.\" Hey Emacs! This file is -*- nroff -*- source. .\" .\" This manpage is Copyright (C) 1992 Drew Eckhardt; .\" 1993 Michael Haardt, Ian Jackson. .\" .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" .\" Modified 1993-07-21 by Rik Faith .\" Modified 1994-08-21 by Michael Haardt .\" Modified 1996-04-13 by Andries Brouwer .\" Modified 1996-05-13 by Thomas Koenig .\" Modified 1996-12-20 by Michael Haardt .\" Modified 1999-02-19 by Andries Brouwer .\" Modified 1998-11-28 by Joseph S. Myers .\" Modified 1999-06-03 by Michael Haardt .\" Modified 2002-05-07 by Michael Kerrisk .\" Modified 2004-06-23 by Michael Kerrisk .\" .TH OPEN 2 2004-06-23 "Linux 2.6.7" "Linux Programmer's Manual" .SH NAME open, creat \- open and possibly create a file or device .SH SYNOPSIS .nf .B #include .B #include .B #include .sp .BI "int open(const char *" pathname ", int " flags ); .BI "int open(const char *" pathname ", int " flags ", mode_t " mode ); .BI "int creat(const char *" pathname ", mode_t " mode ); .fi .SH DESCRIPTION The .B open() system call is used to convert a pathname into a file descriptor (a small, non-negative integer for use in subsequent I/O as with .BR read ", " write ", etc.)." When the call is successful, the file descriptor returned will be the lowest file descriptor not currently open for the process. This call creates a new open file, not shared with any other process. (But shared open files may arise via the .BR fork (2) system call.) The new file descriptor is set to remain open across exec functions (see .BR fcntl (2)). The file offset is set to the beginning of the file. The parameter .I flags is one of .BR O_RDONLY ", " O_WRONLY " or " O_RDWR which request opening the file read-only, write-only or read/write, respectively, .RI bitwise- or 'd with zero or more of the following: .TP .B O_CREAT If the file does not exist it will be created. The owner (user ID) of the file is set to the effective user ID of the process. The group ownership (group ID) is set either to the effective group ID of the process or to the group ID of the parent directory (depending on filesystem type and mount options, and the mode of the parent directory, see, e.g., the mount options .I bsdgroups and .I sysvgroups of the ext2 filesystem, as described in .BR mount (8)). .TP .B O_EXCL When used with .BR O_CREAT , if the file already exists it is an error and the .B open will fail. In this context, a symbolic link exists, regardless of where its points to. .B O_EXCL is broken on NFS file systems, programs which rely on it for performing locking tasks will contain a race condition. The solution for performing atomic file locking using a lockfile is to create a unique file on the same fs (e.g., incorporating hostname and pid), use .BR link (2) to make a link to the lockfile. If \fBlink()\fP returns 0, the lock is successful. Otherwise, use .BR stat (2) on the unique file to check if its link count has increased to 2, in which case the lock is also successful. .TP .B O_NOCTTY If .I pathname refers to a terminal device \(em see .BR tty (4) \(em it will not become the process's controlling terminal even if the process does not have one. .TP .B O_TRUNC If the file already exists and is a regular file and the open mode allows writing (i.e., is O_RDWR or O_WRONLY) it will be truncated to length 0. If the file is a FIFO or terminal device file, the O_TRUNC flag is ignored. Otherwise the effect of O_TRUNC is unspecified. .TP .B O_APPEND The file is opened in append mode. Before each .BR write , the file pointer is positioned at the end of the file, as if with .BR lseek . .B O_APPEND may lead to corrupted files on NFS file systems if more than one process appends data to a file at once. This is because NFS does not support appending to a file, so the client kernel has to simulate it, which can't be done without a race condition. .TP .BR O_NONBLOCK " or " O_NDELAY When possible, the file is opened in non-blocking mode. Neither the .B open nor any subsequent operations on the file descriptor which is returned will cause the calling process to wait. For the handling of FIFOs (named pipes), see also .BR fifo (4). This mode need not have any effect