.\" Hey Emacs! This file is -*- nroff -*- source. .\" .\" This manpage is Copyright (C) 1992 Drew Eckhardt; .\" 1993 Michael Haardt, Ian Jackson. .\" 2008 Greg Banks .\" .\" 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 .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date.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 .\" 2004-12-08, mtk, reordered flags list alphabetically .\" 2004-12-08, Martin Pool (& mtk), added O_NOATIME .\" 2007-09-18, mtk, Added description of O_CLOEXEC + other minor edits .\" 2008-01-03, mtk, with input from Trond Myklebust .\" and Timo Sirainen .\" Rewrite description of O_EXCL. .\" 2008-01-11, Greg Banks : add more detail .\" on O_DIRECT. .\" 2008-02-26, Michael Haardt: Reorganized text for O_CREAT and mode .\" .\" FIXME . Apr 08: The next POSIX revision has O_EXEC, O_SEARCH, and .\" O_TTYINIT. Eventually these may need to be documented. --mtk .\" FIXME Linux 2.6.33 has O_DSYNC, and a hidden __O_SYNC. .\" FIXME: Linux 2.6.39 added O_PATH .\" .TH OPEN 2 2012-05-01 "Linux" "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 Given a .I pathname for a file, .BR open () returns a file descriptor, a small, nonnegative integer for use in subsequent system calls .RB ( read "(2), " write "(2), " lseek "(2), " fcntl "(2), etc.)." The file descriptor returned by a successful call will be the lowest-numbered file descriptor not currently open for the process. .PP By default, the new file descriptor is set to remain open across an .BR execve (2) (i.e., the .B FD_CLOEXEC file descriptor flag described in .BR fcntl (2) is initially disabled; the .B O_CLOEXEC flag, described below, can be used to change this default). The file offset is set to the beginning of the file (see .BR lseek (2)). .PP A call to .BR open () creates a new .IR "open file description" , an entry in the system-wide table of open files. This entry records the file offset and the file status flags (modifiable via the .BR fcntl (2) .B F_SETFL operation). A file descriptor is a reference to one of these entries; this reference is unaffected if .I pathname is subsequently removed or modified to refer to a different file. The new open file description is initially not shared with any other process, but sharing may arise via .BR fork (2). .PP The argument .I flags must include one of the following .IR "access modes" : .BR O_RDONLY ", " O_WRONLY ", or " O_RDWR . These request opening the file read-only, write-only, or read/write, respectively. In addition, zero or more file creation flags and file status flags can be .RI bitwise- or 'd in .IR flags . The .I file creation flags are .BR O_CREAT ", " O_EXCL ", " O_NOCTTY ", and " O_TRUNC . The .I file status flags are all of the remaining flags listed below. .\" FIXME . Actually is it true that the "file status flags" are all of the .\" remaining flags listed below? SUSv4 divides the flags into: .\" * Access mode .\" * File creation .\" * File status .\" * Other (O_CLOEXEC, O_DIRECTORY, O_NOFOLLOW) .\" though it's not clear what the difference between "other" and .\" "File creation" flags is. (I've raised an Aardvark to see if this .\" can be clarified in SUSv4; 10 Oct 2008.) The distinction between these two groups of flags is that the file status flags can be retrieved and (in some cases) modified using .BR fcntl (2). The full list of file creation flags and file status flags is as follows: .TP .B O_APPEND The file is opened in append mode. Before each .BR write (2), the file offset is positioned at the end of the file, as if with .BR lseek (2). .B O_APPEND may lead to corrupted files on NFS file systems if more than one process appends data to a file at once. .\" For more background, see .\" http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=453946 .\" http://nfs.sourceforge.net/ 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 .B O_ASYNC Enable signal-driven I/O: generate a signal .RB ( 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, pseudoterminals, sockets, and (since Linux 2.6) pipes and FIFOs. See .BR fcntl (2) for further details. .TP .BR O_CLOEXEC " (Since Linux 2.6.23)" Enable the close-on-exec flag for the new file descriptor. Specifying this flag permits a program to avoid additional .BR fcntl (2) .B F_SETFD operations to set the .B FD_CLOEXEC flag. Additionally, use of this flag is essential in some multithreaded programs since using a separate .BR fcntl (2) .B F_SETFD operation to set the .B FD_CLOEXEC flag does not suffice to avoid race conditions where one thread opens a file descriptor at the same time as another thread does a .BR fork (2) plus .BR execve (2). .\" This flag fixes only one form of the race condition; .\" The race can also occur with, for example, descriptors .\" returned by accept(), pipe(), etc. .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 file system type and mount options, and the mode of the parent directory, see the mount options .I bsdgroups and .I sysvgroups described in .BR mount (8)). .