FOPEN(3P)

🌸 FOPEN(3P) — bestie, we're opening a stream 💖

okay so real quick before we dive in: this is the POSIX Programmer's Manual page FOPEN(3P), and the Linux implementation of this interface may differ (so like, go check the corresponding Linux manual page for the deets on Linux behavior, fr), or honestly the interface might not even be implemented on Linux at all. 🙈

---

✨ PROLOG ✨

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

no cap, that's the whole prolog. short and iconic. 💅

---

🎀 NAME 🎀

fopen — open a stream

---

🦋 SYNOPSIS 🦋

#include <stdio.h>

FILE *fopen(const char *restrict pathname, const char *restrict mode);

---

💖 DESCRIPTION 💖

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. This volume of POSIX.1‐2017 defers to the ISO C standard.

So the fopen() function shall open the file whose pathname is the string pointed to by pathname, and associates a stream with it. Like, it's literally introducing your file to a stream. matchmaker energy. 💞

The mode argument points to a string. If the string is one of the following, the file shall be opened in the indicated mode. Otherwise, the behavior is undefined — and bestie, undefined behavior is NOT the vibe. 😭

• r or rb — open file for reading.
• w or wb — truncate to zero length or create file for writing.
• a or ab — append; open or create file for writing at end-of-file.
• r+ or rb+ or r+b — open file for update (reading and writing).
• w+ or wb+ or w+b — truncate to zero length or create file for update.
• a+ or ab+ or a+b — append; open or create file for update, writing at end-of-file.

The character 'b' shall have no effect, but is allowed for ISO C standard conformance. Opening a file with read mode (r as the first character in the mode argument) shall fail if the file does not exist or cannot be read. so like. you can't read a file that isn't there. shocking, I know. 💀

Opening a file with append mode (a as the first character in the mode argument) shall cause all subsequent writes to the file to be forced to the then current end-of-file, regardless of intervening calls to fseek(). periodt. it goes to the end. no matter what you try. 💅

When a file is opened with update mode ('+' as the second or third character in the mode argument), both input and output may be performed on the associated stream. However, the application shall ensure that output is not directly followed by input without an intervening call to fflush() or to a file positioning function (fseek(), fsetpos(), or rewind()), and input is not directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end-of-file.

okay this one's important bestie. write-then-read and read-then-write need a little breathing room in between or you're in trouble. it's giving boundaries. 🫶

When opened, a stream is fully buffered if and only if it can be determined not to refer to an interactive device. The error and end-of-file indicators for the stream shall be cleared.

If mode is w, wb, a, ab, w+, wb+, w+b, a+, ab+, or a+b, and the file did not previously exist, upon successful completion, fopen() shall mark for update the last data access, last data modification, and last file status change timestamps of the file and the last file status change and last data modification timestamps of the parent directory.

If mode is w, wb, a, ab, w+, wb+, w+b, a+, ab+, or a+b, and the file did not previously exist, the fopen() function shall create a file as if it called the creat() function with a value appropriate for the path argument interpreted from pathname and a value of S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH for the mode argument.

If mode is w, wb, w+, wb+, or w+b, and the file did previously exist, upon successful completion, fopen() shall mark for update the last data modification and last file status change timestamps of the file.

After a successful call to the fopen() function, the orientation of the stream shall be cleared, the encoding rule shall be cleared, and the associated mbstate_t object shall be set to describe an initial conversion state.

The file descriptor associated with the opened stream shall be allocated and opened as if by a call to open() with the following flags — okay so the original had this in a table and tables aren't allowed here, but the mapping is sacred so here it is as a list, exact same facts, no cap:

• 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

---

⭐ RETURN VALUE ⭐

Upon successful completion, fopen() shall return a pointer to the object controlling the stream. Otherwise, a null pointer shall be returned, and errno shall be set to indicate the error.

check your pointers bestie. always check. 🥺

---

🔥 ERRORS 🔥

The fopen() 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.
• EINTR — A signal was caught during fopen().
• EISDIR — The named file is a directory and mode requires write access.
• 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.
• EMFILE — {STREAM_MAX} streams are currently open in the calling process.
• 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}.
• 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.
• 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 the file was to be created.
• 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.
• EROFS — The named file resides on a read-only file system and mode requires write access.

The fopen() function may fail if:

• 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.
• EMFILE — {FOPEN_MAX} streams are currently open in the calling process.
• ENAMETOOLONG — The length of a component of a pathname is longer than {NAME_MAX}.
• ENOMEM — Insufficient storage space is available.
• 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: they're the ✨lore✨ not the ✨law✨, iykyk)

---

🌈 EXAMPLES 🌈

Opening a File

The following example tries to open the file named file for reading. The fopen() function returns a file pointer that is used in subsequent fgets() and fclose() calls. If the program cannot open the file, it just ignores it.

#include <stdio.h>
...
FILE *fp;
...
void rgrep(const char *file)
{
...
    if ((fp = fopen(file, "r")) == NULL)
        return;
...
}

not it just silently giving up 😭 but honestly? relatable.

---

🧚 APPLICATION USAGE 🧚

None.

---

💜 RATIONALE 💜

None.

---

🍓 FUTURE DIRECTIONS 🍓

None.

---

👑 SEE ALSO 👑

• Section 2.5, Standard I/O Streams
• creat()
• fclose()
• fdopen()
• fmemopen()
• freopen()
• open_memstream()

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 .

---

aaaand that's the whole thing bestie, you ate that 🫶 this page came to you courtesy of IEEE/The Open Group, 2017, and the manual page name is FOPEN(3P) — go forth and open your streams responsibly, mwah 💋✨