.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "OPEN" 3P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" open 
.SH 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.
.SH NAME
open \- open a file
.SH SYNOPSIS
.LP
\fB#include <sys/stat.h> \fP
\fB
.br
#include <fcntl.h>
.br
.sp
int open(const char *\fP\fIpath\fP\fB, int\fP \fIoflag\fP\fB, ...
);
.br
\fP
.SH DESCRIPTION
.LP
The \fIopen\fP() function shall establish the connection between a
file and a file descriptor. It shall create an open file
description that refers to a file and a file descriptor that refers
to that open file description. The file descriptor is used by
other I/O functions to refer to that file. The \fIpath\fP argument
points to a pathname naming the file.
.LP
The \fIopen\fP() function shall return a file descriptor for the named
file that is the lowest file descriptor not currently
open for that process. The open file description is new, and therefore
the file descriptor shall not share it with any other
process in the system. The FD_CLOEXEC file descriptor flag associated
with the new file descriptor shall be cleared.
.LP
The file offset used to mark the current position within the file
shall be set to the beginning of the file.
.LP
The file status flags and file access modes of the open file description
shall be set according to the value of
\fIoflag\fP.
.LP
Values for \fIoflag\fP are constructed by a bitwise-inclusive OR of
flags from the following list, defined in \fI<fcntl.h>\fP. Applications
shall specify exactly one of the first three values (file
access modes) below in the value of \fIoflag\fP:
.TP 7
O_RDONLY
Open for reading only.
.TP 7
O_WRONLY
Open for writing only.
.TP 7
O_RDWR
Open for reading and writing. The result is undefined if this flag
is applied to a FIFO.
.sp
.LP
Any combination of the following may be used:
.TP 7
O_APPEND
If set, the file offset shall be set to the end of the file prior
to each write.
.TP 7
O_CREAT
If the file exists, this flag has no effect except as noted under
O_EXCL below. Otherwise, the file shall be created; the user
ID of the file shall be set to the effective user ID of the process;
the group ID of the file shall be set to the group ID of the
file's parent directory or to the effective group ID of the process;
and the access permission bits (see \fI<sys/stat.h>\fP) of the file
mode shall be set to the value of the third argument taken
as type \fBmode_t\fP modified as follows: a bitwise AND is performed
on the file-mode bits and the corresponding bits in the
complement of the process' file mode creation mask. Thus, all bits
in the file mode whose corresponding bit in the file mode
creation mask is set are cleared. When bits other than the file permission
bits are set, the effect is unspecified. The third
argument does not affect whether the file is open for reading, writing,
or for both. Implementations shall provide a way to
initialize the file's group ID to the group ID of the parent directory.
Implementations may, but need not, provide an
implementation-defined way to initialize the file's group ID to the
effective group ID of the calling process.
.TP 7
O_DSYNC
Write I/O operations on the file descriptor shall complete as defined
by synchronized I/O data integrity completion. 
.TP 7
O_EXCL
If O_CREAT and O_EXCL are set, \fIopen\fP() shall fail if the file
exists. The check for the existence of the file and the
creation of the file if it does not exist shall be atomic with respect
to other threads executing \fIopen\fP() naming the same
filename in the same directory with O_EXCL and O_CREAT set. If O_EXCL
and O_CREAT are set, and \fIpath\fP names a symbolic link,
\fIopen\fP() shall fail and set \fIerrno\fP to [EEXIST], regardless
of the contents of the symbolic link. If O_EXCL is set and
O_CREAT is not set, the result is undefined.
.TP 7
O_NOCTTY
If set and \fIpath\fP identifies a terminal device, \fIopen\fP() shall
not cause the terminal device to become the
controlling terminal for the process.
.TP 7
O_NONBLOCK
When opening a FIFO with O_RDONLY or O_WRONLY set: 
.RS
.IP " *" 3
If O_NONBLOCK is set, an \fIopen\fP() for reading-only shall return
without delay. An \fIopen\fP() for writing-only shall
return an error if no process currently has the file open for reading.
.LP
.IP " *" 3
If O_NONBLOCK is clear, an \fIopen\fP() for reading-only shall block
the calling thread until a thread opens the file for
writing. An \fIopen\fP() for writing-only shall block the calling
thread until a thread opens the file for reading.