\" As at 2.6.25, bsdgroups is supported by ext2, ext3, ext4, and .\" XFS (since 2.6.14). .RS .PP .I mode specifies the permissions to use in case a new file is created. This argument must be supplied when .B O_CREAT is specified in .IR flags ; if .B O_CREAT is not specified, then .I mode is ignored. The effective permissions are modified by the process's .I umask in the usual way: The permissions of the created file are .IR "(mode\ &\ ~umask)" . Note that this mode only applies to future accesses of the newly created file; the .BR 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 9 .B S_IRWXU 00700 user (file owner) has read, write and execute permission .TP .B S_IRUSR 00400 user has read permission .TP .B S_IWUSR 00200 user has write permission .TP .B S_IXUSR 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 permission .TP .B S_IXOTH 00001 others have execute permission .RE .TP .BR O_DIRECT " (Since Linux 2.4.10)" 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 .B O_DIRECT flag on its own makes an effort to transfer data synchronously, but does not give the guarantees of the .B O_SYNC flag that data and necessary metadata are transferred. To guarantee synchronous I/O, .B O_SYNC must be used in addition to .BR O_DIRECT . See NOTES below for further discussion. .sp A semantically similar (but deprecated) interface for block devices is described in .BR raw (8). .TP .B O_DIRECTORY If \fIpathname\fP is not a directory, cause the open to fail. .\" But see the following and its replies: .\" http://marc.theaimsgroup.com/?t=112748702800001&r=1&w=2 .\" [PATCH] open: O_DIRECTORY and O_CREAT together should fail .\" O_DIRECTORY | O_CREAT causes O_DIRECTORY to be ignored. This flag is Linux-specific, and was added in kernel version 2.1.126, to avoid denial-of-service problems if .BR opendir (3) is called on a FIFO or tape device, but should not be used outside of the implementation of .BR opendir (3). .TP .B O_EXCL Ensure that this call creates the file: if this flag is specified in conjunction with .BR O_CREAT , and .I pathname already exists, then .BR open () will fail. When these two flags are specified, symbolic links are not followed: .\" POSIX.1-2001 explicitly requires this behavior. if .I pathname is a symbolic link, then .BR open () fails regardless of where the symbolic link points to. In general, the behavior of .B O_EXCL is undefined if it is used without .BR O_CREAT . There is one exception: on Linux 2.6 and later, .B O_EXCL can be used without .B O_CREAT if .I pathname refers to a block device. If the block device is in use by the system (e.g., mounted), .BR open () fails with the error .BR EBUSY . On NFS, .B O_EXCL is only supported when using NFSv3 or later on kernel 2.6 or later. In NFS environments where .B O_EXCL support is not provided, programs that rely on it for performing locking tasks will contain a race condition. Portable programs that want to perform atomic file locking using a lockfile, and need to avoid reliance on NFS support for .BR O_EXCL , can create a unique file on the same file system (e.g., incorporating hostname and PID), and use .BR link (2) to make a link to the lockfile. If .BR link (2) 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_LARGEFILE (LFS) Allow files whose sizes cannot be represented in an .I off_t (but can be represented in an .IR off64_t ) to be opened. The .B _LARGEFILE64_SOURCE macro must be defined (before including .I any header files) in order to obtain this definition. Setting the .B _FILE_OFFSET_BITS feature test macro to 64 (rather than using .BR O_LARGEFILE ) is the preferred method of accessing large files on 32-bit systems (see .BR feature_test_macros (7)). .TP .BR O_NOATIME " (Since Linux 2.6.8)" Do not update the file last access time (st_atime in the inode) when the file is .BR read (2). This flag is intended for use by indexing or backup programs, where its use can significantly reduce the amount of disk activity. This flag may not be effective on all file systems. One example is NFS, where the server maintains the access time. .\" The O_NOATIME flag also affects the treatment of st_atime .\" by mmap() and readdir(2), MTK, Dec 04. .TP .B O_NOCTTY If .I pathname refers to a terminal device\(emsee .BR tty (4)\(em it will not become the process's controlling terminal even if the process does not have one. .TP .B O_NOFOLLOW If \fIpathname\fP 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\fP. .TP .BR O_NONBLOCK " or " O_NDELAY When possible, the file is opened in nonblocking mode. Neither the .BR 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 (7). For a discussion of the effect of .B O_NONBLOCK in conjunction with mandatory file locks and with file leases, see .BR fcntl (2). .TP .B O_SYNC The file is opened for synchronous I/O. Any .BR write (2)s on the resulting file descriptor will block the calling process until