FREOPEN(3P)

✨ freopen(3P) ✨

okay bestie, sit down, hydrate, and let me put you on — this is the manpage for freopen(3P), and it comes to you straight from the POSIX Programmer's Manual. She takes a stream you already own and re-homes it onto a whole new file. iconic behaviour, honestly. 💅

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🌸 PROLOG 🌸

Real talk before we get into it: this manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (so pls consult the corresponding Linux manual page for the actual Linux lore ☕), or the interface may just straight up not be implemented on Linux at all. no cap. 🙈

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💖 NAME 💖

freopen — open a stream. that's the whole job. she's employed and thriving. 🫶

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🦋 SYNOPSIS 🦋

#include <stdio.h>

FILE *freopen(const char *restrict pathname, const char *restrict mode,
    FILE *restrict stream);

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👑 DESCRIPTION 👑

So here's the tea: the functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional — and this volume of POSIX.1‐2017 defers to the ISO C standard. she's not trying to start drama with ISO C, ok?? 😭

The freopen() function shall first attempt to flush the stream associated with stream as if by a call to fflush(stream). Failure to flush the stream successfully shall be ignored (she's not gonna make it a whole thing). If pathname is not a null pointer, freopen() shall close any file descriptor associated with stream. Failure to close the file descriptor successfully shall also be ignored. And the error and end-of-file indicators for the stream shall be cleared. ✨ clean slate energy ✨

Then, freopen() shall open the file whose pathname is the string pointed to by pathname, and associate the stream pointed to by stream with it. The mode argument shall be used just as in fopen(). same rules, same vibe.

⚠️ The original stream shall be closed regardless of whether the subsequent open succeeds. she is leaving that stream behind no matter what, bestie. 💀

If pathname is a null pointer, freopen() shall attempt to change the mode of the stream to that specified by mode, as if the name of the file currently associated with the stream had been used. In this case, the file descriptor associated with the stream need not be closed if the call to freopen() succeeds. It is implementation-defined which changes of mode are permitted (if any), and under what circumstances. so like… your mileage may vary, don't @ me. 🫠

After a successful call to freopen(), the following shall be reset to factory settings, no notes:

• the orientation of the stream shall be cleared
• the encoding rule shall be cleared
• the associated mbstate_t object shall be set to describe an initial conversion state

And now for the ✨ mapping ✨. If pathname is not a null pointer, or if pathname is a null pointer and the specified mode change necessitates the file descriptor associated with the stream to be closed and reopened, the file descriptor associated with the reopened stream shall be allocated and opened as if by a call to open() with the following flags:

                  ┌──────────────────┬───────────────────────────┐
                  │  freopen() Mode  │       open() Flags        │
                  ├──────────────────┼───────────────────────────┤
                  │ r or rb          │ O_RDONLY                  │
                  │ w or wb          │ O_WRONLY|O_CREAT|O_TRUNC  │
                  │ a or ab          │ O_WRONLY|O_CREAT|O_APPEND │
                  │ r+ or rb+ or r+b │ O_RDWR                    │
                  │ w+ or wb+ or w+b │ O_RDWR|O_CREAT|O_TRUNC    │
                  │ a+ or ab+ or a+b │ O_RDWR|O_CREAT|O_APPEND   │
                  └──────────────────┴───────────────────────────┘

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🎀 RETURN VALUE 🎀

Upon successful completion, freopen() shall return the value of stream. otherwise? a null pointer shall be returned, and errno shall be set to indicate the error. so check errno, bestie, she's telling you what went wrong. 🚨

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💅 ERRORS 💅

The freopen() function shall fail if:

