.\" Copyright (c) 1980, 1991, 1993 .\" The Regents of the University of California. All rights reserved. .\" .\" %sccs.include.redist.man% .\" .\" @(#)open.2 8.2 (Berkeley) 11/16/93 .\" .Dd .Dt OPEN 2 .Os BSD 4 .Sh NAME .Nm open .Nd open or create a file for reading or writing .Sh SYNOPSIS .Fd #include .Ft int .Fn open "const char *path" "int flags" "mode_t mode" .Sh DESCRIPTION The file name specified by .Fa path is opened for reading and/or writing as specified by the argument .Fa flags and the file descriptor returned to the calling process. The .Fa flags argument may indicate the file is to be created if it does not exist (by specifying the .Dv O_CREAT flag), in which case the file is created with mode .Fa mode as described in .Xr chmod 2 and modified by the process' umask value (see .Xr umask 2 ) . .Pp The flags specified are formed by .Em or Ns 'ing the following values .Pp .Bd -literal -offset indent -compact O_RDONLY open for reading only O_WRONLY open for writing only O_RDWR open for reading and writing O_NONBLOCK do not block on open O_APPEND append on each write O_CREAT create file if it does not exist O_TRUNC truncate size to 0 O_EXCL error if create and file exists O_SHLOCK atomically obtain a shared lock O_EXLOCK atomically obtain an exclusive lock .Ed .Pp Opening a file with .Dv O_APPEND set causes each write on the file to be appended to the end. If .Dv O_TRUNC is specified and the file exists, the file is truncated to zero length. If .Dv O_EXCL is set with .Dv O_CREAT and the file already exists, .Fn open returns an error. This may be used to implement a simple exclusive access locking mechanism. If .Dv O_EXCL is set and the last component of the pathname is a symbolic link, .Fn open will fail even if the symbolic link points to a non-existent name. If the .Dv O_NONBLOCK flag is specified and the .Fn open call would result in the process being blocked for some reason (e.g., waiting for carrier on a dialup line), .Fn open returns immediately. The first time the process attempts to perform I/O on the open file it will block (not currently implemented). .Pp When opening a file, a lock with .Xr flock 2 semantics can be obtained by setting .Dv O_SHLOCK for a shared lock, or .Dv O_EXLOCK for an exclusive lock. If creating a file with .Dv O_CREAT , the request for the lock will never fail (provided that the underlying filesystem supports locking). .Pp If successful, .Fn open returns a non-negative integer, termed a file descriptor. It returns -1 on failure. The file pointer used to mark the current position within the file is set to the beginning of the file. .Pp When a new file is created it is given the group of the directory which contains it. .Pp The new descriptor is set to remain open across .Xr execve system calls; see .Xr close 2 and .Xr fcntl 2 . .Pp The system imposes a limit on the number of file descriptors open simultaneously by one process. .Xr Getdtablesize 2 returns the current system limit. .Sh ERRORS The named file is opened unless: .Bl -tag -width Er .It Bq Er ENOTDIR A component of the path prefix is not a directory. .It Bq Er ENAMETOOLONG A component of a pathname exceeded 255 characters, or an entire path name exceeded 1023 characters. .It Bq Er ENOENT .Dv O_CREAT is not set and the named file does not exist. .It Bq Er ENOENT A component of the path name that must exist does not exist. .It Bq Er EACCES Search permission is denied for a component of the path prefix. .It Bq Er EACCES The required permissions (for reading and/or writing) are denied for the given flags. .It Bq Er EACCES .Dv O_CREAT is specified, the file does not exist, and the directory in which it is to be created does not permit writing. .It Bq Er ELOOP Too many symbolic links were encountered in translating the pathname. .It Bq Er EISDIR The named file is a directory, and the arguments specify it is to be opened for writing. .It Bq Er EROFS The named file resides on a read-only file system, and the file is to be modified. .It Bq Er EMFILE The process has already reached its limit for open file descriptors. .It Bq Er ENFILE The system file table is full. .It Bq Er ENXIO The named file is a character special or block special file, and the device associated with this special file does not exist. .It Bq Er EINTR The .Nm operation was interrupted by a signal. .It Bq Er EOPNOTSUPP .Dv O_SHLOCK or .Dv O_EXLOCK is specified but the underlying filesystem does not support locking. .It Bq Er ENOSPC .Dv O_CREAT is specified, the file does not exist, and 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. .It Bq Er ENOSPC .Dv O_CREAT is specified, the file does not exist, and there are no free inodes on the file system on which the file is being created. .It Bq Er EDQUOT .Dv O_CREAT is specified, the file does not exist, and the directory in which the entry for the new file is being placed cannot be extended because the user's quota of disk blocks on the file system containing the directory has been exhausted. .It Bq Er EDQUOT .Dv O_CREAT is specified, the file does not exist, and the user's quota of inodes on the file system on which the file is being created has been exhausted. .It Bq Er EIO An I/O error occurred while making the directory entry or allocating the inode for .Dv O_CREAT . .It Bq Er ETXTBSY The file is a pure procedure (shared text) file that is being executed and the .Fn open call requests write access. .It Bq Er EFAULT .Fa Path points outside the process's allocated address space. .It Bq Er EEXIST .Dv O_CREAT and .Dv O_EXCL were specified and the file exists. .It Bq Er EOPNOTSUPP An attempt was made to open a socket (not currently implemented). .El .Sh SEE ALSO .Xr chmod 2 , .Xr close 2 , .Xr dup 2 , .Xr getdtablesize 2 , .Xr lseek 2 , .Xr read 2 , .Xr write 2 , .Xr umask 2 .Sh HISTORY An .Nm function call appeared in Version 6 AT&T UNIX.