The Open Group Base Specifications Issue 7
IEEE Std 1003.1-2008, 2016 Edition
Copyright © 2001-2016 The IEEE and The Open Group


close - close a file descriptor


#include <unistd.h>

int close(int


The close() function shall deallocate the file descriptor indicated by fildes. To deallocate means to make the file descriptor available for return by subsequent calls to open() or other functions that allocate file descriptors. All outstanding record locks owned by the process on the file associated with the file descriptor shall be removed (that is, unlocked).

If close() is interrupted by a signal that is to be caught, it shall return -1 with errno set to [EINTR] and the state of fildes is unspecified. If an I/O error occurred while reading from or writing to the file system during close(), it may return -1 with errno set to [EIO]; if this error is returned, the state of fildes is unspecified.

When all file descriptors associated with a pipe or FIFO special file are closed, any data remaining in the pipe or FIFO shall be discarded.

When all file descriptors associated with an open file description have been closed, the open file description shall be freed.

If the link count of the file is 0, when all file descriptors associated with the file are closed, the space occupied by the file shall be freed and the file shall no longer be accessible.

[OB XSR] [Option Start] If a STREAMS-based fildes is closed and the calling process was previously registered to receive a SIGPOLL signal for events associated with that STREAM, the calling process shall be unregistered for events associated with the STREAM. The last close() for a STREAM shall cause the STREAM associated with fildes to be dismantled. If O_NONBLOCK is not set and there have been no signals posted for the STREAM, and if there is data on the module's write queue, close() shall wait for an unspecified time (for each module and driver) for any output to drain before dismantling the STREAM. The time delay can be changed via an I_SETCLTIME ioctl() request. If the O_NONBLOCK flag is set, or if there are any pending signals, close() shall not wait for output to drain, and shall dismantle the STREAM immediately.

If the implementation supports STREAMS-based pipes, and fildes is associated with one end of a pipe, the last close() shall cause a hangup to occur on the other end of the pipe. In addition, if the other end of the pipe has been named by fattach(), then the last close() shall force the named end to be detached by fdetach(). If the named end has no open file descriptors associated with it and gets detached, the STREAM associated with that end shall also be dismantled. [Option End]

[XSI] [Option Start] If fildes refers to the master side of a pseudo-terminal, and this is the last close, a SIGHUP signal shall be sent to the controlling process, if any, for which the slave side of the pseudo-terminal is the controlling terminal. It is unspecified whether closing the master side of the pseudo-terminal flushes all queued input and output. [Option End]

[OB XSR] [Option Start] If fildes refers to the slave side of a STREAMS-based pseudo-terminal, a zero-length message may be sent to the master. [Option End]

When there is an outstanding cancelable asynchronous I/O operation against fildes when close() is called, that I/O operation may be canceled. An I/O operation that is not canceled completes as if the close() operation had not yet occurred. All operations that are not canceled shall complete as if the close() blocked until the operations completed. The close() operation itself need not block awaiting such I/O completion. Whether any I/O operation is canceled, and which I/O operation may be canceled upon close(), is implementation-defined.

If a memory mapped file [SHM] [Option Start]  or a shared memory object [Option End]  remains referenced at the last close (that is, a process has it mapped), then the entire contents of the memory object shall persist until the memory object becomes unreferenced. If this is the last close of a memory mapped file [SHM] [Option Start]  or a shared memory object [Option End]  and the close results in the memory object becoming unreferenced, and the memory object has been unlinked, then the memory object shall be removed.

If fildes refers to a socket, close() shall cause the socket to be destroyed. If the socket is in connection-mode, and the SO_LINGER option is set for the socket with non-zero linger time, and the socket has untransmitted data, then close() shall block for up to the current linger interval until all data is transmitted.


Upon successful completion, 0 shall be returned; otherwise, -1 shall be returned and errno set to indicate the error.


The close() function shall fail if:

The fildes argument is not a open file descriptor.
The close() function was interrupted by a signal.

The close() function may fail if:

An I/O error occurred while reading from or writing to the file system.

The following sections are informative.


