#include <sys/stat.h>
#include <fcntl.h>
int open(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 that is the lowest file descriptor not currently open for that process. 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.
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 three 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 behavior of O_NONBLOCK is unspecified.
If O_CREAT is set and the file did not previously exist, upon successful completion, open() shall mark for update the st_atime, st_ctime, and st_mtime fields of the file and the st_ctime and st_mtime fields of the parent directory.
If O_TRUNC is set and the file did previously exist, upon successful completion, open() shall mark for update the st_ctime and st_mtime fields 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.
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.
Upon successful completion, the function shall open the file and return a non-negative integer representing the lowest numbered unused file descriptor. Otherwise, -1 shall be returned and errno set to indicate the error. No files shall be created or modified if the function returns -1.
The open() function shall fail if:
The open() function 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 *filename = "/tmp/file"; ... fd = open(filename, 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 filename[PATH_MAX+1];
...
if ((pfd = open(filename, O_WRONLY | O_CREAT | O_TRUNC,
S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)) == -1)
{
perror("Cannot open output file\n"); exit(1);
}
...
Except as specified in this volume of IEEE Std 1003.1-2001, the flags allowed in oflag are not mutually-exclusive and any number of them may be used simultaneously.
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.
IEEE Std 1003.1-2001 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 IEEE Std 1003.1-2001 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 IEEE Std 1003.1-2001, Chapter 11, General Terminal Interface are met. The O_NOCTTY flag is an effective no-op if the file being opened is not a terminal device.
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
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.
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.
chmod(), close(), creat(), dup(), fcntl(), lseek(), read(), umask(), unlockpt(), write(), the Base Definitions volume of IEEE Std 1003.1-2001, <fcntl.h>, <sys/stat.h>, <sys/types.h>