.\" This manpage is Copyright (C) 1992 Drew Eckhardt; .\" and Copyright (C) 1993 Michael Haardt, Ian Jackson. .\" and Copyright (C) 2008 Greg Banks .\" and Copyright (C) 2006, 2008, 2013, 2014 Michael Kerrisk .\" .\" %%%LICENSE_START(VERBATIM) .\" 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 .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date.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. .\" %%%LICENSE_END .\" .\" 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 .\" .TH OPEN 2 2018-04-30 "Linux" "Linux Programmer's Manual" .SH NAME open, openat, creat \- open and possibly create a file .SH SYNOPSIS .nf .B #include .B #include .B #include .PP .BI "int open(const char *" pathname ", int " flags ); .BI "int open(const char *" pathname ", int " flags ", mode_t " mode ); .PP .BI "int creat(const char *" pathname ", mode_t " mode ); .PP .BI "int openat(int " dirfd ", const char *" pathname ", int " flags ); .BI "int openat(int " dirfd ", const char *" pathname ", int " flags \ ", mode_t " mode ); .fi .PP .in -4n Feature Test Macro Requirements for glibc (see .BR feature_test_macros (7)): .in .PP .BR openat (): .PD 0 .ad l .RS 4 .TP 4 Since glibc 2.10: _POSIX_C_SOURCE\ >=\ 200809L .TP Before glibc 2.10: _ATFILE_SOURCE .RE .ad .PD .SH DESCRIPTION The .BR open () system call opens the file specified by .IR pathname . If the specified file does not exist, it may optionally (if .B O_CREAT is specified in .IR flags ) be created by .BR open (). .PP The return value of .BR open () is a file descriptor, a small, nonnegative integer that is used in subsequent system calls .RB ( read "(2), " write "(2), " lseek "(2), " fcntl (2), etc.) to refer to the open file. 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. The open file description records the file offset and the file status flags (see below). A file descriptor is a reference to an open file description; this reference is unaffected if .I pathname is subsequently removed or modified to refer to a different file. For further details on open file descriptions, see NOTES. .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. .PP 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_CLOEXEC , .BR O_CREAT , .BR O_DIRECTORY , .BR O_EXCL , .BR O_NOCTTY , .BR O_NOFOLLOW , .BR O_TMPFILE , and .BR O_TRUNC . The .I 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 raised an Aardvark to see if this .\" can be clarified in SUSv4; 10 Oct 2008. .\" http://thread.gmane.org/gmane.comp.standards.posix.austin.general/64/focus=67 .\" TC1 (balloted in 2013), resolved this, so that those three constants .\" are also categorized" as file status flags. .\" The distinction between these two groups of flags is that the file creation flags affect the semantics of the open operation itself, while the file status flags affect the semantics of subsequent I/O operations. The file status flags can be retrieved and (in some cases) modified; see .BR fcntl (2) for details. .PP 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). The modification of the file offset and the write operation are performed as a single atomic step. .IP .B O_APPEND may lead to corrupted files on NFS filesystems 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 available only for terminals, pseudoterminals, sockets, and (since Linux 2.6) pipes and FIFOs. See .BR fcntl (2) for further details. See also BUGS, below. .TP .BR O_CLOEXEC " (since Linux 2.6.23)" .\" NOTE! several other man pages refer to this text Enable the close-on-exec flag for the new file descriptor. .\" FIXME . for later review when Issue 8 is one day released... .\" POSIX proposes to fix many APIs that provide hidden FDs .\" http://austingroupbugs.net/tag_view_page.php?tag_id=8 .\" http://austingroupbugs.net/view.php?id=368 Specifying this flag permits a program to avoid additional .BR fcntl (2) .B F_SETFD operations to set the .B FD_CLOEXEC flag. .IP Note that the use of this flag is essential in some multithreaded programs, because 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 and attempts to set its close-on-exec flag using .BR fcntl (2) at the same time as another thread does a .BR fork (2) plus .BR execve (2). Depending on the order of execution, the race may lead to the file descriptor returned by .BR open () being unintentionally leaked to the program executed by the child process created by .BR fork (2). (This kind of race is in principle possible for any system call that creates a file descriptor whose close-on-exec flag should be set, and various other Linux system calls provide an equivalent of the .BR O_CLOEXEC flag to deal with this problem.) .