NAME

open − open a file for reading or writing, or create a new file

USAGE

#include <sys/file.h>
 
open(path, flags, mode) char *path; int flags, mode;

DESCRIPTION

Open opens the file named by path for reading and/or writing, as specified by the flags argument and returns a descriptor for that file.  The flags argument may indicate that the file is to be created if it does not already exist (the O_CREAT flag).  In this case, the file is created with mode mode, as described in chmod(2) and as modified by the process’s umask value (see umask(2)). 

Path is the address of a null-terminated string of ASCII characters representing a pathname.  The flags are formed from the logical OR of the following values: 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 O_APPEND append on each write O_CREAT create file if it does not exist O_TRUNC truncate size to zero O_EXCL error if create and file exists Opening a file with O_APPEND set causes each write on the file to be appended to the end.  If O_TRUNC is specified and the file exists, the file is truncated to zero length.  If O_EXCL is set with O_CREAT and the file already exists, the open returns an error.  This can be used to implement a simple exclusive access locking mechanism.  If the O_NDELAY flag is specified and the open call would result in the process being blocked for some reason (e.g., waiting for carrier on a dial-up line), the open returns immediately.  The first time the process attempts to perform I/O on the open file, it will block. 

NOTES

The DOMAIN System’s single level store architecture requires that all filesystem objects be readable in order to be writable or executable.  Since write-only or execute-only files would be unusable in DOMAIN/IX, modes that specify 02 (write-only) or 01 (execute-only) are ORed with 0400 to force read permission.  This applies to the owner, group, and world portions of the mode word.  For example, if mode 0631 were specified, the mode applied to the file would actually be 0675. 

No process may have more than {OPEN_MAX} file descriptors open simultaneously. 

RETURN VALUE

Upon successful completion, a non-negative integer 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).  A failed call returns -1 and sets errno as indicated below. 

ERRORS

The named file is opened unless one or more of the following are true:

[EPERM] The pathname 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 that 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] {OPEN_MAX} (usually 20) file descriptors are currently open. 

[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] The call encountered too many symbolic links in translating the pathname. 

[EEXIST] O_EXCL was specified and the file exists. 

[ENXIO] The O_NDELAY flag is given, and the file is a communications device on which no carrier is present. 

RELATED INFORMATION

chmod(2), close(2), dup(2), lseek(2), read(2), write(2), umask(2)