.LP
.RE
.LP
When opening a block special or character special file that supports
non-blocking opens:
.RS
.IP " *" 3
If O_NONBLOCK is set, the \fIopen\fP() function shall return without
blocking for the device to be ready or available.
Subsequent behavior of the device is device-specific.
.LP
.IP " *" 3
If O_NONBLOCK is clear, the \fIopen\fP() function shall block the
calling thread until the device is ready or available before
returning.
.LP
.RE
.LP
Otherwise, the behavior of O_NONBLOCK is unspecified.
.TP 7
O_RSYNC
Read I/O operations on the file descriptor shall complete at the same
level of integrity as specified by the O_DSYNC and O_SYNC
flags. If both O_DSYNC and O_RSYNC are set in \fIoflag\fP, all I/O
operations on the file descriptor shall complete as defined by
synchronized I/O data integrity completion. If both O_SYNC and O_RSYNC
are set in flags, all I/O operations on the file descriptor
shall complete as defined by synchronized I/O file integrity completion.
.TP 7
O_SYNC
Write I/O operations on the file descriptor shall complete as defined
by synchronized I/O file integrity completion. 
.TP 7
O_TRUNC
If the file exists and is a regular file, and the file is successfully
opened O_RDWR or O_WRONLY, its length shall be truncated
to 0, and the mode and owner shall be unchanged. It shall have no
effect on FIFO special files or terminal device files. Its effect
on other file types is implementation-defined. The result of using
O_TRUNC with O_RDONLY is undefined.
.sp
.LP
If O_CREAT is set and the file did not previously exist, upon successful
completion, \fIopen\fP() shall mark for update the
\fIst_atime,\fP \fIst_ctime\fP, and \fIst_mtime\fP fields of the file
and the \fIst_ctime\fP and \fIst_mtime\fP fields of the
parent directory.
.LP
If O_TRUNC is set and the file did previously exist, upon successful
completion, \fIopen\fP() shall mark for update the
\fIst_ctime\fP and \fIst_mtime\fP fields of the file.
.LP
If both the O_SYNC and O_DSYNC flags are set, the effect is as if
only the O_SYNC flag was set. 
.LP
If \fIpath\fP refers to a STREAMS file, \fIoflag\fP may be constructed
from O_NONBLOCK OR'ed with either O_RDONLY, O_WRONLY, or
O_RDWR. Other flag values are not applicable to STREAMS devices and
shall have no effect on them. The value O_NONBLOCK affects the
operation of STREAMS drivers and certain functions applied to file
descriptors associated with STREAMS files. For STREAMS drivers,
the implementation of O_NONBLOCK is device-specific. 
.LP
If \fIpath\fP names the master side of a pseudo-terminal device, then
it is unspecified whether \fIopen\fP() locks the slave side
so that it cannot be opened. Conforming applications shall call \fIunlockpt\fP()
before
opening the slave side. 
.LP
The largest value that can be represented correctly in an object of
type \fBoff_t\fP shall be established as the offset maximum
in the open file description.
.SH RETURN VALUE
.LP
Upon successful completion, the function shall open the file and return
a non-negative integer representing the lowest numbered
unused file descriptor. Otherwise, -1 shall be returned and \fIerrno\fP
set to indicate the error. No files shall be created or
modified if the function returns -1.
.SH ERRORS
.LP
The \fIopen\fP() function shall fail if:
.TP 7
.B EACCES
Search permission is denied on a component of the path prefix, or
the file exists and the permissions specified by \fIoflag\fP
are denied, or the file does not exist and write permission is denied
for the parent directory of the file to be created, or
O_TRUNC is specified and write permission is denied.
.TP 7
.B EEXIST
O_CREAT and O_EXCL are set, and the named file exists.
.TP 7
.B EINTR
A signal was caught during \fIopen\fP().
.TP 7
.B EINVAL
The implementation does not support synchronized I/O for this file.
.TP 7
.B EIO
The \fIpath\fP argument names a STREAMS file and a hangup or error
occurred during the \fIopen\fP(). 
.TP 7
.B EISDIR
The named file is a directory and \fIoflag\fP includes O_WRONLY or
O_RDWR.
.TP 7
.B ELOOP
A loop exists in symbolic links encountered during resolution of the
\fIpath\fP argument.