on files other than FIFOs. .TP .B O_SYNC The file is opened for synchronous I/O. Any .BR write s on the resulting file descriptor will block the calling process until the data has been physically written to the underlying hardware. .I See RESTRICTIONS below, though. .TP .B O_NOFOLLOW If \fIpathname\fR is a symbolic link, then the open fails. This is a FreeBSD extension, which was added to Linux in version 2.1.126. Symbolic links in earlier components of the pathname will still be followed. The headers from glibc 2.0.100 and later include a definition of this flag; \fIkernels before 2.1.126 will ignore it if used\fR. .TP .B O_DIRECTORY If \fIpathname\fR is not a directory, cause the open to fail. This flag is Linux-specific, and was added in kernel version 2.1.126, to avoid denial-of-service problems if \fBopendir\fR(3) is called on a FIFO or tape device, but should not be used outside of the implementation of \fBopendir\fR. .TP .B O_DIRECT Try to minimize cache effects of the I/O to and from this file. In general this will degrade performance, but it is useful in special situations, such as when applications do their own caching. File I/O is done directly to/from user space buffers. The I/O is synchronous, i.e., at the completion of the .BR read (2) or .BR write (2) system call, data is guaranteed to have been transferred. Under Linux 2.4 transfer sizes, and the alignment of user buffer and file offset must all be multiples of the logical block size of the file system. Under Linux 2.6 alignment to 512-byte boundaries suffices. .\" Alignment should satisfy requirements for the underlying device .\" There may be coherency problems. .br A semantically similar interface for block devices is described in .BR raw (8). .TP .B O_ASYNC Generate a signal (SIGIO by default, but this can be changed via .BR fcntl (2)) when input or output becomes possible on this file descriptor. This feature is only available for terminals, pseudo-terminals, and sockets. See .BR fcntl (2) for further details. .TP .B O_LARGEFILE (LFS) Allow files whose sizes cannot be represented in an .B off_t (but can be represented in an .BR off64_t ) to be opened. .PP Some of these optional flags can be altered using .B fcntl after the file has been opened. The argument .I mode specifies the permissions to use in case a new file is created. It is modified by the process's .BR umask in the usual way: the permissions of the created file are .BR "(mode & ~umask)" . Note that this mode only applies to future accesses of the newly created file; the .B open call that creates a read-only file may well return a read/write file descriptor. .PP The following symbolic constants are provided for .IR mode : .TP .B S_IRWXU 00700 user (file owner) has read, write and execute permission .TP .B S_IRUSR (S_IREAD) 00400 user has read permission .TP .B S_IWUSR (S_IWRITE) 00200 user has write permission .TP .B S_IXUSR (S_IEXEC) 00100 user has execute permission .TP .B S_IRWXG 00070 group has read, write and execute permission .TP .B S_IRGRP 00040 group has read permission .TP .B S_IWGRP 00020 group has write permission .TP .B S_IXGRP 00010 group has execute permission .TP .B S_IRWXO 00007 others have read, write and execute permission .TP .B S_IROTH 00004 others have read permission .TP .B S_IWOTH 00002 others have write permisson .TP .B S_IXOTH 00001 others have execute permission .PP .I mode must be specified when .B O_CREAT is in the .IR flags , and is ignored otherwise. .B creat is equivalent to .B open with .I flags equal to .BR O_CREAT|O_WRONLY|O_TRUNC . .SH "RETURN VALUE" .BR open " and " creat return the new file descriptor, or \-1 if an error occurred (in which case, .I errno is set appropriately). Note that .B open can open device special files, but .B creat cannot create them - use .BR mknod (2) instead. .LP On NFS file systems with UID mapping enabled, \fBopen\fP may return a file descriptor but e.g. \fBread\fP(2) requests are denied with \fBEACCES\fP. This is because the client performs \fBopen\fP by checking the permissions, but UID mapping is performed by the server upon read and write requests. If the file is newly created, its atime, ctime, mtime fields are