October 23, 2006 OPEN 2

### NAME

open - open or create a file for reading or writing

### LIBRARY

Standard C Library (libc, -lc)

### SYNOPSIS

```int` open(```const char *path``, int flags``, mode_t mode`)

### DESCRIPTION

The file name specified by ```path` is opened for reading and/or writing as specified by the argument ```flags` and the file descriptor returned to the calling process. The ```flags` are specified by or'ing the values listed below. Applications must specify exactly one of the first three values (file access methods):

```O_RDONLY`
  Open for reading only.

```O_WRONLY`
  Open for writing only.

```O_RDWR`
  Open for reading and writing.

Any combination of the following may be used:

```O_NONBLOCK`
  Do not block on open or for data to become available.

```O_APPEND`
  Append to the file on each write.

```O_CREAT`
  Create the file if it does not exist, in which case the file is created with mode mode as described in chmod(2) and modified by the process' umask value (see umask(2)).

```O_TRUNC`
  Truncate size to 0.

```O_EXCL`
  Error if ```O_CREAT` and the file already exists.

```O_SHLOCK`
  Atomically obtain a shared lock.

```O_EXLOCK`
  Atomically obtain an exclusive lock.

```O_NOFOLLOW`
  If last path element is a symlink, don't follow it. This option is provided for compatibility with other operating systems, but its security value is questionable.

```O_DSYNC`
  If set, write operations will be performed according to synchronized I/O data integrity completion: each write will wait for the file data to be committed to stable storage.

```O_SYNC`
  If set, write operations will be performed according to synchronized I/O file integrity completion: each write will wait for both the file data and file status to be committed to stable storage.

```O_RSYNC`
  If set, read operations will complete at the same level of integrity which is in effect for write operations: if specified together with ```O_SYNC`, each read will wait for the file status to be committed to stable storage.

  Combining ```O_RSYNC` with ```O_DSYNC` only, or specifying it without any other synchronized I/O integrity completion flag set, has no further effect.

```O_ALT_IO`
  Alternate I/O semantics will be used for read and write operations on the file descriptor. Alternate semantics are defined by the underlying layers and will not have any alternate effect in most cases.

```O_NOCTTY`
  If the file is a terminal device, the opened device is not made the controlling terminal for the session. This flag has no effect on NetBSD, since the system defaults to the abovementioned behaviour. The flag is present only for standards conformance.

```O_DIRECT`
  If set on a regular file, data I/O operations will not buffer the data being transferred in the kernel's cache, but rather transfer the data directly between user memory and the underlying device driver if possible. This flag is advisory; the request may be performed in the normal buffered fashion if certain conditions are not met, e.g. if the request is not sufficiently aligned or if the file is mapped.

  To meet the alignment requirements for direct I/O, the file offset, the length of the I/O and the address of the buffer in memory must all be multiples of ```DEV_BSIZE` (512 bytes). If the I/O request is made using an interface that supports scatter/gather via struct iovec, each element of the request must meet the above alignment constraints.

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, open(``) returns an error. This may be used to implement a simple exclusive access locking mechanism. If ```O_EXCL` is set and the last component of the pathname is a symbolic link, open(``) will fail even if the symbolic link points to a non-existent name.

If the ```O_NONBLOCK` flag is specified, do not wait for the device or file to be ready or available. If the open(``) call would result in the process being blocked for some reason (e.g., waiting for carrier on a dialup line), open(``) returns immediately. This flag also has the effect of making all subsequent I/O on the open file non-blocking.

When opening a file, a lock with flock(2) semantics can be obtained by setting ```O_SHLOCK` for a shared lock, or ```O_EXLOCK` for an exclusive lock. If creating a file with ```O_CREAT`, the request for the lock will never fail (provided that the underlying filesystem supports locking).

If open(``) is successful, the file pointer used to mark the current position within the file is set to the beginning of the file.

When a new file is created it is given the group of the directory which contains it.

The new descriptor is set to remain open across execve(2) system calls; see close(2) and fcntl(2).

The system imposes a limit on the number of file descriptors open simultaneously by one process. Calling getdtablesize(3) returns the current system limit.

### RETURN VALUES

If successful, open(``) returns a non-negative integer, termed a file descriptor. Otherwise, a value of -1 is returned and errno is set to indicate the error.

### ERRORS

The named file is opened unless:

[```EPERM`]
  The file's flags (see chflags(2)) don't allow the file to be opened.

[```ENOTDIR`]
  A component of the path prefix is not a directory.

[```ENAMETOOLONG`]
  A component of a pathname exceeded ```NAME_MAX` characters, or an entire path name exceeded ```PATH_MAX` characters.

[```ENOENT`]
  ```O_CREAT` is not set and the named file does not exist, or a component of the path name that must exist does not exist.

[```EACCES`]
  Search permission is denied for a component of the path prefix, the required permissions (for reading and/or writing) are denied for the given flags, or ```O_CREAT` is specified, the file does not exist, and the directory in which it is to be created does not permit writing.

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

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

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

[```EMFILE`]
  The process has already reached its limit for open file descriptors.

[```ENFILE`]
  The system file table is full.

[```ENXIO`]
  The named file is a character special or block special file, and the device associated with this special file does not exist, or the named file is a FIFO, ```O_NONBLOCK` and ```O_WRONLY` is set and no process has the file open for reading.

[```EINTR`]
  The open(``) operation was interrupted by a signal.

[```EOPNOTSUPP`]
  ```O_SHLOCK` or ```O_EXLOCK` is specified but the underlying filesystem does not support locking.

[```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.

[```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.

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

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

[```EFAULT`]
  ```path` points outside the process's allocated address space.

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

[```EOPNOTSUPP`]
  An attempt was made to open a socket (not currently implemented).

### SEE ALSO

chmod(2), close(2), dup(2), lseek(2), read(2), umask(2), write(2), getdtablesize(3)

### STANDARDS

The open(``) function conforms to ISO/IEC 9945-1:1990 (``POSIX.1'') . The ```flags` values ```O_DSYNC`, ```O_SYNC` and ```O_RSYNC` are extensions defined in IEEE Std 1003.1b-1993 (``POSIX.1'') .

The ```O_SHLOCK`, ```O_EXLOCK`, and ```O_NOFOLLOW` flags are non-standard extensions and should not be used if portability is of concern.

### HISTORY

An open(``) function call appeared in Version 6 `AT&T UNIX`.