OPEN(2)

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

SYNOPSIS
#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 bitwise-or'd 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. 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.

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_APPEND may lead to corrupted files on NFS file systems if more than one process appends data to a file at once. This is because NFS does not support appending to a file, so the client kernel has to simulate it, which can't be done without a race condition.

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 RESTRICTIONS 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).

The following symbolic constants are provided for mode:

S_IRWXU
    00700 user (file owner) has read, write and execute permission

S_IRUSR (S_IREAD)
    00400 user has read permission

S_IWUSR (S_IWRITE)
    00200 user has write permission

S_IXUSR (S_IEXEC)
    00100 user has execute permission

S_IRWXG
    00070 group has read, write and execute permission

S_IRGRP
    00040 group has read permission

S_IWGRP
    00020 group has write permission

S_IXGRP
    00010 group has execute permission

S_IRWXO
    00007 others have read, write and execute permission

S_IROTH
    00004 others have read permission

S_IWOTH
    00002 others have write permisson

S_IXOTH
    00001 others have execute permission

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). Note that open can open device special files, but creat cannot create them - use mknod(2) instead.

On NFS file systems with UID mapping enabled, open may return a file descriptor but e.g. read(2) requests are denied with EACCES. This is because the client performs open by checking the permissions, but UID mapping is performed by the server upon read and write requests.

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
    Too many symbolic links were encountered in resolving pathname.

ENOSPC
    pathname was to be created but the device containing pathname has no room for the new file.

CONFORMING TO
SVr4, SVID, POSIX, X/OPEN, BSD 4.3

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

SEE ALSO
read(2), write(2), fcntl(2), close(2), unlink(2), mknod(2), stat(2), umask(2), mount(2), socket(2), socket(2), fopen(3), link(2).