\" This flag fixes only one form of the race condition; .\" The race can also occur with, for example, file descriptors .\" returned by accept(), pipe(), etc. .TP .B O_CREAT If .I pathname does not exist, create it as a regular file. .IP The owner (user ID) of the new file is set to the effective user ID of the process. .IP The group ownership (group ID) of the new file is set either to the effective group ID of the process (System V semantics) or to the group ID of the parent directory (BSD semantics). On Linux, the behavior depends on whether the set-group-ID mode bit is set on the parent directory: if that bit is set, then BSD semantics apply; otherwise, System V semantics apply. For some filesystems, the behavior also depends on the .I bsdgroups and .I sysvgroups mount options 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 The .I mode argument specifies the file mode bits be applied when a new file is created. This argument must be supplied when .B O_CREAT or .B O_TMPFILE is specified in .IR flags ; if neither .B O_CREAT nor .B O_TMPFILE is specified, then .I mode is ignored. The effective mode is modified by the process's .I umask in the usual way: in the absence of a default ACL, the mode of the created file is .IR "(mode\ &\ ~umask)" . Note that this mode applies only 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 .IP According to POSIX, the effect when other bits are set in .I mode is unspecified. On Linux, the following bits are also honored in .IR mode : .RS .TP 9 .B S_ISUID 0004000 set-user-ID bit .TP .B S_ISGID 0002000 set-group-ID bit (see .BR inode (7)). .TP .B S_ISVTX 0001000 sticky bit (see .BR inode (7)). .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. .IP 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 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. .TP .B O_DSYNC Write operations on the file will complete according to the requirements of synchronized I/O .I data integrity completion. .IP By the time .BR write (2) (and similar) return, the output data has been transferred to the underlying hardware, along with any file metadata that would be required to retrieve that data (i.e., as though each .BR write (2) was followed by a call to .BR fdatasync (2)). .IR "See NOTES below" . .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 () fails with the error .BR EEXIST . .IP 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. .IP 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 . .IP On NFS, .B O_EXCL is supported only 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 filesystem (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 .RI ( st_atime in the inode) when the file is .BR read (2). .IP This flag can be employed only if one of the following conditions is true: .RS .IP * 3 The effective UID of the process .\" Strictly speaking: the filesystem UID matches the owner UID of the file. .IP * The calling process has the .BR CAP_FOWNER capability in its user namespace and the owner UID of the file has a mapping in the namespace. .RE .IP 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 filesystems. 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)\(emit 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, with the error .BR ELOOP . Symbolic links in earlier components of the pathname will still be followed. (Note that the .B ELOOP error that can occur in this case is indistinguishable from the case where an open fails because there are too many symbolic links found while resolving components in the prefix part of the pathname.) .IP This flag is a FreeBSD extension, which was added to Linux in version 2.1.126, and has subsequently been standardized in POSIX.1-2008. .IP See also .BR O_PATH below. .\" 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. .IP Note that this flag has no effect for regular files and block devices; that is, I/O operations will (briefly) block when device activity is required, regardless of whether .B O_NONBLOCK is set. Since .B O_NONBLOCK semantics might eventually be implemented, applications should not depend upon blocking behavior when specifying this flag for regular files and block devices. .IP 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 .BR O_PATH " (since Linux 2.6.39)" .