NAME
open, creat - open and possibly create a file or device SYNOPSIS
#include
#include
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int open(const char *pathname, int flags); int open(const char *pathname, int flags, modet mode); int creat(const char *pathname, modet mode); DESCRIPTION Given a pathname for a file, open() returns a file descriptor, a small, nonnegative integer for use in subsequent system calls (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 cur‐ rently open for the process. By default, the new file descriptor is set to remain open across an execve(2) (i.e., the FDCLOEXEC file descriptor flag described in fcntl(2) is initially disabled; the OCLOEXEC flag, described below, can be used to change this default). The file offset is set to the beginning of the file (see lseek(2)). A call to open() creates a new 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 fcntl(2) FSETFL opera‐ tion). A file descriptor is a reference to one of these entries; this reference is unaffected if pathname is subsequently removed or modified to refer to a different file. The new open file description is ini‐ tially not shared with any other process, but sharing may arise via fork(2). The argument flags must include one of the following access modes:
ORDONLY, OWRONLY, or ORDWR. 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 bitwise-or'd in flags. The file creation flags are OCLOEXEC, OCREAT, ODIRECTORY, OEXCL, ONOCTTY, ONOFOLLOW, OTRUNC, and OTTYINIT. The file status flags are all of the remaining flags listed below. The distinction between these two groups of flags is that the file status flags can be retrieved and (in some cases) modi‐ fied using fcntl(2). The full list of file creation flags and file status flags is as follows: OAPPEND The file is opened in append mode. Before each write(2), the file offset is positioned at the end of the file, as if with lseek(2). OAPPEND may lead to corrupted files on NFS file sys‐ tems 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. OASYNC
Enable signal-driven I/O: generate a signal (SIGIO by default, but this can be changed via 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 fcntl(2) for further details. OCLOEXEC (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 fcntl(2) FSETFD operations to set the FDCLOEXEC flag. Addi‐ tionally, use of this flag is essential in some multithreaded programs since using a separate fcntl(2) FSETFD operation to set the FDCLOEXEC flag does not suffice to avoid race condi‐ tions where one thread opens a file descriptor at the same time as another thread does a fork(2) plus execve(2). OCREAT 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 direc‐ tory (depending on file system type and mount options, and the mode of the parent directory, see the mount options bsdgroups and sysvgroups described in mount(8)). mode specifies the permissions to use in case a new file is cre‐ ated. This argument must be supplied when OCREAT is specified in flags; if OCREAT is not specified, then mode is ignored. The effective permissions are modified by the process's umask in the usual way: The permissions of the created file are (mode & ~umask). Note that this mode applies only to future accesses of the newly created file; the open() call that creates
a read-only file may well return a read/write file descriptor. The following symbolic constants are provided for mode: SIRWXU 00700 user (file owner) has read, write and execute permission SIRUSR 00400 user has read permission SIWUSR 00200 user has write permission SIXUSR 00100 user has execute permission SIRWXG 00070 group has read, write and execute permission SIRGRP 00040 group has read permission SIWGRP 00020 group has write permission SIXGRP 00010 group has execute permission SIRWXO 00007 others have read, write and execute permission SIROTH 00004 others have read permission SIWOTH 00002 others have write permission SIXOTH 00001 others have execute permission ODIRECT (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 ODIRECT flag on its own makes an effort to transfer data synchronously, but does not give the guarantees of the OSYNC flag that data and necessary metadata are transferred. To guar‐ antee synchronous I/O, OSYNC must be used in addition to ODIRECT. See NOTES below for further discussion. A semantically similar (but deprecated) interface for block devices is described in raw(8). ODIRECTORY If pathname 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 opendir(3) is called on a FIFO or tape device. OEXCL Ensure that this call creates the file: if this flag is speci‐ fied in conjunction with OCREAT, and pathname already exists, then open() will fail. When these two flags are specified, symbolic links are not fol‐ lowed: if pathname is a symbolic link, then open() fails regard‐ less of where the symbolic link points to. In general, the behavior of OEXCL is undefined if it is used without OCREAT. There is one exception: on Linux 2.6 and later, OEXCL can be used without OCREAT if pathname refers to a block device. If the block device is in use by the system (e.g., mounted), open() fails with the error EBUSY. On NFS, OEXCL is supported only when using NFSv3 or later on kernel 2.6 or later. In NFS environments where OEXCL 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 OEXCL, can create a unique file on the same file system (e.g., incorporating hostname and PID), and use link(2) to make a link to the lockfile. If link(2) returns 0, the lock is successful. Otherwise, use stat(2) on the unique file to check if its link count has increased to 2, in which case the lock is also successful. OLARGEFILE (LFS) Allow files whose sizes cannot be represented in an offt (but can be represented in an off64t) to be opened. The LARGEFILE64SOURCE macro must be defined (before including any header files) in order to obtain this definition. Setting the FILEOFFSETBITS feature test macro to 64 (rather than using OLARGEFILE) is the preferred method of accessing large files on
