| OPEN(3POSIX) | POSIX Programmer's Manual | OPEN(3POSIX) |
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.
open, openat — open file
#include <sys/stat.h> #include <fcntl.h>
int open(const char *path, int oflag, ...); int openat(int fd, const char *path, int oflag, ...);
The open() function shall establish the connection between a file and a file descriptor. It shall create an open file description that refers to a file and a file descriptor that refers to that open file description. The file descriptor is used by other I/O functions to refer to that file. The path argument points to a pathname naming the file.
The open() function shall return a file descriptor for the named file, allocated as described in Section 2.14, File Descriptor Allocation. The open file description is new, and therefore the file descriptor shall not share it with any other process in the system. The FD_CLOEXEC file descriptor flag associated with the new file descriptor shall be cleared unless the O_CLOEXEC flag is set in oflag.
The file offset used to mark the current position within the file shall be set to the beginning of the file.
The file status flags and file access modes of the open file description shall be set according to the value of oflag.
Values for oflag are constructed by a bitwise-inclusive OR of flags from the following list, defined in <fcntl.h>. Applications shall specify exactly one of the first five values (file access modes) below in the value of oflag:
Any combination of the following may be used:
When opening a block special or character special file that supports non-blocking opens:
Otherwise, the O_NONBLOCK flag shall not cause an error, but it is unspecified whether the file status flags will include the O_NONBLOCK flag.
The O_SYNC flag shall be supported for regular files, even if the Synchronized Input and Output option is not supported.
If O_CREAT and O_DIRECTORY are set and the requested access mode is neither O_WRONLY nor O_RDWR, the result is unspecified.
If O_CREAT is set and the file did not previously exist, upon successful completion, open() shall mark for update the last data access, last data modification, and last file status change timestamps of the file and the last data modification and last file status change timestamps of the parent directory.
If O_TRUNC is set and the file did previously exist, upon successful completion, open() shall mark for update the last data modification and last file status change timestamps of the file.
If both the O_SYNC and O_DSYNC flags are set, the effect is as if only the O_SYNC flag was set.
If path refers to a STREAMS file, oflag may be constructed from O_NONBLOCK OR'ed with either O_RDONLY, O_WRONLY, or O_RDWR. Other flag values are not applicable to STREAMS devices and shall have no effect on them. The value O_NONBLOCK affects the operation of STREAMS drivers and certain functions applied to file descriptors associated with STREAMS files. For STREAMS drivers, the implementation of O_NONBLOCK is device-specific.
The application shall ensure that it specifies the O_TTY_INIT flag on the first open of a terminal device since system boot or since the device was closed by the process that last had it open. The application need not specify the O_TTY_INIT flag when opening pseudo-terminals. If path names the master side of a pseudo-terminal device, then it is unspecified whether open() locks the slave side so that it cannot be opened. Conforming applications shall call unlockpt() before opening the slave side.
The largest value that can be represented correctly in an object of type off_t shall be established as the offset maximum in the open file description.
The openat() function shall be equivalent to the open() function except in the case where path specifies a relative path. In this case the file to be opened is determined relative to the directory associated with the file descriptor fd instead of the current working directory. If the access mode of the open file description associated with the file descriptor is not O_SEARCH, the function shall check whether directory searches are permitted using the current permissions of the directory underlying the file descriptor. If the access mode is O_SEARCH, the function shall not perform the check.
The oflag parameter and the optional fourth parameter correspond exactly to the parameters of open().
If openat() is passed the special value AT_FDCWD in the fd parameter, the current working directory shall be used and the behavior shall be identical to a call to open().
Upon successful completion, these functions shall open the file and return a non-negative integer representing the file descriptor. Otherwise, these functions shall return -1 and set errno to indicate the error. If -1 is returned, no files shall be created or modified.
These functions shall fail if:
The openat() function shall fail if:
These functions may fail if:
The following sections are informative.
The following example opens the file /tmp/file, either by creating it (if it does not already exist), or by truncating its length to 0 (if it does exist). In the former case, if the call creates a new file, the access permission bits in the file mode of the file are set to permit reading and writing by the owner, and to permit reading only by group members and others.
If the call to open() is successful, the file is opened for writing.
#include <fcntl.h> ... int fd; mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH; char *pathname = "/tmp/file"; ... fd = open(pathname, O_WRONLY | O_CREAT | O_TRUNC, mode); ...
The following example uses the open() function to try to create the LOCKFILE file and open it for writing. Since the open() function specifies the O_EXCL flag, the call fails if the file already exists. In that case, the program assumes that someone else is updating the password file and exits.
#include <fcntl.h> #include <stdio.h> #include <stdlib.h>
#define LOCKFILE "/etc/ptmp"
...
int pfd; /* Integer for file descriptor returned by open() call. */
...
if ((pfd = open(LOCKFILE, O_WRONLY | O_CREAT | O_EXCL,
S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)) == -1)
{
fprintf(stderr, "Cannot open /etc/ptmp. Try again later.\n");
exit(1);
}
...
The following example opens a file for writing, creating the file if it does not already exist. If the file does exist, the system truncates the file to zero bytes.
#include <fcntl.h> #include <stdio.h> #include <stdlib.h>
#define LOCKFILE "/etc/ptmp"
...
int pfd;
char pathname[PATH_MAX+1];
...
if ((pfd = open(pathname, O_WRONLY | O_CREAT | O_TRUNC,
S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)) == -1)
{
perror("Cannot open output file\n"); exit(1);
}
...