\" commit 1abf0c718f15a56a0a435588d1b104c7a37dc9bd .\" commit 326be7b484843988afe57566b627fb7a70beac56 .\" commit 65cfc6722361570bfe255698d9cd4dccaf47570d .\" .\" http://thread.gmane.org/gmane.linux.man/2790/focus=3496 .\" Subject: Re: [PATCH] open(2): document O_PATH .\" Newsgroups: gmane.linux.man, gmane.linux.kernel .\" Obtain a file descriptor that can be used for two purposes: to indicate a location in the filesystem tree and to perform operations that act purely at the file descriptor level. The file itself is not opened, and other file operations (e.g., .BR read (2), .BR write (2), .BR fchmod (2), .BR fchown (2), .BR fgetxattr (2), .BR ioctl (2), .BR mmap (2)) fail with the error .BR EBADF . .IP The following operations .I can be performed on the resulting file descriptor: .RS .IP * 3 .BR close (2). .IP * .BR fchdir (2), if the file descriptor refers to a directory (since Linux 3.5). .\" commit 332a2e1244bd08b9e3ecd378028513396a004a24 .IP * .BR fstat (2) (since Linux 3.6). .IP * .\" fstat(): commit 55815f70147dcfa3ead5738fd56d3574e2e3c1c2 .BR fstatfs (2) (since Linux 3.12). .\" fstatfs(): commit 9d05746e7b16d8565dddbe3200faa1e669d23bbf .IP * Duplicating the file descriptor .RB ( dup (2), .BR fcntl (2) .BR F_DUPFD , etc.). .IP * Getting and setting file descriptor flags .RB ( fcntl (2) .BR F_GETFD and .BR F_SETFD ). .IP * Retrieving open file status flags using the .BR fcntl (2) .BR F_GETFL operation: the returned flags will include the bit .BR O_PATH . .IP * Passing the file descriptor as the .IR dirfd argument of .BR openat () and the other "*at()" system calls. This includes .BR linkat (2) with .BR AT_EMPTY_PATH (or via procfs using .BR AT_SYMLINK_FOLLOW ) even if the file is not a directory. .IP * Passing the file descriptor to another process via a UNIX domain socket (see .BR SCM_RIGHTS in .BR unix (7)). .RE .IP When .B O_PATH is specified in .IR flags , flag bits other than .BR O_CLOEXEC , .BR O_DIRECTORY , and .BR O_NOFOLLOW are ignored. .IP Opening a file or directory with the .B O_PATH flag requires no permissions on the object itself (but does require execute permission on the directories in the path prefix). Depending on the subsequent operation, a check for suitable file permissions may be performed (e.g., .BR fchdir (2) requires execute permission on the directory referred to by its file descriptor argument). By contrast, obtaining a reference to a filesystem object by opening it with the .B O_RDONLY flag requires that the caller have read permission on the object, even when the subsequent operation (e.g., .BR fchdir (2), .BR fstat (2)) does not require read permission on the object. .IP If .I pathname is a symbolic link and the .BR O_NOFOLLOW flag is also specified, then the call returns a file descriptor referring to the symbolic link. This file descriptor can be used as the .I dirfd argument in calls to .BR fchownat (2), .BR fstatat (2), .BR linkat (2), and .BR readlinkat (2) with an empty pathname to have the calls operate on the symbolic link. .IP If .I pathname refers to an automount point that has not yet been triggered, so no other filesystem is mounted on it, then the call returns a file descriptor referring to the automount directory without triggering a mount. .BR fstatfs (2) can then be used to determine if it is, in fact, an untriggered automount point .RB ( ".f_type == AUTOFS_SUPER_MAGIC" ). .IP One use of .B O_PATH for regular files is to provide the equivalent of POSIX.1's .B O_EXEC functionality. This permits us to open a file for which we have execute permission but not read permission, and then execute that file, with steps something like the following: .IP .in +4n .EX char buf[PATH_MAX]; fd = open("some_prog", O_PATH); snprintf(buf, PATH_MAX, "/proc/self/fd/%d", fd); execl(buf, "some_prog", (char *) NULL); .EE .in .IP An .B O_PATH file descriptor can also be passed as the argument of .BR fexecve (3). .TP .B O_SYNC Write operations on the file will complete according to the requirements of synchronized I/O .I file integrity completion (by contrast with the synchronized I/O .I data integrity completion provided by .BR O_DSYNC .) .IP By the time .BR write (2) (or similar) returns, the output data and associated file metadata have been transferred to the underlying hardware (i.e., as though each .BR write (2) was followed by a call to .BR fsync (2)). .IR "See NOTES below" . .TP .BR O_TMPFILE " (since Linux 3.11)" .