.\" $OpenBSD: open.2,v 1.26 2002/04/30 16:31:42 mpech Exp $ .\" $NetBSD: open.2,v 1.8 1995/02/27 12:35:14 cgd Exp $ .\" .\" Copyright (c) 1980, 1991, 1993 .\" The Regents of the University of California. All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. All advertising materials mentioning features or use of this software .\" must display the following acknowledgement: .\" This product includes software developed by the University of .\" California, Berkeley and its contributors. .\" 4. Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" @(#)open.2 8.2 (Berkeley) 11/16/93 .\" .Dd November 16, 1993 .Dt OPEN 2 .Os .Sh NAME .Nm open .Nd open or create a file for reading or writing .Sh SYNOPSIS .Fd #include .Ft int .Fn open "const char *path" "int flags" "mode_t mode" .Sh DESCRIPTION The file name specified by .Fa path is opened for reading and/or writing as specified by the argument .Fa flags and the file descriptor returned to the calling process. The .Fa flags argument may indicate the file is to be created if it does not exist (by specifying the .Dv O_CREAT flag), in which case the file is created with mode .Fa mode as described in .Xr chmod 2 and modified by the process' umask value (see .Xr umask 2 ) . .Pp The flags specified are formed by .Tn OR Ns 'ing the following values .Pp .Bl -tag -width O_NONBLOCK -offset indent -compact .It Dv O_RDONLY Open for reading only. .It Dv O_WRONLY Open for writing only. .It Dv O_RDWR Open for reading and writing. .It Dv O_NONBLOCK Do not block on open or for data to become available. .It Dv O_APPEND Append on each write. .It Dv O_CREAT Create file if it does not exist. .It Dv O_TRUNC Truncate size to 0. .It Dv O_EXCL Error if create and file exists. .It Dv O_SYNC Perform synchronous I/O operations. .It Dv O_SHLOCK Atomically obtain a shared lock. .It Dv O_EXLOCK Atomically obtain an exclusive lock. .It Dv O_NOFOLLOW If last path element is a symlink, don't follow it. .El .Pp Opening a file with .Dv O_APPEND set causes each write on the file to be appended to the end. If .Dv O_TRUNC and a writing mode are specified and the file exists, the file is truncated to zero length. If .Dv O_EXCL is set with .Dv O_CREAT and the file already exists, .Fn open returns an error. This may be used to implement a simple exclusive access locking mechanism. If either of .Dv O_EXCL or .Dv O_NOFOLLOW are set and the last component of the pathname is a symbolic link, .Fn open will fail even if the symbolic link points to a non-existent name. If the .Dv O_NONBLOCK flag is specified, do not wait for the device or file to be ready or available. If the .Fn open call would result in the process being blocked for some reason (e.g., waiting for carrier on a dialup line), .Fn open returns immediately. This flag also has the effect of making all subsequent I/O on the open file non-blocking. If the .Dv O_SYNC flag is set, all I/O operations on the file will be done synchronously. .Pp A fifo should either be opened with .Dv O_RDONLY or with .Dv O_WRONLY . The behavior for opening a fifo with .Dv O_RDWR is undefined. .Pp When opening a file, a lock with .Xr flock 2 semantics can be obtained by setting .Dv O_SHLOCK for a shared lock, or .Dv O_EXLOCK for an exclusive lock. If creating a file with .Dv O_CREAT , the request for the lock will never fail (provided that the underlying filesystem supports locking). .Pp If .Fn open is successful, the file pointer used to mark the current position within the file is set to the beginning of the file. .Pp When a new file is created it is given the group of the directory which contains it. .Pp The new descriptor is set to remain open across .Xr execve 2 system calls; see .Xr close 2 and .Xr fcntl 2 . .Pp The system imposes a limit on the number of file descriptors open simultaneously by one process. .Xr getdtablesize 3 returns the current system limit. .Sh RETURN VALUES If successful, .Fn open returns a non-negative integer, termed a file descriptor. Otherwise, a