the data has been physically written to the underlying hardware. .IR "But see NOTES below" . .TP .B O_TRUNC If the file already exists and is a regular file and the open mode allows writing (i.e., is .B O_RDWR or .BR O_WRONLY ) it will be truncated to length 0. If the file is a FIFO or terminal device file, the .B O_TRUNC flag is ignored. Otherwise the effect of .B O_TRUNC is unspecified. .PP Some of these optional flags can be altered using .BR fcntl (2) after the file has been opened. .BR creat () is equivalent to .BR open () with .I flags equal to .BR O_CREAT|O_WRONLY|O_TRUNC . .SH "RETURN VALUE" .BR open () and .BR creat () return the new file descriptor, or \-1 if an error occurred (in which case, .I errno is set appropriately). .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 (7).) .TP .B EEXIST .I pathname already exists and .BR O_CREAT " and " O_EXCL were used. .TP .B EFAULT .I pathname points outside your accessible address space. .TP .B EFBIG See .BR EOVERFLOW . .TP .B EINTR While blocked waiting to complete an open of a slow device (e.g., a FIFO; see .BR fifo (7)), the call was interrupted by a signal handler; see .BR signal (7). .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\fP 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 .I 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 .B ENXIO must be returned.) .TP .B ENOENT .B 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\fP was specified and .I pathname was not a directory. .TP .B ENXIO .BR 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 that is too large to be opened. The usual scenario here is that an application compiled on a 32-bit platform without .I -D_FILE_OFFSET_BITS=64 tried to open a file whose size exceeds .I (2<<31)-1 bits; see also .B O_LARGEFILE above. This is the error specified by POSIX.1-2001; in kernels before 2.6.24, Linux gave the error .B EFBIG for this case. .\" See http://bugzilla.kernel.org/show_bug.cgi?id=7253 .\" "Open of a large file on 32-bit fails with EFBIG, should be EOVERFLOW" .\" Reported 2006-10-03 .TP .B EPERM The .B O_NOATIME flag was specified, but the effective user ID of the caller .\" Strictly speaking, it's the file system UID... (MTK) did not match the owner of the file and the caller was not privileged .RB ( CAP_FOWNER ). .TP .B EROFS .I pathname refers to a file on a read-only file system 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. .TP .B EWOULDBLOCK The .B O_NONBLOCK flag was specified, and an incompatible lease was held on the file (see .BR fcntl (2)). .SH "CONFORMING TO" SVr4, 4.3BSD, POSIX.1-2001. The .BR O_DIRECTORY , .BR O_NOATIME , and .B O_NOFOLLOW flags are Linux-specific, and one may need to define .B _GNU_SOURCE (before including .I any header files) to obtain their definitions. The .BR O_CLOEXEC flag is not specified in POSIX.1-2001, but is specified in POSIX.1-2008. .B O_DIRECT is not specified in POSIX; one has to define .B _GNU_SOURCE (before including .I any header files) to get its definition. .SH NOTES Under Linux, the .B 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). Unlike the other values that can be specified in .IR flags , the .I "access mode" values .BR O_RDONLY ", " O_WRONLY ", and " O_RDWR , do not specify individual bits. Rather, they define the low order two bits of .IR flags , and are defined respectively as 0, 1, and 2. In other words, the combination .B "O_RDONLY | O_WRONLY" is a logical error, and certainly does not have the same meaning as .BR O_RDWR . Linux reserves the special, nonstandard access mode 3 (binary 11) in .I flags to mean: check for read and write permission on the file and return a descriptor that can't be used for reading or writing. This nonstandard access mode is used by some Linux drivers to return a descriptor that is only to be used for device-specific .BR ioctl (2) operations. .\" See for example util-linux's disk-utils/setfdprm.c .\" For some background on access mode 3, see .\" http://thread.gmane.org/gmane.linux.kernel/653123 .\" "[RFC] correct flags to f_mode conversion in __dentry_open" .\" LKML, 12 Mar 2008 .LP The (undefined) effect of .B O_RDONLY | O_TRUNC varies 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 .PP 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 synchronized I/O, corresponding to the flags .BR O_SYNC , .BR O_DSYNC , and .BR O_RSYNC . Currently (2.6.31), Linux only implements .BR O_SYNC , but glibc maps .B O_DSYNC and .B O_RSYNC to the same numerical value as .BR O_SYNC . Most Linux file systems don't actually implement the POSIX .B O_SYNC semantics, which require all metadata updates of a write to be on disk on returning to user space, but only the .B O_DSYNC semantics, which require only actual file data and metadata necessary to retrieve it to be on disk by the time the system call returns. Note that .BR open () can open device special files, but .BR creat () cannot create them; use .BR mknod (2) instead. .LP On NFS file systems with UID mapping enabled, .BR open () may return a file descriptor but, for example, .BR read (2) requests are denied with \fBEACCES\fP. This is because the client performs .BR open () by checking the permissions, but UID mapping is performed by the server upon read and write requests. If the file is newly created, its .IR st_atime , .IR st_ctime , .I st_mtime fields (respectively, time of last access, time of last status change, and time of last modification; see .BR stat (2)) are set to the current time, and so are the .I st_ctime and .I st_mtime fields of the parent directory. Otherwise, if the file is modified because of the .B O_TRUNC flag, its st_ctime and st_mtime fields are set to the current time. .SS O_DIRECT .LP The .B O_DIRECT flag may impose alignment restrictions on the length and address of user-space buffers and the file offset of I/Os. In Linux alignment restrictions vary by file system and kernel version and might be absent entirely. However there is currently no file system\-independent interface for an application to discover these restrictions for a given file or file system. Some file systems provide their own interfaces for doing so, for example the .B XFS_IOC_DIOINFO operation in .BR xfsctl (3). .LP Under Linux 2.4, transfer sizes, and the alignment of the user buffer and the 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. .LP .B O_DIRECT I/Os should never be run concurrently with the .BR fork (2) system call, if the memory buffer is a private mapping (i.e., any mapping created with the .BR mmap (2) .BR MAP_PRIVATE flag; this includes memory allocated on the heap and statically allocated buffers). Any such I/Os, whether submitted via an asynchronous I/O interface or from another thread in the process, should be completed before .BR fork (2) is called. Failure to do so can result in data corruption and undefined behavior in parent and child processes. This restriction does not apply when the memory buffer for the .B O_DIRECT I/Os was created using .BR shmat (2) or .BR mmap (2) with the .B MAP_SHARED flag. Nor does this restriction apply when the memory buffer has been advised as .B MADV_DONTFORK with .BR madvise (2), ensuring that it will not be available to the child after .BR fork (2). .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 .BR fcntl (2) call to query appropriate alignments, and sizes. FreeBSD 4.x introduced a flag of the same name, but without alignment restrictions. .LP .B O_DIRECT support was added under Linux in kernel version 2.4.10. Older Linux kernels simply ignore this flag. Some file systems may not implement the flag and .BR open () will fail with .B EINVAL if it is used. .LP Applications should avoid mixing .B O_DIRECT and normal I/O to the same file, and especially to overlapping byte regions in the same file. Even when the file system correctly handles the coherency issues in this situation, overall I/O throughput is likely to be slower than using either mode alone. Likewise, applications should avoid mixing .BR mmap (2) of files with direct I/O to the same files. .LP The behaviour of .B O_DIRECT with NFS will differ from local file systems. Older kernels, or kernels configured in certain ways, may not support this combination. The NFS protocol does not support passing the flag to the server, so .B O_DIRECT I/O will only bypass the page cache on the client; the server may still cache the I/O. The client asks the server to make the I/O synchronous to preserve the synchronous semantics of .BR O_DIRECT . Some servers will perform poorly under these circumstances, especially if the I/O size is small. Some servers may also be configured to lie to clients about the I/O having reached stable storage; this will avoid the performance penalty at some risk to data integrity in the event of server power failure. The Linux NFS client places no alignment restrictions on .B O_DIRECT I/O. .PP In summary, .B O_DIRECT is a potentially powerful tool that should be used with caution. It is recommended that applications treat use of .B O_DIRECT as a performance option which is disabled by default. .PP .RS "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."\(emLinus .RE .SH BUGS Currently, it is not possible to enable signal-driven I/O by specifying .B O_ASYNC when calling .BR open (); use .BR fcntl (2) to enable this flag. .\" FIXME . Check bugzilla report on open(O_ASYNC) .\" See http://bugzilla.kernel.org/show_bug.cgi?id=5993 .SH "SEE ALSO" .BR chmod (2), .BR chown (2), .BR close (2), .BR dup (2), .BR fcntl (2), .BR link (2), .BR lseek (2), .BR mknod (2), .BR mmap (2), .BR mount (2), .BR openat (2), .BR read (2), .BR socket (2), .BR stat (2), .BR umask (2), .BR unlink (2), .BR write (2), .BR fopen (3), .BR fifo (7), .BR path_resolution (7), .BR symlink (7) .SH COLOPHON This page is part of release 3.44 of the Linux .I man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.