\" commit 60545d0d4610b02e55f65d141c95b18ccf855b6e .\" commit f4e0c30c191f87851c4a53454abb55ee276f4a7e .\" commit bb458c644a59dbba3a1fe59b27106c5e68e1c4bd Create an unnamed temporary regular file. The .I pathname argument specifies a directory; an unnamed inode will be created in that directory's filesystem. Anything written to the resulting file will be lost when the last file descriptor is closed, unless the file is given a name. .IP .B O_TMPFILE must be specified with one of .B O_RDWR or .B O_WRONLY and, optionally, .BR O_EXCL . If .B O_EXCL is not specified, then .BR linkat (2) can be used to link the temporary file into the filesystem, making it permanent, using code like the following: .IP .in +4n .EX char path[PATH_MAX]; fd = open("/path/to/dir", O_TMPFILE | O_RDWR, S_IRUSR | S_IWUSR); /* File I/O on 'fd'... */ snprintf(path, PATH_MAX, "/proc/self/fd/%d", fd); linkat(AT_FDCWD, path, AT_FDCWD, "/path/for/file", AT_SYMLINK_FOLLOW); .EE .in .IP In this case, the .BR open () .I mode argument determines the file permission mode, as with .BR O_CREAT . .IP Specifying .B O_EXCL in conjunction with .B O_TMPFILE prevents a temporary file from being linked into the filesystem in the above manner. (Note that the meaning of .B O_EXCL in this case is different from the meaning of .B O_EXCL otherwise.) .IP There are two main use cases for .\" Inspired by http://lwn.net/Articles/559147/ .BR O_TMPFILE : .RS .IP * 3 Improved .BR tmpfile (3) functionality: race-free creation of temporary files that (1) are automatically deleted when closed; (2) can never be reached via any pathname; (3) are not subject to symlink attacks; and (4) do not require the caller to devise unique names. .IP * Creating a file that is initially invisible, which is then populated with data and adjusted to have appropriate filesystem attributes .RB ( fchown (2), .BR fchmod (2), .BR fsetxattr (2), etc.) before being atomically linked into the filesystem in a fully formed state (using .BR linkat (2) as described above). .RE .IP .B O_TMPFILE requires support by the underlying filesystem; only a subset of Linux filesystems provide that support. In the initial implementation, support was provided in the ext2, ext3, ext4, UDF, Minix, and shmem filesystems. .\" To check for support, grep for "tmpfile" in kernel sources Support for other filesystems has subsequently been added as follows: XFS (Linux 3.15); .\" commit 99b6436bc29e4f10e4388c27a3e4810191cc4788 .\" commit ab29743117f9f4c22ac44c13c1647fb24fb2bafe Btrfs (Linux 3.16); .\" commit ef3b9af50bfa6a1f02cd7b3f5124b712b1ba3e3c F2FS (Linux 3.16); .\" commit 50732df02eefb39ab414ef655979c2c9b64ad21c and ubifs (Linux 4.9) .TP .B O_TRUNC If the file already exists and is a regular file and the access 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. .SS creat() A call to .BR creat () is equivalent to calling .BR open () with .I flags equal to .BR O_CREAT|O_WRONLY|O_TRUNC . .SS openat() The .BR openat () system call operates in exactly the same way as .BR open (), except for the differences described here. .PP If the pathname given in .I pathname is relative, then it is interpreted relative to the directory referred to by the file descriptor .I dirfd (rather than relative to the current working directory of the calling process, as is done by .BR open () for a relative pathname). .PP If .I pathname is relative and .I dirfd is the special value .BR AT_FDCWD , then .I pathname is interpreted relative to the current working directory of the calling process (like .BR open ()). .PP If .I pathname is absolute, then .I dirfd is ignored. .SH RETURN VALUE .BR open (), .BR openat (), and .BR creat () return the new file descriptor, or \-1 if an error occurred (in which case, .I errno is set appropriately). .SH ERRORS .BR open (), .BR openat (), and .BR creat () can fail with the following 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 EDQUOT Where .B O_CREAT is specified, the file does not exist, and the user's quota of disk blocks or inodes on the filesystem has been exhausted. .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 EINVAL The filesystem does not support the .BR O_DIRECT flag. See .BR NOTES for more information. .TP .B EINVAL Invalid value in .