• EACCES — Search permission is denied on a component of the path prefix, or the file exists and the permissions specified by mode are denied, or the file does not exist and write permission is denied for the parent directory of the file to be created. 🚫
• EBADF — The file descriptor underlying the stream is not a valid file descriptor when pathname is a null pointer.
• EINTR — A signal was caught during freopen(). rude interruption fr.
• EISDIR — The named file is a directory and mode requires write access. not a directory, girl. 📁
• ELOOP — A loop exists in symbolic links encountered during resolution of the path argument. 🌀
• EMFILE — All file descriptors available to the process are currently open. no more fd's left in the group chat.
• ENAMETOOLONG — The length of a component of a pathname is longer than {NAME_MAX}.
• ENFILE — The maximum allowable number of files is currently open in the system.
• ENOENT — The mode string begins with 'r' and a component of pathname does not name an existing file, or mode begins with 'w' or 'a' and a component of the path prefix of pathname does not name an existing file, or pathname is an empty string. she's not there, she never was. 😭
• ENOENT or ENOTDIR — The pathname argument contains at least one non-<slash> character and ends with one or more trailing <slash> characters. If pathname without the trailing <slash> characters would name an existing file, an [ENOENT] error shall not occur.
• ENOSPC — The directory or file system that would contain the new file cannot be expanded, the file does not exist, and it was to be created. no room left, sorry babe. 💀
• ENOTDIR — A component of the path prefix names an existing file that is neither a directory nor a symbolic link to a directory, or the pathname argument contains at least one non-<slash> character and ends with one or more trailing <slash> characters and the last pathname component names an existing file that is neither a directory nor a symbolic link to a directory.
• ENXIO — The named file is a character special or block special file, and the device associated with this special file does not exist.
• EOVERFLOW — The named file is a regular file and the size of the file cannot be represented correctly in an object of type off_t. too big to even describe. iconically large. 📏
• EROFS — The named file resides on a read-only file system and mode requires write access. 🔒

The freopen() function may fail if:

• EBADF — The mode with which the file descriptor underlying the stream was opened does not support the requested mode when pathname is a null pointer.
• EINVAL — The value of the mode argument is not valid.
• ELOOP — More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the path argument.
• ENAMETOOLONG — The length of a pathname exceeds {PATH_MAX}, or pathname resolution of a symbolic link produced an intermediate result with a length that exceeds {PATH_MAX}.
• ENOMEM — Insufficient storage space is available.
• ENXIO — A request was made of a nonexistent device, or the request was outside the capabilities of the device.
• ETXTBSY — The file is a pure procedure (shared text) file that is being executed and mode requires write access.

The following sections are informative. (translation: this is the fun bonus content, not the contract 💖)

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🌈 EXAMPLES 🌈

Directing Standard Output to a File

The following example logs all standard output to the /tmp/logfile file. look at her go. 👑

#include <stdio.h>
...
FILE *fp;
...
fp = freopen ("/tmp/logfile", "a+", stdout);
...

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🦋 APPLICATION USAGE 🦋

The freopen() function is typically used to attach the pre-opened streams associated with stdin, stdout, and stderr to other files. that's her main character moment. ✨

Since implementations are not required to support any stream mode changes when the pathname argument is NULL, portable applications cannot rely on the use of freopen() to change the stream mode, and use of this feature is discouraged. 🚩 The feature was originally added to the ISO C standard in order to facilitate changing stdin and stdout to binary mode. Since a 'b' character in the mode has no effect on POSIX systems, this use of the feature is unnecessary in POSIX applications.

⚠️ But hold up — even though the 'b' is ignored, a successful call to freopen(NULL, "wb", stdout) does have an effect. In particular, for regular files it truncates the file and sets the file-position indicator for the stream to the start of the file. 😭

It is possible that these side-effects are an unintended consequence of the way the feature is specified in the ISO/IEC 9899:1999 standard, but unless or until the ISO C standard is changed, applications which successfully call freopen(NULL, "wb", stdout) will behave in unexpected ways on conforming systems in situations such as:

{ appl file1; appl file2; } > file3

which will result in file3 containing only the output from the second invocation of appl. the first one got ate. 💀 periodt.

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🧚 RATIONALE 🧚

None. she said no comment, your honour. 😌

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⭐ FUTURE DIRECTIONS ⭐

None. the future is unwritten and this function is not in it. 🫶

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🔥 SEE ALSO 🔥

• Section 2.5, Standard I/O Streams
• fclose()
• fdopen()
• fflush()
• fmemopen()
• fopen()
• mbsinit()
• open()
• open_memstream()

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!! 💖 she's brought to you by IEEE/The Open Group, 2017 — POSIX Programmer's Manual, freopen(3P), and she ate and left no crumbs. now go flush a stream or something. mwah. 💅✨🎀