Reassigning a File Descriptor

The following example closes the file descriptor associated with standard output for the current process, re-assigns standard output to a new file descriptor, and closes the original file descriptor to clean up. This example assumes that the file descriptor 0 (which is the descriptor for standard input) is not closed.

#include <unistd.h>
int pfd;

Incidentally, this is exactly what could be achieved using:

dup2(pfd, 1);

Closing a File Descriptor

In the following example, close() is used to close a file descriptor after an unsuccessful attempt is made to associate that file descriptor with a stream.

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>

#define LOCKFILE "/etc/ptmp" ... int pfd; FILE *fpfd; ... if ((fpfd = fdopen (pfd, "w")) == NULL) { close(pfd); unlink(LOCKFILE); exit(1); } ...


An application that had used the stdio routine fopen() to open a file should use the corresponding fclose() routine rather than close(). Once a file is closed, the file descriptor no longer exists, since the integer corresponding to it no longer refers to a file.

Implementations may use file descriptors that must be inherited into child processes for the child process to remain conforming, such as for message catalog or tracing purposes. Therefore, an application that calls close() on an arbitrary integer risks non-conforming behavior, and close() can only portably be used on file descriptor values that the application has obtained through explicit actions, as well as the three file descriptors corresponding to the standard file streams. In multi-threaded parent applications, the practice of calling close() in a loop after fork() and before an exec call in order to avoid a race condition of leaking an unintended file descriptor into a child process, is therefore unsafe, and the race should instead be combatted by opening all file descriptors with the FD_CLOEXEC bit set unless the file descriptor is intended to be inherited across exec.

Usage of close() on file descriptors STDIN_FILENO, STDOUT_FILENO, or STDERR_FILENO should immediately be followed by an operation to reopen these file descriptors. Unexpected behavior will result if any of these file descriptors is left in a closed state (for example, an [EBADF] error from perror()) or if an unrelated open() or similar call later in the application accidentally allocates a file to one of these well-known file descriptors. Furthermore, a close() followed by a reopen operation (e.g., open(), dup(), etc.) is not atomic; dup2() should be used to change standard file descriptors.


The use of interruptible device close routines should be discouraged to avoid problems with the implicit closes of file descriptors by exec and exit(). This volume of POSIX.1-2008 only intends to permit such behavior by specifying the [EINTR] error condition.

Note that the requirement for close() on a socket to block for up to the current linger interval is not conditional on the O_NONBLOCK setting.

The standard developers rejected a proposal to add closefrom() to the standard. Because the standard permits implementations to use inherited file descriptors as a means of providing a conforming environment for the child process, it is not possible to standardize an interface that closes arbitrary file descriptors above a certain value while still guaranteeing a conforming environment.




STREAMS, dup, exec, exit, fattach, fclose, fdetach, fopen, fork, ioctl, open, perror, unlink

XBD <unistd.h>


First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 5

The DESCRIPTION is updated for alignment with the POSIX Realtime Extension.

Issue 6

The DESCRIPTION related to a STREAMS-based file or pseudo-terminal is marked as part of the XSI STREAMS Option Group.

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

Text referring to sockets is added to the DESCRIPTION.

The DESCRIPTION is updated for alignment with IEEE Std 1003.1j-2000 by specifying that shared memory objects and memory mapped files (and not typed memory objects) are the types of memory objects to which the paragraph on last closes applies.

IEEE Std 1003.1-2001/Cor 1-2002, item XSH/TC1/D6/12 is applied, correcting the XSH shaded text relating to the master side of a pseudo-terminal. The reason for the change is that the behavior of pseudo-terminals and regular terminals should be as much alike as possible in this case; the change achieves that and matches historical behavior.

Issue 7

Functionality relating to the XSI STREAMS option is marked obsolescent.

Functionality relating to the Asynchronous Input and Output and Memory Mapped Files options is moved to the Base.

Austin Group Interpretation 1003.1-2001 #139 is applied, clarifying that the requirement for close() on a socket to block for up to the current linger interval is not conditional on the O_NONBLOCK setting.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0059 [419], XSH/TC1-2008/0060 [149], and XSH/TC1-2008/0061 [149] are applied.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0069 [555] is applied.

End of informative text.


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