fcntl.h(7POSIX)

💖 fcntl.h — file control options 💖

okay bestie, welcome to the fcntl.h page from the POSIX Programmer's Manual (7POSIX, she's iconic). this is THE header for file control options and honestly she does so much, let's get into it 🫶

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✨ PROLOG ✨

real talk first: 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 honestly the interface may not be implemented on Linux at all. just so you know 💅

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🌸 NAME 🌸

fcntl.h — file control options. that's her. that's the whole vibe.

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🦋 SYNOPSIS 🦋

#include <fcntl.h>

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✨ DESCRIPTION ✨

okay so the <fcntl.h> header shall define these symbolic constants for the cmd argument used by fcntl(). the values shall be unique and shall be suitable for use in #if preprocessing directives. no cap.

• F_DUPFD — Duplicate file descriptor.
• F_DUPFD_CLOEXEC — Duplicate file descriptor with the close-on-exec flag FD_CLOEXEC set.
• F_GETFD — Get file descriptor flags.
• F_SETFD — Set file descriptor flags.
• F_GETFL — Get file status flags and file access modes.
• F_SETFL — Set file status flags.
• F_GETLK — Get record locking information.
• F_SETLK — Set record locking information.
• F_SETLKW — Set record locking information; wait if blocked.
• F_GETOWN — Get process or process group ID to receive SIGURG signals.
• F_SETOWN — Set process or process group ID to receive SIGURG signals.

the <fcntl.h> header shall define the following symbolic constant used for the fcntl() file descriptor flags, which shall be suitable for use in #if preprocessing directives:

• FD_CLOEXEC — Close the file descriptor upon execution of an exec family function.

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the <fcntl.h> header shall also define these symbolic constants for the l_type argument used for record locking with fcntl(). values shall be unique and suitable for use in #if preprocessing directives 💖

• F_RDLCK — Shared or read lock.
• F_UNLCK — Unlock.
• F_WRLCK — Exclusive or write lock.

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the <fcntl.h> header shall define the values used for l_whence, SEEK_SET, SEEK_CUR, and SEEK_END as described in <stdio.h>.

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now for the file creation flags 🎀 these get used in the oflag value to open() and openat(). the values shall be bitwise-distinct and shall be suitable for use in #if preprocessing directives:

• O_CLOEXEC — The FD_CLOEXEC flag associated with the new descriptor shall be set to close the file descriptor upon execution of an exec family function.
• O_CREAT — Create file if it does not exist.
• O_DIRECTORY — Fail if file is a non-directory file.
• O_EXCL — Exclusive use flag.
• O_NOCTTY — Do not assign controlling terminal.
• O_NOFOLLOW — Do not follow symbolic links.
• O_TRUNC — Truncate flag.
• O_TTY_INIT — Set the termios structure terminal parameters to a state that provides conforming behavior; see Section 11.2, Parameters that Can be Set.

heads up bestie: the O_TTY_INIT flag can have the value zero, and in this case it need not be bitwise-distinct from the other flags. yes, that's a whole thing.

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next up, the file status flags ✨ for use with open(), openat(), and fcntl(). the values shall be suitable for use in #if preprocessing directives:

• O_APPEND — Set append mode.
• O_DSYNC — Write according to synchronized I/O data integrity completion.
• O_NONBLOCK — Non-blocking mode.
• O_RSYNC — Synchronized read I/O operations.
• O_SYNC — Write according to synchronized I/O file integrity completion.

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and the mask for file access modes (the value shall be suitable for use in #if preprocessing directives):

• O_ACCMODE — Mask for file access modes.

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okay now the file access modes 🌈 for use with open(), openat(), and fcntl(). the values shall be unique — except that O_EXEC and O_SEARCH may have equal values. and they shall be suitable for use in #if preprocessing directives:

• O_EXEC — Open for execute only (non-directory files). The result is unspecified if this flag is applied to a directory. 🫣
• O_RDONLY — Open for reading only.
• O_RDWR — Open for reading and writing.
• O_SEARCH — Open directory for search only. The result is unspecified if this flag is applied to a non-directory file. 🫣
• O_WRONLY — Open for writing only.

