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🎀 SYNOPSIS 🎀

#include <stdio.h>

int fflush(FILE *stream);

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💅 DESCRIPTION 💅

so the functionality on this reference page is aligned with the ISO C standard. If anything here conflicts with the ISO C standard, that conflict is unintentional — no cap. This volume of POSIX.1‐2017 defers to the ISO C standard, periodt.

Here's the tea ☕ — if stream points to an output stream, or an update stream in which the most recent operation was not input, then fflush() shall cause any unwritten data for that stream to be written to the file. And the last data modification and last file status change timestamps of the underlying file shall be marked for update. so basically: it commits. it locks in. 💖

Now for the reading case, which is giving subtler main character energy ✨ — for a stream open for reading with an underlying file description, if the file is not already at EOF, and the file is one capable of seeking, then:

• the file offset of the underlying open file description shall be set to the file position of the stream, and
• any characters pushed back onto the stream by ungetc() or ungetwc() that have not subsequently been read from the stream shall be discarded — without further changing the file offset. bye bestie, gone 🙈

And if stream is a null pointer? fflush() shall perform this flushing action on all streams for which the behavior is defined above. all of them. every single one. group flush 💫

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🔥 RETURN VALUE 🔥

Upon successful completion, fflush() shall return 0. Slay.

Otherwise, it shall set the error indicator for the stream, return EOF, and set errno to indicate the error. so if you see EOF, that's not a vibe — go check errno 😭

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💀 ERRORS 💀

The fflush() function shall fail if:

• EAGAIN — The O_NONBLOCK flag is set for the file descriptor underlying stream and the thread would be delayed in the write operation. the descriptor said "not right now" 🙈
• EBADF — The file descriptor underlying stream is not valid. that fd is fake, babe.
• 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. yes, that's EFBIG three separate times, three separate reasons — the manpage said it thrice and so do I, no editing bestie 💅
• EINTR — The fflush() function was interrupted by a signal. rude.
• EIO — The process is a member of a background process group attempting to 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. truly the most specific paragraph you'll ever read 🫠
• ENOMEM — The underlying stream was created by open_memstream() or open_wmemstream() and insufficient memory is available.
• ENOSPC — There was no free space remaining on the device containing the file or in the buffer used by the fmemopen() function.
• EPIPE — An attempt is 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. 🚨 and yes, the signal really does get sent, so heads up.

The fflush() 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 bestie: the following sections are informative. they're context, not requirements. 📖✨

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🍓 EXAMPLES 🍓

🧚 Sending Prompts to Standard Output 🧚

The following example uses printf() calls to print a series of prompts for information the user must enter from standard input. The fflush() calls force the output to standard output. The fflush() function is used because standard output is usually buffered and the prompt may not immediately be printed on the output or terminal. The getline() function calls read strings from standard input and place the results in variables, for use later in the program.

char *user;
char *oldpasswd;
char *newpasswd;
ssize_t llen;
size_t blen;
struct termios term;
tcflag_t saveflag;

printf("User name: ");
fflush(stdout);
blen = 0;
llen = getline(&user, &blen, stdin);
user[llen-1] = 0;
tcgetattr(fileno(stdin), &term);
saveflag = term.c_lflag;
term.c_lflag &= ~ECHO;
tcsetattr(fileno(stdin), TCSANOW, &term);
printf("Old password: ");
fflush(stdout);
blen = 0;
llen = getline(&oldpasswd, &blen, stdin);
oldpasswd[llen-1] = 0;

printf("\nNew password: ");
fflush(stdout);
blen = 0;
llen = getline(&newpasswd, &blen, stdin);
newpasswd[llen-1] = 0;
term.c_lflag = saveflag;
tcsetattr(fileno(stdin), TCSANOW, &term);
free(user);
free(oldpasswd);
free(newpasswd);

like, that's the whole bit — prompt, flush, read. the flush is the part that makes the prompt actually show up before the program sits there waiting on you. 🎀

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🌈 APPLICATION USAGE 🌈

None.

(yeah. literally none. it's that self-explanatory. iconic behavior honestly 👑)

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💜 RATIONALE 💜

Data buffered by the system may make determining the validity of the position of the current file descriptor impractical. Thus, enforcing the repositioning of the file descriptor after fflush() on streams open for read() is not mandated by POSIX.1‐2008. so don't @ the standard about it, it already told you why 💅

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⭐ FUTURE DIRECTIONS ⭐

None.

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🫶 SEE ALSO 🫶

• Section 2.5, Standard I/O Streams
• fmemopen()
• getrlimit()
• open_memstream()
• ulimit()

And from 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 the whole page, bestie 💖 go flush your streams, they deserve it. ✧･ﾟ IEEE/The Open Group — 2017 — FFLUSH(3POSIX) ✧･ﾟ 🦋✨