32-bit systems (see featuretestmacros(7)). ONOATIME (Since Linux 2.6.8) Do not update the file last access time (statime in the inode) when the file is 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 effec‐ tive on all file systems. One example is NFS, where the server maintains the access time. ONOCTTY If pathname refers to a terminal device—see tty(4)—it will not become the process's controlling terminal even if the process does not have one. ONOFOLLOW If pathname 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. See also ONOPATH below. ONONBLOCK or ONDELAY When possible, the file is opened in nonblocking mode. Neither the 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 fifo(7). For a discussion of the effect of ONONBLOCK in conjunction with mandatory file locks and with file leases, see fcntl(2). OPATH (since Linux 2.6.39) Obtain a file descriptor that can be used for two purposes: to
indicate a location in the file-system tree and to perform oper‐ ations that act purely at the file descriptor level. The file itself is not opened, and other file operations (e.g., read(2), write(2), fchmod(2), fchown(2), fgetxattr(2)) fail with the error EBADF. The following operations can be performed on the resulting file descriptor: * close(2); fchdir(2) (since Linux 3.5); fstat(2) (since Linux 3.6). * Duplicating the file descriptor (dup(2), fcntl(2) FDUPFD, etc.). * Getting and setting file descriptor flags (fcntl(2) FGETFD and FSETFD). * Retrieving open file status flags using the fcntl(2) FGETFL operation: the returned flags will include the bit OPATH. * Passing the file descriptor as the dirfd argument of ope‐ nat(2) and the other "*at()" system calls. * Passing the file descriptor to another process via a UNIX domain socket (see SCMRIGHTS in unix(7)). When OPATH is specified in flags, flag bits other than ODIREC‐ TORY and ONOFOLLOW are ignored. If the ONOFOLLOW flag is also specified, then the call returns a file descriptor referring to the symbolic link. This file descriptor can be used as the dirfd argument in calls to fchow‐ nat(2), fstatat(2), linkat(2), and readlinkat(2) with an empty pathname to have the calls operate on the symbolic link. OSYNC The file is opened for synchronous I/O. Any write(2)s on the resulting file descriptor will block the calling process until the data has been physically written to the underlying hardware. But see NOTES below. OTRUNC If the file already exists and is a regular file and the open mode allows writing (i.e., is ORDWR or OWRONLY) it will be truncated to length 0. If the file is a FIFO or terminal device file, the OTRUNC flag is ignored. Otherwise the effect of OTRUNC is unspecified. Some of these optional flags can be altered using fcntl(2) after the file has been opened. creat() is equivalent to open() with flags equal to OCREAT|OWRONLY|OTRUNC. RETURN VALUE
open() and creat() return the new file descriptor, or -1 if an error occurred (in which case, errno is set appropriately). ERRORS EACCES The requested access to the file is not allowed, or search per‐ mission is denied for one of the directories in the path prefix of pathname, or the file did not exist yet and write access to the parent directory is not allowed. (See also pathresolu‐ tion(7).) EDQUOT Where OCREAT is specified, the file does not exist, and the user's quota of disk blocks or inodes on the file system has been exhausted. EEXIST pathname already exists and OCREAT and OEXCL were used. EFAULT pathname points outside your accessible address space. EFBIG See EOVERFLOW. EINTR While blocked waiting to complete an open of a slow device (e.g., a FIFO; see fifo(7)), the call was interrupted by a sig‐ nal handler; see signal(7). EISDIR pathname refers to a directory and the access requested involved writing (that is, OWRONLY or ORDWR is set). ELOOP Too many symbolic links were encountered in resolving pathname, or ONOFOLLOW was specified but pathname was a symbolic link. EMFILE The process already has the maximum number of files open. ENAMETOOLONG pathname was too long. ENFILE The system limit on the total number of open files has been reached. ENODEV 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.) ENOENT OCREAT is not set and the named file does not exist. Or, a directory component in pathname does not exist or is a dangling symbolic link. ENOMEM Insufficient kernel memory was available. ENOSPC pathname was to be created but the device containing pathname has no room for the new file. ENOTDIR A component used as a directory in pathname is not, in fact, a directory, or ODIRECTORY was specified and pathname was not a directory. ENXIO ONONBLOCK | OWRONLY 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. EOVERFLOW 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 -DFILEOFFSETBITS=64 tried to
open a file whose size exceeds (2<<31)-1 bits; see also OLARGE‐
FILE above. This is the error specified by POSIX.1-2001; in kernels before 2.6.24, Linux gave the error EFBIG for this case. EPERM The ONOATIME flag was specified, but the effective user ID of the caller did not match the owner of the file and the caller was not privileged (CAPFOWNER).