\" In particular, __O_TMPFILE instead of O_TMPFILE .IR flags . .TP .B EINVAL .B O_TMPFILE was specified in .IR flags , but neither .B O_WRONLY nor .B O_RDWR was specified. .TP .B EINVAL .B O_CREAT was specified in .I flags and the final component ("basename") of the new file's .I pathname is invalid (e.g., it contains characters not permitted by the underlying filesystem). .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 EISDIR .I pathname refers to an existing directory, .B O_TMPFILE and one of .B O_WRONLY or .B O_RDWR were specified in .IR flags , but this kernel version does not provide the .B O_TMPFILE functionality. .TP .B ELOOP Too many symbolic links were encountered in resolving .IR pathname . .TP .B ELOOP .I pathname was a symbolic link, and .I flags specified .BR O_NOFOLLOW but not .BR O_PATH . .TP .B EMFILE The per-process limit on the number of open file descriptors has been reached (see the description of .BR RLIMIT_NOFILE in .BR getrlimit (2)). .TP .B ENAMETOOLONG .I pathname was too long. .TP .B ENFILE The system-wide 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 ENOENT .I pathname refers to a nonexistent directory, .B O_TMPFILE and one of .B O_WRONLY or .B O_RDWR were specified in .IR flags , but this kernel version does not provide the .B O_TMPFILE functionality. .TP .B ENOMEM The named file is a FIFO, but memory for the FIFO buffer can't be allocated because the per-user hard limit on memory allocation for pipes has been reached and the caller is not privileged; see .BR pipe (7). .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 FIFO open for reading. .TP .B ENXIO The file is a device special file and no corresponding device exists. .TP .BR EOPNOTSUPP The filesystem containing .I pathname does not support .BR O_TMPFILE . .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 (1<<31)-1 bytes; see also .B O_LARGEFILE above. This is the error specified by POSIX.1; 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 filesystem UID... (MTK) did not match the owner of the file and the caller was not privileged. .TP .B EPERM The operation was prevented by a file seal; see .BR fcntl (2). .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. .TP .B ETXTBSY .I pathname refers to a file that is currently in use as a swap file, and the .B O_TRUNC flag was specified. .TP .B ETXTBSY .I pathname refers to a file that is currently being read by the kernel (e.g. for module/firmware loading), 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)). .PP The following additional errors can occur for .BR openat (): .TP .B EBADF .I dirfd is not a valid file descriptor. .TP .B ENOTDIR .I pathname is a relative pathname and .I dirfd is a file descriptor referring to a file other than a directory. .SH VERSIONS .BR openat () was added to Linux in kernel 2.6.16; library support was added to glibc in version 2.4. .SH CONFORMING TO .BR open (), .BR creat () SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008. .PP .BR openat (): POSIX.1-2008. .PP The .BR O_DIRECT , .BR O_NOATIME , .BR O_PATH , and .BR O_TMPFILE flags are Linux-specific. One must define .B _GNU_SOURCE to obtain their definitions. .PP The .BR O_CLOEXEC , .BR O_DIRECTORY , and .BR O_NOFOLLOW flags are not specified in POSIX.1-2001, but are specified in POSIX.1-2008. Since glibc 2.12, one can obtain their definitions by defining either .B _POSIX_C_SOURCE with a value greater than or equal to 200809L or .BR _XOPEN_SOURCE with a value greater than or equal to 700. In glibc 2.11 and earlier, one obtains the definitions by defining .BR _GNU_SOURCE . .PP As noted in .BR feature_test_macros (7), feature test macros such as .BR _POSIX_C_SOURCE , .BR _XOPEN_SOURCE , and .B _GNU_SOURCE must be defined before including .I any header files. .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). .PP 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 Note that .BR open () can open device special files, but .BR creat () cannot create them; use .BR mknod (2) instead. .PP 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 .I st_ctime and .I st_mtime fields are set to the current time. .PP The files in the .I /proc/[pid]/fd directory show the open file descriptors of the process with the PID .IR pid . The files in the .I /proc/[pid]/fdinfo directory show even more information about these files descriptors. See .BR proc (5) for further details of both of these directories. .\" .