POSIX.1‐2008 does not require that terminal parameters be automatically set to any state on first open, nor that they be reset after the last close. It is possible for a non-conforming application to leave a terminal device in a state where the next process to use that device finds it in a non-conforming state, but has no way of determining this. To ensure that the device is set to a conforming initial state, applications which perform a first open of a terminal (other than a pseudo-terminal) should do so using the O_TTY_INIT flag to set the parameters associated with the terminal to a conforming state.
Except as specified in this volume of POSIX.1‐2017, the flags allowed in oflag are not mutually-exclusive and any number of them may be used simultaneously. Not all combinations of flags make sense. For example, using O_SEARCH | O_CREAT will successfully open a pre-existing directory for searching, but if there is no existing file by that name, then it is unspecified whether a regular file will be created. Likewise, if a non-directory file descriptor is successfully returned, it is unspecified whether that descriptor will have execute permissions as if by O_EXEC (note that it is unspecified whether O_EXEC and O_SEARCH have the same value).
Some implementations permit opening FIFOs with O_RDWR. Since FIFOs could be implemented in other ways, and since two file descriptors can be used to the same effect, this possibility is left as undefined.
See getgroups() about the group of a newly created file.
The use of open() to create a regular file is preferable to the use of creat(), because the latter is redundant and included only for historical reasons.
The use of the O_TRUNC flag on FIFOs and directories (pipes cannot be open()-ed) must be permissible without unexpected side-effects (for example, creat() on a FIFO must not remove data). Since terminal special files might have type-ahead data stored in the buffer, O_TRUNC should not affect their content, particularly if a program that normally opens a regular file should open the current controlling terminal instead. Other file types, particularly implementation-defined ones, are left implementation-defined.
POSIX.1‐2008 permits [EACCES] to be returned for conditions other than those explicitly listed.
The O_NOCTTY flag was added to allow applications to avoid unintentionally acquiring a controlling terminal as a side-effect of opening a terminal file. This volume of POSIX.1‐2017 does not specify how a controlling terminal is acquired, but it allows an implementation to provide this on open() if the O_NOCTTY flag is not set and other conditions specified in the Base Definitions volume of POSIX.1‐2017, Chapter 11, General Terminal Interface are met.
In historical implementations the value of O_RDONLY is zero. Because of that, it is not possible to detect the presence of O_RDONLY and another option. Future implementations should encode O_RDONLY and O_WRONLY as bit flags so that:
O_RDONLY | O_WRONLY == O_RDWR
O_EXEC and O_SEARCH are specified as two of the five file access modes. Since O_EXEC does not apply to directories, and O_SEARCH only applies to directories, their values need not be distinct. Since O_RDONLY has historically had the value zero, implementations are not able to distinguish between O_SEARCH and O_SEARCH | O_RDONLY, and similarly for O_EXEC.
In general, the open() function follows the symbolic link if path names a symbolic link. However, the open() function, when called with O_CREAT and O_EXCL, is required to fail with [EEXIST] if path names an existing symbolic link, even if the symbolic link refers to a nonexistent file. This behavior is required so that privileged applications can create a new file in a known location without the possibility that a symbolic link might cause the file to be created in a different location.
For example, a privileged application that must create a file with a predictable name in a user-writable directory, such as the user's home directory, could be compromised if the user creates a symbolic link with that name that refers to a nonexistent file in a system directory. If the user can influence the contents of a file, the user could compromise the system by creating a new system configuration or spool file that would then be interpreted by the system. The test for a symbolic link which refers to a nonexisting file must be atomic with the creation of a new file.
In addition, the open() function refuses to open non-directories if the O_DIRECTORY flag is set. This avoids race conditions whereby a user might compromise the system by substituting a hard link to a sensitive file (e.g., a device or a FIFO) while a privileged application is running, where opening a file even for read access might have undesirable side-effects.
In addition, the open() function does not follow symbolic links if the O_NOFOLLOW flag is set. This avoids race conditions whereby a user might compromise the system by substituting a symbolic link to a sensitive file (e.g., a device) while a privileged application is running, where opening a file even for read access might have undesirable side-effects.
The POSIX.1‐1990 standard required that the group ID of a newly created file be set to the group ID of its parent directory or to the effective group ID of the creating process. FIPS 151‐2 required that implementations provide a way to have the group ID be set to the group ID of the containing directory, but did not prohibit implementations also supporting a way to set the group ID to the effective group ID of the creating process. Conforming applications should not assume which group ID will be used. If it matters, an application can use chown() to set the group ID after the file is created, or determine under what conditions the implementation will set the desired group ID.
The purpose of the openat() function is to enable opening files in directories other than the current working directory without exposure to race conditions. Any part of the path of a file could be changed in parallel to a call to open(), resulting in unspecified behavior. By opening a file descriptor for the target directory and using the openat() function it can be guaranteed that the opened file is located relative to the desired directory. Some implementations use the openat() function for other purposes as well. In some cases, if the oflag parameter has the O_XATTR bit set, the returned file descriptor provides access to extended attributes. This functionality is not standardized here.
None.
chmod(), close(), creat(), dirfd(), dup(), exec, fcntl(), fdopendir(), link(), lseek(), mkdtemp(), mknod(), read(), symlink(), umask(), unlockpt(), write()
The Base Definitions volume of POSIX.1‐2017, Chapter 11, General Terminal Interface, <fcntl.h>, <sys_stat.h>, <sys_types.h>
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .
| 2017 | IEEE/The Open Group |