EROFS pathname refers to a file on a read-only file system and write access was requested. ETXTBSY pathname refers to an executable image which is currently being executed and write access was requested. EWOULDBLOCK The ONONBLOCK flag was specified, and an incompatible lease was held on the file (see fcntl(2)). CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001. The ODIRECTORY, ONOATIME, ONOFOLLOW,
and OPATH flags are Linux-specific, and one may need to define GNUSOURCE (before including any header files) to obtain their defini‐ tions.
The OCLOEXEC flag is not specified in POSIX.1-2001, but is specified
in POSIX.1-2008. ODIRECT is not specified in POSIX; one has to define GNUSOURCE (before including any header files) to get its definition. NOTES Under Linux, the ONONBLOCK flag indicates that one wants to open but does not necessarily have the intention to read or write. This is typ‐ ically used to open devices in order to get a file descriptor for use with ioctl(2). Unlike the other values that can be specified in flags, the access mode values ORDONLY, OWRONLY, and ORDWR, do not specify individual bits. Rather, they define the low order two bits of flags, and are defined respectively as 0, 1, and 2. In other words, the combination ORDONLY | OWRONLY is a logical error, and certainly does not have the same meaning as ORDWR. Linux reserves the special, nonstandard access mode 3 (binary 11) in 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 to be used only for device-specific ioctl(2) operations. The (undefined) effect of ORDONLY | OTRUNC varies among implementa‐ tions. On many systems the file is actually truncated. There are many infelicities in the protocol underlying NFS, affecting amongst others OSYNC and ONDELAY. POSIX provides for three different variants of synchronized I/O, corre‐ sponding to the flags OSYNC, ODSYNC, and ORSYNC. Currently (2.6.31), Linux implements only OSYNC, but glibc maps ODSYNC and ORSYNC to the same numerical value as OSYNC. Most Linux file systems don't actually implement the POSIX OSYNC semantics, which require all metadata updates of a write to be on disk on returning to user space, but only the ODSYNC 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 open() can open device special files, but creat() cannot cre‐ ate them; use mknod(2) instead. On NFS file systems with UID mapping enabled, open() may return a file descriptor but, for example, read(2) requests are denied with EACCES. This is because the client performs 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 statime, stctime, stmtime fields (respectively, time of last access, time of last status change, and time of last modification; see stat(2)) are set to the current time, and so are the stctime and stmtime fields of the parent directory. Otherwise, if the file is modified because of the OTRUNC flag, its stctime and stmtime fields are set to the current time. ODIRECT The ODIRECT 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-indepen‐ dent interface for an application to discover these restrictions for a given file or file system. Some file systems provide their own inter‐ faces for doing so, for example the XFSIOCDIOINFO operation in xfsctl(3). 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 ioctl(2) BLKSSZGET operation or from the shell using the command: blockdev getss ODIRECT I/Os should never be run concurrently with the fork(2) system call, if the memory buffer is a private mapping (i.e., any mapping cre‐ ated with the mmap(2) MAPPRIVATE 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 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 ODIRECT I/Os was created using shmat(2) or mmap(2) with the MAPSHARED flag. Nor does this restriction apply when the memory buffer has been advised as MADVDONTFORK with madvise(2), ensuring that it will not be available to the child after fork(2). The ODIRECT 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 intro‐ duced a flag of the same name, but without alignment restrictions. ODIRECT 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 open() will fail with EINVAL if it is used. Applications should avoid mixing ODIRECT 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 mmap(2) of files with direct I/O to the same files. The behaviour of ODIRECT 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 ODIRECT 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 ODIRECT. Some servers will perform poorly under these circumstances, especially if the I/O size is small. Some servers may also be config‐ ured 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 ODIRECT I/O. In summary, ODIRECT is a potentially powerful tool that should be used with caution. It is recommended that applications treat use of ODIRECT as a performance option which is disabled by default. "The thing that has always disturbed me about ODIRECT is that the whole interface is just stupid, and was probably designed by
a deranged monkey on some serious mind-controlling sub‐ stances."—Linus BUGS
Currently, it is not possible to enable signal-driven I/O by specifying OASYNC when calling open(); use fcntl(2) to enable this flag. SEE ALSO chmod(2), chown(2), close(2), dup(2), fcntl(2), link(2), lseek(2), mknod(2), mmap(2), mount(2), openat(2), read(2), socket(2), stat(2), umask(2), unlink(2), write(2), fopen(3), fifo(7), pathresolution(7), symlink(7), blockdev(8) COLOPHON
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Linux 2013-07-21 OPEN(2)