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the <fcntl.h> header shall define the symbolic constants for file modes for use as values of mode_t as described in <sys/stat.h>.

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the <fcntl.h> header shall define the following symbolic constant as a special value used in place of a file descriptor for the *at() functions which take a directory file descriptor as a parameter:

• AT_FDCWD — Use the current working directory to determine the target of relative file paths.

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the <fcntl.h> header shall define the following symbolic constant as a value for the flag used by faccessat():

• AT_EACCESS — Check access using effective user and group ID.

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the <fcntl.h> header shall define the following symbolic constant as a value for the flag used by fstatat(), fchmodat(), fchownat(), and utimensat():

• AT_SYMLINK_NOFOLLOW — Do not follow symbolic links.

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the <fcntl.h> header shall define the following symbolic constant as a value for the flag used by linkat():

• AT_SYMLINK_FOLLOW — Follow symbolic link.

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the <fcntl.h> header shall define the following symbolic constant as a value for the flag used by unlinkat():

• AT_REMOVEDIR — Remove directory instead of file.

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now for the advice argument used by posix_fadvise() 💅 the header shall define these:

• POSIX_FADV_DONTNEED — The application expects that it will not access the specified data in the near future.
• POSIX_FADV_NOREUSE — The application expects to access the specified data once and then not reuse it thereafter.
• POSIX_FADV_NORMAL — The application has no advice to give on its behavior with respect to the specified data. It is the default characteristic if no advice is given for an open file.
• POSIX_FADV_RANDOM — The application expects to access the specified data in a random order.
• POSIX_FADV_SEQUENTIAL — The application expects to access the specified data sequentially from lower offsets to higher offsets.
• POSIX_FADV_WILLNEED — The application expects to access the specified data in the near future.

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the <fcntl.h> header shall define the flock structure describing a file lock 👑 it shall include the following members:

short  l_type   Type of lock; F_RDLCK, F_WRLCK, F_UNLCK.
short  l_whence Flag for starting offset.
off_t  l_start  Relative offset in bytes.
off_t  l_len    Size; if 0 then until EOF.
pid_t  l_pid    Process ID of the process holding the lock; returned with F_GETLK.

the <fcntl.h> header shall define the mode_t, off_t, and pid_t types as described in <sys/types.h>.

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the following shall be declared as functions and may also be defined as macros. function prototypes shall be provided, bestie:

int  creat(const char *, mode_t);
int  fcntl(int, int, ...);
int  open(const char *, int, ...);
int  openat(int, const char *, int, ...);
int  posix_fadvise(int, off_t, off_t, int);
int  posix_fallocate(int, off_t, off_t);

inclusion of the <fcntl.h> header may also make visible all symbols from <sys/stat.h> and <unistd.h>. so many friends showing up uninvited, love that for them 🧚

the following sections are informative. iykyk.

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💜 APPLICATION USAGE 💜

Although no existing implementation defines AT_SYMLINK_FOLLOW and AT_SYMLINK_NOFOLLOW as the same numeric value, POSIX.1‐2008 does not prohibit that, since the two constants are not used with the same interfaces. so like, technically allowed, but nobody's doing it. 👀

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🍓 RATIONALE 🍓

While many of the symbolic constants introduced in the <fcntl.h> header do not strictly need to be used in #if preprocessor directives, widespread historic practice has defined them as macros that are usable in such constructs, and examination of existing applications has shown that they are occasionally used in such a way. Therefore it was decided to retain this requirement on an implementation in POSIX.1‐2008. tradition, basically 💅

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⭐ FUTURE DIRECTIONS ⭐

None. that's it. nothing. moving on 💀

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🌈 SEE ALSO 🌈

• <stdio.h>
• <sys_stat.h>
• <sys_types.h>
• <unistd.h>

The System Interfaces volume of POSIX.1‐2017: creat(), exec, fcntl(), futimens(), open(), posix_fadvise(), posix_fallocate(), posix_madvise()

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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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okay that's the whole page bestie, you ate it 💅✨ this one came to you from IEEE/The Open Group, 2017, and yes, fcntl.h(7POSIX) is officially a whole queen. go forth and control your files 🫶💖