value of \-1 is returned and .Va errno is set to indicate the error. .Sh ERRORS The named file is opened unless: .Bl -tag -width Er .It Bq Er ENOTDIR A component of the path prefix is not a directory. .It Bq Er ENAMETOOLONG A component of a pathname exceeded .Dv {NAME_MAX} characters, or an entire path name exceeded .Dv {PATH_MAX} characters. .It Bq Er ENOENT .Dv O_CREAT is not set and the named file does not exist. .It Bq Er ENOENT A component of the path name that must exist does not exist. .It Bq Er EACCES Search permission is denied for a component of the path prefix. .It Bq Er EACCES The required permissions (for reading and/or writing) are denied for the given flags. .It Bq Er EACCES .Dv O_CREAT is specified, the file does not exist, and the directory in which it is to be created does not permit writing. .It Bq Er ELOOP Too many symbolic links were encountered in translating the pathname, or the .Dv O_NOFOLLOW flag was specified and the target is a symbolic link. .It Bq Er EISDIR The named file is a directory, and the arguments specify it is to be opened for writing. .It Bq Er EINVAL The flags specified for opening the file are not valid. .It Bq Er EROFS The named file resides on a read-only file system, and the file is to be modified. .It Bq Er EMFILE The process has already reached its limit for open file descriptors. .It Bq Er ENFILE The system file table is full. .It Bq Er ENXIO The named file is a character special or block special file, and the device associated with this special file does not exist. .It Bq Er EINTR The .Fn open operation was interrupted by a signal. .It Bq Er EOPNOTSUPP .Dv O_SHLOCK or .Dv O_EXLOCK is specified but the underlying filesystem does not support locking. .It Bq Er EWOULDBLOCK .Dv O_NONBLOCK and one of .Dv O_SHLOCK or .Dv O_EXLOCK is specified and the file is locked. .It Bq Er ENOSPC .Dv O_CREAT is specified, the file does not exist, and the directory in which the entry for the new file is being placed cannot be extended because there is no space left on the file system containing the directory. .It Bq Er ENOSPC .Dv O_CREAT is specified, the file does not exist, and there are no free inodes on the file system on which the file is being created. .It Bq Er EDQUOT .Dv O_CREAT is specified, the file does not exist, and the directory in which the entry for the new file is being placed cannot be extended because the user's quota of disk blocks on the file system containing the directory has been exhausted. .It Bq Er EDQUOT .Dv O_CREAT is specified, the file does not exist, and the user's quota of inodes on the file system on which the file is being created has been exhausted. .It Bq Er EIO An I/O error occurred while making the directory entry or allocating the inode for .Dv O_CREAT . .It Bq Er ETXTBSY The file is a pure procedure (shared text) file that is being executed and the .Fn open call requests write access. .It Bq Er EFAULT .Fa path points outside the process's allocated address space. .It Bq Er EEXIST .Dv O_CREAT and .Dv O_EXCL were specified and the file exists. .It Bq Er EOPNOTSUPP An attempt was made to open a socket (not currently implemented). .It Bq Er EAGAIN .Dv O_NONBLOCK and either .Dv O_EXLOCK or .Dv O_SHLOCK are set and the file is already locked. .El .Sh SEE ALSO .Xr chmod 2 , .Xr close 2 , .Xr dup 2 , .Xr flock 2 , .Xr lseek 2 , .Xr read 2 , .Xr umask 2 , .Xr write 2 , .Xr getdtablesize 3 .Sh STANDARDS The .Fn open function conforms to .St -ansiC , .St -p1003.1-90 and .St -xpg4.2 . .Pp .Dv POSIX specifies three different flavors for synchronous I/O: .Dv O_SYNC , .Dv O_DSYNC , and .Dv O_RSYNC . In .Ox , these are all equivalent. .Pp The .Dv O_SHLOCK , .Dv O_EXLOCK , and .Dv O_NOFOLLOW flags are non-standard extensions and should not be used if portability is of concern. .Sh HISTORY An .Fn open function call appeared in .At v2 . .Sh CAVEATS The .Dv O_TRUNC flag requires that one of .Dv O_RDWR or .Dv O_WRONLY also be specified, else .Dv EINVAL is returned.