\" .SS Open file descriptions The term open file description is the one used by POSIX to refer to the entries in the system-wide table of open files. In other contexts, this object is variously also called an "open file object", a "file handle", an "open file table entry", or\(emin kernel-developer parlance\(ema .IR "struct file" . .PP When a file descriptor is duplicated (using .BR dup (2) or similar), the duplicate refers to the same open file description as the original file descriptor, and the two file descriptors consequently share the file offset and file status flags. Such sharing can also occur between processes: a child process created via .BR fork (2) inherits duplicates of its parent's file descriptors, and those duplicates refer to the same open file descriptions. .PP Each .BR open () of a file creates a new open file description; thus, there may be multiple open file descriptions corresponding to a file inode. .PP On Linux, one can use the .BR kcmp (2) .B KCMP_FILE operation to test whether two file descriptors (in the same process or in two different processes) refer to the same open file description. .\" .\" .SS Synchronized I/O The POSIX.1-2008 "synchronized I/O" option specifies different variants of synchronized I/O, and specifies the .BR open () flags .BR O_SYNC , .BR O_DSYNC , and .BR O_RSYNC for controlling the behavior. Regardless of whether an implementation supports this option, it must at least support the use of .BR O_SYNC for regular files. .PP Linux implements .BR O_SYNC and .BR O_DSYNC , but not .BR O_RSYNC . (Somewhat incorrectly, glibc defines .BR O_RSYNC to have the same value as .BR O_SYNC .) .PP .BR O_SYNC provides synchronized I/O .I file integrity completion, meaning write operations will flush data and all associated metadata to the underlying hardware. .BR O_DSYNC provides synchronized I/O .I data integrity completion, meaning write operations will flush data to the underlying hardware, but will only flush metadata updates that are required to allow a subsequent read operation to complete successfully. Data integrity completion can reduce the number of disk operations that are required for applications that don't need the guarantees of file integrity completion. .PP To understand the difference between the two types of completion, consider two pieces of file metadata: the file last modification timestamp .RI ( st_mtime ) and the file length. All write operations will update the last file modification timestamp, but only writes that add data to the end of the file will change the file length. The last modification timestamp is not needed to ensure that a read completes successfully, but the file length is. Thus, .BR O_DSYNC would only guarantee to flush updates to the file length metadata (whereas .BR O_SYNC would also always flush the last modification timestamp metadata). .PP Before Linux 2.6.33, Linux implemented only the .BR O_SYNC flag for .BR open (). However, when that flag was specified, most filesystems actually provided the equivalent of synchronized I/O .I data integrity completion (i.e., .BR O_SYNC was actually implemented as the equivalent of .BR O_DSYNC ). .PP Since Linux 2.6.33, proper .BR O_SYNC support is provided. However, to ensure backward binary compatibility, .BR O_DSYNC was defined with the same value as the historical .BR O_SYNC , and .BR O_SYNC was defined as a new (two-bit) flag value that includes the .BR O_DSYNC flag value. This ensures that applications compiled against new headers get at least .BR O_DSYNC semantics on pre-2.6.33 kernels. .\" .SS C library/kernel differences Since version 2.26, the glibc wrapper function for .BR open () employs the .BR openat () system call, rather than the kernel's .BR open () system call. For certain architectures, this is also true in glibc versions before 2.26. .\" .SS NFS There are many infelicities in the protocol underlying NFS, affecting amongst others .BR O_SYNC " and " O_NDELAY . .PP On NFS filesystems 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. .\" .\" .SS FIFOs Opening the read or write end of a FIFO blocks until the other end is also opened (by another process or thread). See .BR fifo (7) for further details. .\" .\" .SS File access mode 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 . .PP 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 file descriptor that can't be used for reading or writing. This nonstandard access mode is used by some Linux drivers to return a file descriptor that is to be used only 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 .