NAME open, creat - open and possibly create a file or device SYNOPSIS #include #include #include 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. See BUGS below, though. O_NOCTTY If pathname refers to a terminal device -- see tty(4) -- it will not become the process's control- ling 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 appropri- ately). 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 sym- bolic link, ie a symbolic link whose expansion con- tains a reference to itself. ENOSPC pathname was to be created but the device contain- ing 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 lock- ing 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. 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).