OPEN(2) COMMAND REFERENCE OPEN(2) NAME open - open a file for reading or writing, or create a new file SYNOPSIS #include <sys/file.h> fd = open(path, flags, mode) int fd; char *path; int flags, mode; DESCRIPTION The command open opens the file named by path as specified by the flags argument and returns a descriptor for that file in fd. The flags argument may indicate the file is to be created if it does not already exist (by specifying the O_CREAT flag), 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)). Refer to the CAVEATS section for specifics on setting modes. The path name path is the address of a string of ASCII characters representing a path name, terminated by a null character; flags is constructed by or'ing the following values, defined in <sys/file.h>: O_RDONLY Open for reading only. O_WRONLY Open for writing only. O_RDWR Open for reading and writing. O_NDELAY Do not block on open. If the open call would result in the process being blocked for some reason (e.g., waiting for carrier on a dialup line), the open returns immediately. O_APPEND Append on each write. If set, the file pointer is set to the end-of-file prior to each write. O_CREAT Create file if it does not exist. When a file is created, the owner of the file is the effective user-ID of the creating process, and the group is the group of the file's parent directory. O_TRUNC Truncate size to 0. Printed 4/6/89 1
OPEN(2) COMMAND REFERENCE OPEN(2) If the file exists, it is truncated to zero length. O_EXCL Error if create and file exists. If O_EXCL and O_CREAT are set, open fails if the file exists. Upon successful completion a non-negative integer fd, termed a file descriptor, is returned. The file pointer used to mark the current position within the file is set to the beginning of the file. The new descriptor is set to remain open across execve system calls; see close(2). There is a limit on the number of file descriptors a process may have open simultaneously. This number is NOFILE, defined in <sys/max.h>. The getdtablesize(2) call returns the current value of NOFILE. DIAGNOSTICS The named file is opened unless one or more of the following are true: [ENAMETOOLONG] The argument path is too long. [ENOASCII] The path name contains a character with the high-order bit set. [ENOTDIR] A component of the path prefix is not a directory. [ENOENT] O_CREAT is not set and the named file does not exist. [EACCES] A component of the path prefix denies search permission. [EACCES] The required permissions (for reading and/or writing) are denied for the named flag. [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] NOFILE files are currently open (see getdtablesize(2)). Printed 4/6/89 2
OPEN(2) COMMAND REFERENCE OPEN(2) [ENXIO] The named file is a character special or block special file, and the device associated with this special file does not exist. [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. [ELOOP] Too many symbolic links were encountered in translating the path name. [EEXIST] O_EXCL is specified and the file exists. [ENOSPC] O_CREAT is specified, and the file system is out of inodes. [ENOSPC] 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, the file does not exist and O_CREAT is specified. [ENFILE] O_CREAT is specified, and the system inode table is full. [EIO] An I/O error occurred while reading from or writing to the file system. [ENXIO] The O_NDELAY flag is given, and the file is a communications device on which their is no carrier present. [EBUSY] An exclusively-opened port is already opened. [EOPNOTSUPP] An attempt is made to open a socket (not currently implemented). RETURN VALUE If no error occurred, open returns the file descriptor in fd. Otherwise, -1 is returned and errno is set to indicate the error. CAVEATS The command open will not allow a superuser to set a mode including the sticky-bit (save text image after execution); this prevents unreasonably large files from being created within another process. (For example, the command open(name,O_CREAT,01777) results in the mode being set to 0777 instead of 01777.) Printed 4/6/89 3
OPEN(2) COMMAND REFERENCE OPEN(2) SEE ALSO chmod(2), close(2), dup(2), getdtablesize(2), lseek(2), read(2), write(2), umask(2), and unlink(2). Printed 4/6/89 4
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