\" .\" .SS Rationale for openat() and other "directory file descriptor" APIs .BR openat () and the other system calls and library functions that take a directory file descriptor argument (i.e., .BR execveat (2), .BR faccessat (2), .BR fanotify_mark (2), .BR fchmodat (2), .BR fchownat (2), .BR fstatat (2), .BR futimesat (2), .BR linkat (2), .BR mkdirat (2), .BR mknodat (2), .BR name_to_handle_at (2), .BR readlinkat (2), .BR renameat (2), .BR statx (2), .BR symlinkat (2), .BR unlinkat (2), .BR utimensat (2), .BR mkfifoat (3), and .BR scandirat (3)) address two problems with the older interfaces that preceded them. Here, the explanation is in terms of the .BR openat () call, but the rationale is analogous for the other interfaces. .PP First, .BR openat () allows an application to avoid race conditions that could occur when using .BR open () to open files in directories other than the current working directory. These race conditions result from the fact that some component of the directory prefix given to .BR open () could be changed in parallel with the call to .BR open (). Suppose, for example, that we wish to create the file .I dir1/dir2/xxx.dep if the file .I dir1/dir2/xxx exists. The problem is that between the existence check and the file-creation step, .I dir1 or .I dir2 (which might be symbolic links) could be modified to point to a different location. Such races can be avoided by opening a file descriptor for the target directory, and then specifying that file descriptor as the .I dirfd argument of (say) .BR fstatat (2) and .BR openat (). The use of the .I dirfd file descriptor also has other benefits: .IP * 3 the file descriptor is a stable reference to the directory, even if the directory is renamed; and .IP * the open file descriptor prevents the underlying filesystem from being dismounted, just as when a process has a current working directory on a filesystem. .PP Second, .BR openat () allows the implementation of a per-thread "current working directory", via file descriptor(s) maintained by the application. (This functionality can also be obtained by tricks based on the use of .IR /proc/self/fd/ dirfd, but less efficiently.) .\" .\" .SS O_DIRECT .PP 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 filesystem and kernel version and might be absent entirely. However there is currently no filesystem\-independent interface for an application to discover these restrictions for a given file or filesystem. Some filesystems provide their own interfaces for doing so, for example the .B XFS_IOC_DIOINFO operation in .BR xfsctl (3). .PP 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 filesystem. Since Linux 2.6.0, alignment to the logical block size of the underlying storage (typically 512 bytes) suffices. The logical block size can be determined using the .BR ioctl (2) .B BLKSSZGET operation or from the shell using the command: .PP .EX blockdev \-\-getss .EE .PP .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). .PP 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. .PP .B O_DIRECT support was added under Linux in kernel version 2.4.10. Older Linux kernels simply ignore this flag. Some filesystems may not implement the flag, in which case .BR open () fails with the error .B EINVAL if it is used. .PP 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 filesystem 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. .PP The behavior of .B O_DIRECT with NFS will differ from local filesystems. 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 bypass the page cache only 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 .PP One must check for two different error codes, .B EISDIR and .BR ENOENT , when trying to determine whether the kernel supports .B O_TMPFILE functionality. .PP When both .B O_CREAT and .B O_DIRECTORY are specified in .IR flags and the file specified by .I pathname does not exist, .BR open () will create a regular file (i.e., .B O_DIRECTORY is ignored). .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 open_by_handle_at (2), .BR read (2), .BR socket (2), .BR stat (2), .BR umask (2), .BR unlink (2), .BR write (2), .BR fopen (3), .BR acl (5), .BR fifo (7), .BR inode (7), .BR path_resolution (7), .BR symlink (7) .SH COLOPHON This page is part of release 4.16 of the Linux .I man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at \%https://www.kernel.org/doc/man\-pages/.