set to the current time, and so are the ctime and mtime fields of the parent directory. Otherwise, if the file is modified because of the O_TRUNC flag, its ctime and mtime fields are set to the current time. .SH ERRORS .TP .B EACCES The requested access to the file is not allowed, or search permission is denied for one of the directories in the path prefix of .IR pathname , or the file did not exist yet and write access to the parent directory is not allowed. (See also .BR path_resolution (2).) .TP .B EEXIST .I pathname already exists and .BR O_CREAT " and " O_EXCL were used. .TP .B EFAULT .IR pathname " points outside your accessible address space." .TP .B EISDIR .I pathname refers to a directory and the access requested involved writing (that is, .B O_WRONLY or .B O_RDWR is set). .TP .B ELOOP Too many symbolic links were encountered in resolving .IR pathname , or \fBO_NOFOLLOW\fR was specified but .I pathname was a symbolic link. .TP .B EMFILE The process already has the maximum number of files open. .TP .B ENAMETOOLONG .IR pathname " was too long." .TP .B ENFILE The system limit on the total number of open files has been reached. .TP .B ENODEV .I pathname refers to a device special file and no corresponding device exists. (This is a Linux kernel bug - in this situation ENXIO must be returned.) .TP .B ENOENT O_CREAT is not set and the named file does not exist. Or, a directory component in .I pathname does not exist or is a dangling symbolic link. .TP .B ENOMEM Insufficient kernel memory was available. .TP .B ENOSPC .I pathname was to be created but the device containing .I pathname has no room for the new file. .TP .B ENOTDIR A component used as a directory in .I pathname is not, in fact, a directory, or \fBO_DIRECTORY\fR was specified and .I pathname was not a directory. .TP .B ENXIO O_NONBLOCK | O_WRONLY is set, the named file is a FIFO and no process has the file open for reading. Or, the file is a device special file and no corresponding device exists. .TP .B EOVERFLOW .I pathname refers to a regular file, too large to be opened - see O_LARGEFILE above. .TP .B EROFS .I pathname refers to a file on a read-only filesystem and write access was requested. .TP .B ETXTBSY .I pathname refers to an executable image which is currently being executed and write access was requested. .SH NOTE Under Linux, the O_NONBLOCK flag indicates that one wants to open but does not necessarily have the intention to read or write. This is typically used to open devices in order to get a file descriptor for use with .BR ioctl (2). .SH "CONFORMING TO" SVr4, SVID, POSIX, X/OPEN, BSD 4.3. The .B O_NOFOLLOW and .B O_DIRECTORY flags are Linux-specific. One may have to define the .B _GNU_SOURCE macro to get their definitions. .LP The (undefined) effect of .B O_RDONLY | O_TRUNC various among implementations. On many systems the file is actually truncated. .\" Linux 2.0, 2.5: truncate .\" Solaris 5.7, 5.8: truncate .\" Irix 6.5: truncate .\" Tru64 5.1B: truncate .\" HP-UX 11.22: truncate .\" FreeBSD 4.7: truncate .LP The .B O_DIRECT flag was introduced in SGI IRIX, where it has alignment restrictions similar to those of Linux 2.4. IRIX has also a fcntl(2) call to query appropriate alignments, and sizes. FreeBSD 4.x introduced a flag of same name, but without alignment restrictions. Support was added under Linux in kernel version 2.4.10. Older Linux kernels simply ignore this flag. .SH BUGS "The thing that has always disturbed me about O_DIRECT is that the whole interface is just stupid, and was probably designed by a deranged monkey on some serious mind-controlling substances." -- Linus .SH RESTRICTIONS There are many infelicities in the protocol underlying NFS, affecting amongst others .BR O_SYNC " and " O_NDELAY . POSIX provides for three different variants of synchronised I/O, corresponding to the flags \fBO_SYNC\fR, \fBO_DSYNC\fR and \fBO_RSYNC\fR. Currently (2.1.130) these are all synonymous under Linux. .SH "SEE ALSO" .BR close (2), .BR fcntl (2), .BR link (2), .BR mknod (2), .BR mount (2), .BR path_resolution (2), .BR read (2), .BR socket (2), .BR stat (2), .BR umask (2), .BR unlink (2), .BR write (2), .BR fopen (3), .BR fifo (4)