.TP 7
.B EMFILE
{OPEN_MAX} file descriptors are currently open in the calling process.
.TP 7
.B ENAMETOOLONG
The length of the \fIpath\fP argument exceeds {PATH_MAX} or a pathname
component is longer than {NAME_MAX}.
.TP 7
.B ENFILE
The maximum allowable number of files is currently open in the system.
.TP 7
.B ENOENT
O_CREAT is not set and the named file does not exist; or O_CREAT is
set and either the path prefix does not exist or the
\fIpath\fP argument points to an empty string.
.TP 7
.B ENOSR
The \fIpath\fP argument names a STREAMS-based file and the system
is unable to allocate a STREAM. 
.TP 7
.B ENOSPC
The directory or file system that would contain the new file cannot
be expanded, the file does not exist, and O_CREAT is
specified.
.TP 7
.B ENOTDIR
A component of the path prefix is not a directory.
.TP 7
.B ENXIO
O_NONBLOCK is set, the named file is a FIFO, O_WRONLY is set, and
no process has the file open for reading.
.TP 7
.B ENXIO
The named file is a character special or block special file, and the
device associated with this special file does not
exist.
.TP 7
.B EOVERFLOW
The named file is a regular file and the size of the file cannot be
represented correctly in an object of type
\fBoff_t\fP.
.TP 7
.B EROFS
The named file resides on a read-only file system and either O_WRONLY,
O_RDWR, O_CREAT (if the file does not exist), or O_TRUNC
is set in the \fIoflag\fP argument.
.sp
.LP
The \fIopen\fP() function may fail if:
.TP 7
.B EAGAIN
The \fIpath\fP argument names the slave side of a pseudo-terminal
device that is locked. 
.TP 7
.B EINVAL
The value of the \fIoflag\fP argument is not valid.
.TP 7
.B ELOOP
More than {SYMLOOP_MAX} symbolic links were encountered during resolution
of the \fIpath\fP argument.
.TP 7
.B ENAMETOOLONG
As a result of encountering a symbolic link in resolution of the \fIpath\fP
argument, the length of the substituted pathname
string exceeded {PATH_MAX}.
.TP 7
.B ENOMEM
The \fIpath\fP argument names a STREAMS file and the system is unable
to allocate resources. 
.TP 7
.B ETXTBSY
The file is a pure procedure (shared text) file that is being executed
and \fIoflag\fP is O_WRONLY or O_RDWR.
.sp
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.SS Opening a File for Writing by the Owner
.LP
The following example opens the file \fB/tmp/file\fP, either by creating
it (if it does not already exist), or by truncating
its length to 0 (if it does exist). In the former case, if the call
creates a new file, the access permission bits in the file mode
of the file are set to permit reading and writing by the owner, and
to permit reading only by group members and others.
.LP
If the call to \fIopen\fP() is successful, the file is opened for
writing.
.sp
.RS
.nf

\fB#include <fcntl.h>
\&...
int fd;
mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH;
char *filename = "/tmp/file";
\&...
fd = open(filename, O_WRONLY | O_CREAT | O_TRUNC, mode);
\&...
\fP
.fi
.RE
.SS Opening a File Using an Existence Check
.LP
The following example uses the \fIopen\fP() function to try to create
the \fBLOCKFILE\fP file and open it for writing. Since
the \fIopen\fP() function specifies the O_EXCL flag, the call fails
if the file already exists. In that case, the program assumes
that someone else is updating the password file and exits.
.sp
.RS
.nf

\fB#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
.sp

#define LOCKFILE "/etc/ptmp"
\&...
int pfd; /* Integer for file descriptor returned by open() call. */
\&...
if ((pfd = open(LOCKFILE, O_WRONLY | O_CREAT | O_EXCL,
    S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)) == -1)
{
    fprintf(stderr, "Cannot open /etc/ptmp. Try again later.\\n");
    exit(1);
}
\&...
\fP
.fi
.RE
.SS Opening a File for Writing
.LP
The following example opens a file for writing, creating the file
if it does not already exist. If the file does exist, the
system truncates the file to zero bytes.
.sp
.RS
.nf

\fB#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
.sp

#define LOCKFILE "/etc/ptmp"
\&...
int pfd;
char filename[PATH_MAX+1];
\&...
if ((pfd = open(filename, O_WRONLY | O_CREAT | O_TRUNC,
    S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)) == -1)
{
    perror("Cannot open output file\\n"); exit(1);
}
\&...
