open(S)

open, open64 -- open for reading or writing

Synopsis

   #include <sys/types.h>
   #include <sys/stat.h>
   #include <fcntl.h>
   
   int open(const char path, int oflag, . . . /* mode_t mode */);
   

   int open64(const char path, int oflag, . . . /* mode_t mode */);

Description

open opens a file descriptor for the file named path and sets the file status flags according to the value of oflag. oflag values are constructed by OR-ing flags from the following list (only one of the first three flags below may be used):

O_RDONLY

Open for reading only.

O_WRONLY

Open for writing only.

O_RDWR

Open for reading and writing.

O_LARGEFILE

If set, the offset maximum in the open file descriptor will be the largest value that can be represented correctly in an object of type off64_t. Using open with O_LARGEFILE set in oflag is equivalent to using open64.

O_NDELAY or O_NONBLOCK

These flags may affect subsequent reads and writes (see read(S) and write(S)). If both O_NDELAY and O_NONBLOCK are set, O_NONBLOCK will take precedence.

When opening a FIFO with O_RDONLY or O_WRONLY set:

If O_NDELAY or O_NONBLOCK is set: An open for reading-only will return without delay; an open for writing-only will return an error if no process currently has the file open for reading.
If O_NDELAY and O_NONBLOCK are clear: An open for reading-only will block until a process opens the file for writing; an open for writing-only will block until a process opens the file for reading.

When opening a file associated with a terminal line:

If O_NDELAY or O_NONBLOCK is set: The open will return without waiting for the device to be ready or available; subsequent behavior of the device is device specific.
If O_NDELAY and O_NONBLOCK are clear: The open will block until the device is ready or available.

O_APPEND

If set, the file pointer will be set to the end of the file prior to each write.

O_SYNC

When opening a regular file, this flag affects subsequent writes. If set, each write will wait for both the file data and file status to be physically updated.

O_NOCTTY

If set and the file is a terminal, the terminal will not be allocated as the calling process's controlling terminal.

O_CREAT

If the file exists, this flag has no effect, except as noted under O_EXCL below. Otherwise, the file is created and the owner ID of the file is set to the effective user IDs of the process, the group ID of the file is set to the effective group IDs of the process, or if the S_ISGID bit is set in the directory in which the file is being created, the file's group ID is set to the group ID of its parent directory. If the group ID of the new file does not match the effective group ID or one of the supplementary groups IDs, the S_ISGID bit is cleared. The access permission bits of the file mode are set to the value of mode, modified as follows (see creat(S)):

All bits set in the file mode creation mask of the process are cleared (see umask(S)).
The ``save text image after execution bit'' of the mode is cleared (see chmod(S)).
This value, together with entries derived from the default entries of the parent directory's access control list (ACL) are used to create an ACL for the new file (see acl(S)).

O_TRUNC

If the file exists, its length is truncated to 0 and the mode and owner are unchanged. O_TRUNC has no effect on special files or directories.

O_EXCL

If O_EXCL and O_CREAT are set, open will fail if the file exists. The check for the existence of the file and the creation of the file if it does not exist is atomic with respect to other processes executing open naming the same filename in the same directory with O_EXCL and O_CREAT set.

When opening a STREAMS file, oflag may be constructed from O_NDELAY or O_NONBLOCK OR-ed with either O_RDONLY, O_WRONLY, or O_RDWR. Other flag values are not applicable to STREAMS devices and have no effect on them. The values of O_NDELAY and O_NONBLOCK affect the operation of STREAMS drivers and certain system calls (see read(S), getmsg(S), putmsg(S), and write(S)). For drivers, the implementation of O_NDELAY and O_NONBLOCK is device specific. Each STREAMS device driver may treat these options differently.

When open is invoked to open a named stream, and the connld module (see connld(HW)) has been pushed on the pipe, open blocks until the server process has issued an I_RECVFD ioctl (see streamio(HW)) to receive the file descriptor.

If path is a symbolic link and O_CREAT and O_EXCL are set, the link is not followed.

The file pointer used to mark the current position within the file is set to the beginning of the file.

The new file descriptor is the lowest numbered file descriptor available and is set to remain open across exec system calls (see fcntl(S)).

Certain flag values can be set following open as described in fcntl.

Using open on a file adds a reference to the file. This guarantees that the file will continue to be visible to the process until it closes it, even if the file is removed from the directory by unlink.

Return values

On success, open returns the file descriptor of the open file and:

If O_CREAT is set and the file did not previously exist, open marks 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, open marks for update the st_ctime and st_mtime fields of the file.

On failure, open returns -1 and sets errno to identify the error.

In the following conditions, open fails and sets errno to:

EOVERFLOW: The named file is a regular file and either the size of the file cannot be represented correctly in an object of type off_t, or O_LARGEFILE is set and the size of the file cannot be represented correctly in an object of type off64_t.

In the following conditions, open64 fails and sets errno to:

EOVERFLOW: The named file is a regular file and O_LARGEFILE is set and the size of the file cannot be represented correctly in an object of type off64_t.

In the following conditions, open and open64 fail and set errno to:

EACCES: The file does not exist and write permission is denied by the parent directory of the file to be created.
EACCES: O_CREAT or O_TRUNC is specified and write permission is denied.
EACCES: A component of the path prefix denies search permission.
EACCES: oflag permission is denied for an existing file.
EAGAIN: The file exists, mandatory file/record locking is set, and there are outstanding record locks on the file (see chmod(S)).
EBUSY: path points to a device special file and the device is in the closing state.
EEXIST: O_CREAT and O_EXCL are set, and the named file exists.
EFAULT: path points outside the allocated address space of the process.
EINTR: A signal was caught during the open system call.
EINVAL: Returned when there are illegal flag values.
EIO: A hangup or error occurred during the open of the STREAMS-based device.
EISDIR: The named file is a directory and oflag is write or read/write.
ELOOP: Too many symbolic links were encountered in translating path.
EMFILE: The process has too many open files (see getrlimit(S)).
EMULTIHOP: Components of path require hopping to multiple remote machines and the file system does not allow it.
ENAMETOOLONG: The length of the path argument exceeds {PATH_MAX}, or the length of a path component exceeds {NAME_MAX} while {_POSIX_NO_TRUNC} is in effect.
ENFILE: The system file table is full.
ENODEV: path points to a device special file and the device is not in the activated state.
ENOENT: O_CREAT is not set and the named file does not exist.
ENOENT: O_CREAT is set and a component of the path prefix does not exist or is the null pathname.
ENOLINK: path points to a remote machine, and the link to that machine is no longer active.
ENOLOAD: Failure in loading a loadable exec module.
ENOMEM: The system is unable to allocate a send descriptor.
ENOSPC: O_CREAT and O_EXCL are set, and the file system is out of inodes.
ENOSPC: O_CREAT is set and the directory that would contain the file cannot be extended.
ENOSR: Unable to allocate a stream.
ENOTDIR: A component of the path prefix is not a directory.
ENXIO: The named file is a character special or block special file, and the device associated with this special file does not exist.
ENXIO: O_NDELAY or O_NONBLOCK is set, the named file is a FIFO, O_WRONLY is set, and no process has the file open for reading.
ENXIO: A STREAMS module or driver open routine failed.
EPERM: path points to a device special file, the device is in the setup state, and the calling process does not have the P_DEV privilege.
EROFS: The named file resides on a read-only file system and either O_WRONLY, O_RDWR, O_CREAT, or O_TRUNC is set in oflag (if the file does not exist).

Usage

The O_EXCL flag is only a modifier to the O_CREAT flag and has no other meaning. The concept of ``exclusive open'' is not supported by the operating system. Cooperating processes can coordinate their access to a file by file and record locking or by other mechanisms.

References

chmod(S), close(S), creat(S), dup(S), exec(S), fcntl(S), getmsg(S), getrlimit(S), intro(S), lseek(S), open(S), putmsg(S), read(S), stat(S), stat(M), umask(S), write(S)

Notices

Considerations for threads programming

Open file descriptors are a process resource and available to any sibling thread; if used concurrently, actions by one thread can interfere with those of a sibling.

While one thread is blocked, siblings might still be executing.

Access rights are an attribute of the containing process and are shared by sibling threads.

Considerations for large file support

open64 supports large files, but is otherwise identical to open. Large file support is filesystem-dependent. For details on programming for large file capable applications, see ``Large File Support'' on intro(S) and ``Large files'' in Understanding filesystem types in the discussion of the vxfs filesystem type in Understanding filesystem types.