Opening and Closing Files

This section describes the primitives for opening and closing files using file descriptors. The open and creat functions are declared in the header file `fcntl.h', while close is declared in `unistd.h'.

Function: int open (const char *filename, int flags[, mode_t mode])

The open function creates and returns a new file descriptor for the file named by filename. Initially, the file position indicator for the file is at the beginning of the file. The argument mode is used only when a file is created, but it doesn't hurt to supply the argument in any case.

The flags argument controls how the file is to be opened. This is a bit mask; you create the value by the bitwise OR of the appropriate parameters (using the `|' operator in C). See section File Status Flags, for the parameters available.

The normal return value from open is a non-negative integer file descriptor. In the case of an error, a value of -1 is returned instead. In addition to the usual file name errors (see section File Name Errors), the following errno error conditions are defined for this function:

EACCES

The file exists but is not readable/writable as requested by the flags argument, the file does not exist and the directory is unwritable so it cannot be created.

EEXIST

Both O_CREAT and O_EXCL are set, and the named file already exists.

EINTR

The open operation was interrupted by a signal. See section Primitives Interrupted by Signals.

EISDIR

The flags argument specified write access, and the file is a directory.

EMFILE

The process has too many files open. The maximum number of file descriptors is controlled by the RLIMIT_NOFILE resource limit; see section Limiting Resource Usage.

ENFILE

The entire system, or perhaps the file system which contains the directory, cannot support any additional open files at the moment. (This problem cannot happen on the GNU system.)

ENOENT

The named file does not exist, and O_CREAT is not specified.

ENOSPC

The directory or file system that would contain the new file cannot be extended, because there is no disk space left.

ENXIO

O_NONBLOCK and O_WRONLY are both set in the flags argument, the file named by filename is a FIFO (see section Pipes and FIFOs), and no process has the file open for reading.

EROFS

The file resides on a read-only file system and any of O_WRONLY, O_RDWR, and O_TRUNC are set in the flags argument, or O_CREAT is set and the file does not already exist.

The open function is the underlying primitive for the fopen and freopen functions, that create streams.

Obsolete function: int creat (const char *filename, mode_t mode)

This function is obsolete. The call:

creat (filename, mode)

is equivalent to:

open (filename, O_WRONLY | O_CREAT | O_TRUNC, mode)

Function: int close (int filedes)

The function close closes the file descriptor filedes. Closing a file has the following consequences:

• The file descriptor is deallocated.
• Any record locks owned by the process on the file are unlocked.
• When all file descriptors associated with a pipe or FIFO have been closed, any unread data is discarded.

The normal return value from close is 0; a value of -1 is returned in case of failure. The following errno error conditions are defined for this function:

EBADF

The filedes argument is not a valid file descriptor.

EINTR

The close call was interrupted by a signal. See section Primitives Interrupted by Signals. Here is an example of how to handle EINTR properly:

TEMP_FAILURE_RETRY (close (desc));

ENOSPC

EIO

EDQUOT

When the file is accessed by NFS, these errors from write can sometimes not be detected until close. See section Input and Output Primitives, for details on their meaning.

To close a stream, call fclose (see section Closing Streams) instead of trying to close its underlying file descriptor with close. This flushes any buffered output and updates the stream object to indicate that it is closed.

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