\fP
.fi
.RE
.SH APPLICATION USAGE
.LP
None.
.SH RATIONALE
.LP
Except as specified in this volume of IEEE\ Std\ 1003.1-2001, the
flags allowed in \fIoflag\fP are not
mutually-exclusive and any number of them may be used simultaneously.
.LP
Some implementations permit opening FIFOs with O_RDWR. Since FIFOs
could be implemented in other ways, and since two file
descriptors can be used to the same effect, this possibility is left
as undefined.
.LP
See \fIgetgroups\fP() about the group of a newly created file.
.LP
The use of \fIopen\fP() to create a regular file is preferable to
the use of \fIcreat\fP(), because the latter is redundant and included
only for historical reasons.
.LP
The use of the O_TRUNC flag on FIFOs and directories (pipes cannot
be \fIopen\fP()-ed) must be permissible without unexpected
side effects (for example, \fIcreat\fP() on a FIFO must not remove
data). Since terminal
special files might have type-ahead data stored in the buffer, O_TRUNC
should not affect their content, particularly if a program
that normally opens a regular file should open the current controlling
terminal instead. Other file types, particularly
implementation-defined ones, are left implementation-defined.
.LP
IEEE\ Std\ 1003.1-2001 permits [EACCES] to be returned for conditions
other than those explicitly listed.
.LP
The O_NOCTTY flag was added to allow applications to avoid unintentionally
acquiring a controlling terminal as a side effect of
opening a terminal file. This volume of IEEE\ Std\ 1003.1-2001 does
not specify how a controlling terminal is acquired, but
it allows an implementation to provide this on \fIopen\fP() if the
O_NOCTTY flag is not set and other conditions specified in the
Base Definitions volume of IEEE\ Std\ 1003.1-2001, Chapter 11, General
Terminal Interface are met. The O_NOCTTY flag is an effective no-op
if the file being opened is not a terminal device.
.LP
In historical implementations the value of O_RDONLY is zero. Because
of that, it is not possible to detect the presence of
O_RDONLY and another option. Future implementations should encode
O_RDONLY and O_WRONLY as bit flags so that:
.sp
.RS
.nf

\fBO_RDONLY | O_WRONLY == O_RDWR
\fP
.fi
.RE
.LP
In general, the \fIopen\fP() function follows the symbolic link if
\fIpath\fP names a symbolic link. However, the
\fIopen\fP() function, when called with O_CREAT and O_EXCL, is required
to fail with [EEXIST] if \fIpath\fP names an existing
symbolic link, even if the symbolic link refers to a nonexistent file.
This behavior is required so that privileged applications
can create a new file in a known location without the possibility
that a symbolic link might cause the file to be created in a
different location.
.LP
For example, a privileged application that must create a file with
a predictable name in a user-writable directory, such as the
user's home directory, could be compromised if the user creates a
symbolic link with that name that refers to a nonexistent file in
a system directory. If the user can influence the contents of a file,
the user could compromise the system by creating a new system
configuration or spool file that would then be interpreted by the
system. The test for a symbolic link which refers to a
nonexisting file must be atomic with the creation of a new file.
.LP
The POSIX.1-1990 standard required that the group ID of a newly created
file be set to the group ID of its parent directory or
to the effective group ID of the creating process. FIPS 151-2 required
that implementations provide a way to have the group ID be
set to the group ID of the containing directory, but did not prohibit
implementations also supporting a way to set the group ID to
the effective group ID of the creating process. Conforming applications
should not assume which group ID will be used. If it
matters, an application can use \fIchown\fP() to set the group ID
after the file is created,
or determine under what conditions the implementation will set the
desired group ID.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIchmod\fP(), \fIclose\fP(), \fIcreat\fP(), \fIdup\fP(), \fIfcntl\fP(),
\fIlseek\fP(), \fIread\fP(), \fIumask\fP(), \fIunlockpt\fP(),
\fIwrite\fP(), the Base Definitions volume of IEEE\ Std\ 1003.1-2001,
\fI<fcntl.h>\fP, \fI<sys/stat.h>\fP, \fI<sys/types.h>\fP
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .