open(2)

NAME

open − open for reading or writing

SYNTAX

#include <sys/file.h>
#include <limits.h> /* definition of OPEN_MAX */

open(path, flags, mode)
char *path;
int flags, mode;

DESCRIPTION

The open system call opens a specified file and returns a descriptor for that file. The file pointer used to mark the current position within the file is set to the beginning of the file.

The file descriptor remains open across execve() system calls. The close() system call closes the file descriptor.

A process cannot have more than OPEN_MAX file descriptors open simultaneously.

ARGUMENTS

pathis the address of a string of ASCII characters representing a path name, terminated by a null character. The path name identifies the file to be opened.

modeis only used with the O_CREAT flag. The file is created with the specified mode, as described in chmod() and modified by the process’s umask value. For further information, see umask(.).

flagsdefines how the file is to be opened. This argument is formed by or’ing the following values:

O_RDONLYOpen for reading only.

O_WRONLYOpen for writing only.

O_RDWROpen for reading and writing.

O_NDELAYDo not block on open when opening a port (named pipe) with O_RDONLY or O_WRONLY:

If O_NDELAY is set, an open for read only returns without delay. An open for write only returns an error if no process currently has the file open for reading.

If O_NDELAY is clear, an open for read only blocks until a process opens the file for writing. An open for write only blocks until a process opens the file for reading.

O_NONBLOCKPOSIX definition of O_NDELAY. See O_NDELAY for explanation of functionality.

O_APPENDAppend on each write.

O_CREATCreate file if it does not exist.

O_TRUNCTruncate size to 0.

O_EXCLError if create and file exists.

O_BLKINUSEBlock if file is in use.

O_BLKANDSET
Block if file is in use then set in use.

O_FSYNCDo file writes synchronously.

Opening a file with O_APPEND set causes each write on the file to be appended to the end.

If O_TRUNC is specified and the file exists, the file is truncated to zero length.

If O_EXCL is set with O_CREAT and the file already exists, the open returns an error. This can be used to implement a simple exclusive access locking mechanism.

If the O_NDELAY or O_NONBLOCK flag is specified and the open call would result in the process being blocked for some reason, the open returns immediately. For example, if the process were waiting for carrier on a dialup line, an open with the O_NDELAY or O_NONBLOCK flag would return immediately. The first time the process attempts to perform I/O on the open file, it will block.

If the O_FSYNC flag is specified, each subsequent write (see write())) for the file is synchronous, instead of the default asynchronous writes. Use this flag to ensure that the write is complete when the system call returns. With asynchronous writes, the call returns when data is written to the buffer cache. There is no guarantee that the data was actually written out to the device. With synchronous writes, the call returns when the data is written from the buffer cache to the device.

O_BLKINUSE and O_BLKANDSET provide a test and set operation similar to a semaphore. O_BLKINUSE will cause the open to block if another process has marked the file as in use. The open blocks in the system at a point where no references to the file are established.

There are two ways to mark a file as in use:

•Use the ioctl() system call with the request argument set to FIOSINUSE or TIOCSINUSE. For further information, see tty(.).

•Use the O_BLKANDSET flag to open(.).

O_BLKANDSET caused the open to block if another process has marked the file in use. When the open resumes, the file is marked in use by the current process.

If O_NDELAY is used with either O_BLKINUSE or O_BLKANDSET, the open faild if the file is in use. The external variable errno is set to EWOULDBLOCK in this case.

NOTE

The in use flag cannot be inherited by a child process nor can it be replicated by dup(.).

When the in use flag is cleared, all processes that are blocked for that reason resumes. The open continues to block if another process marks the file as in use again.

The in use flag can be cleared in three ways:

•When the file descriptor marked as in use is closed

•When the process that set the in use flag exits

•When an ioctl() system call is issued with the request argument as FIOCINUSE or TIOCCINUSE.

ENVIRONMENT

SYSTEM_FIVE

When your program is compiled using the System V environment, and O_NDELAY is specified, subsequent reads and writes are also affected.

RETURN VALUES

Upon successful completion, an integer value greater than or equal to 0 is returned.

DIAGNOSTICS

The named file is opened unless one or more of the following are true:

[EACCES] The required permissions for reading, writing, or both are denied for the named flag.

[EACCES] Search permission is denied for a component of the path prefix.

[EACCES] O_CREAT is specified, the file does not exist, and the directory in which it is to be created does not permit writing.

[EDQUOT] O_CREAT is specified, the file does not exist, and the directory in which the entry for the new file is being placed cannot be extended because the user’s quota of disk blocks on the file system containing the directory has been exhausted.

[EDQUOT] O_CREAT is specified, the file does not exist, and the user’s quota of inodes on the file system on which the file is being created has been exhausted.

[EEXIST] O_CREAT and O_EXCL were specified and the file exists.

[EFAULT] The path points outside the process’s allocated address space.

[ENFILE] The system file table is full.

[EINVAL] An attempt was made to open a file with the O_RDONLY and O_FSYNC flags set.

[EIO] An I/O error occurred while making the directory entry or allocating the inode for O_CREAT.

[EISDIR] The named file is a directory, and the arguments specify it is to be opened for writing.

[ELOOP] Too many symbolic links were encountered in translating the pathname.

[EMFILE] {OPEN_MAX} file descriptors are currently open.

[ENAMETOOLONG]
A component of a pathname exceeds 255 characters or an entire pathname exceeds 1023 characters.

[ENOENT] O_CREAT is not set and the named file does not exist.

[ENOENT] A necessary component of the path name does not exist.

[ENOSPC] O_CREAT is specified, the file does not exist, and the directory in which the entry for the new file is being placed cannot be extended because there is no space left on the file system containing the directory.

[ENOSPC] O_CREAT is specified, the file does not exist, and there are no free inodes on the file system on which the file is being created.

[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] The O_NDELAY flag is given, and the file is a communications device on which there is no carrier present.

[EOPNOTSUPP]
An attempt was made to open a socket that is not set active.

[EROFS] The named file resides on a read-only file system, and the file is to be modified.

[ESTALE] The file handle given in the argument is invalid. The file referred to by that file handle no longer exists or has been revoked.

[ETIMEDOUT] A connect request or remote file operation failed because the connected party did not respond after a period of time determined by the communications protocol.

[ETXTBSY] The file is a pure procedure (shared text) file that is being executed and the open call requests write access.

[EWOULDBLOCK]
The open would have blocked if the O_NDELAY was not used. The probable cause for the block is that the file was marked in use.

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