OPEN(2) manual page

NAME

open, creat - open and possibly create a file or device

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

DESCRIPTION

open attempts to open a file and return a file descriptor (a small, non-negative integer for use in read, write, etc.)

flags is one of O_RDONLY, O_WRONLY or O_RDWR which request opening the file read-only, write-only or read/write, respectively.

flags may also be with one or more of the following:

O_CREAT: If the file does not exist it will be created.
O_EXCL: When used with O_CREAT, if the file already exists it is an error and the open will fail. See BUGS below, though.
O_NOCTTY: If pathname refers to a terminal device see tty(4) it will not become the process's controlling terminal even if the process does not have one.
O_TRUNC: If the file already exists it will be truncated.
O_APPEND: The file is opened in append mode. Initially, and before each write, the file pointer is positioned at the end of the file, as if with lseek.
O_NONBLOCK or O_NDELAY: The file is opened in non-blocking mode. Neither the open nor any subsequent operations on the file descriptor which is returned will cause the calling process to wait.
O_SYNC: The file is opened for synchronous I/O. Any writes on the resulting file descriptor will block the calling process until the data has been physically written to the underlying hardware. See BUGS below, though.

Some of these optional flags can be altered using fcntl after the file has been opened.

mode specifies the permissions to use if a new file is created. It is modified by the process's umask in the usual way: the permissions of the created file are (mode & ~umask).

mode should always be specified when O_CREAT is in the flags, and is ignored otherwise.

creat is equivalent to open with flags equal to O_CREAT|O_WRONLY|O_TRUNC.

RETURN VALUE

open and creat return the new file descriptor, or -1 if an error occurred (in which case, errno is set appropriately).

ERRORS

EEXIST: pathname already exists and O_CREAT and O_EXCL were used.
EISDIR: pathname refers to a directory and the access requested involved writing.
ETXTBSY: pathname refers to an executable image which is currently being executed and write access was requested..
EFAULT: pathname points outside your accessible address space.
EACCES: The requested access to the file is not allowed, or one of the directories in pathname did not allow search (execute) permission.
ENAMETOOLONG: pathname was too long.
ENOENT: A directory component in pathname does not exist or is a dangling symbolic link.
ENOTDIR: A component used as a directory in pathname is not, in fact, a directory.
EMFILE: The process already has the maximum number of files open.
ENFILE: The limit on the total number of files open on the system has been reached.
ENOMEM: Insufficient kernel memory was available.
EROFS: pathname refers to a file on a read-only filesystem and write access was requested.
ELOOP: pathname contains a reference to a circular symbolic link, ie a symbolic link whose expansion contains a reference to itself.
ENOSPC: pathname was to be created but the device containing pathname has no room for the new file.

CONFORMING TO

SVID, AT&T, POSIX, X/OPEN, BSD 4.3

BUGS

O_SYNC is not currently implemented (as of Linux 0.99pl7).

There are many infelicities in the protocol underlying NFS, affecting amongst others O_SYNC, O_NDELAY, and O_APPEND.

O_EXCL is broken on NFS file systems, programs which rely on it for performing locking tasks will contain a race condition. The solution for performing atomic file locking using a lockfile is to create a unique file on the same fs (e.g., incorporating hostname and pid), use link(2) to make a link to the lockfile and use stat(2) on the unique file to check if its link count has increased to 2. Do not use the return value of the link() call.