include "sys.m";
sys := load Sys Sys->PATH;

create: fn(file: string, omode, perm: int): ref FD;
open:   fn(file: string, omode: int):       ref FD;

Open returns nil if the file does not exist, if the file name is unacceptable, if the user does not have permission to open it as requested (see sys-stat(2) for a description of permissions), or if any other error occurs.

Create creates a new file or prepares to rewrite an existing file, opens it according to omode (as described for open), and returns an associated file descriptor.

If the file is new, the owner is set to the user id of the creating process group, the group to that of the containing directory, and the permissions to perm ANDed with the permissions of the containing directory. The bits in perm are the same as those in the file mode returned by sys-stat(2).

If the file already exists, it is truncated to 0 length, but the permissions, owner, and group remain unchanged.

The created file will be a directory if the Sys->DMDIR bit is set in perm, and omode is Sys->OREAD. The file will be exclusive-use if the Sys->DMEXCL bit is set in perm and the underlying file server supports it; see open(5) for details. It will be append-only if the Sys->DMAPPEND bit is set, and the underlying file server supports it.

Create returns nil if the path up to the last element of file cannot be evaluated, if the file name is unacceptable, if the user does not have write permission in the final directory, if the file already exists and does not permit the access defined by omode, or if any other error occurs.

If the file is new and the directory in which it is created is a union directory (see sys-intro(2)) then the constituent directory where the file is created depends on the structure of the union: see sys-bind(2).

Since create may succeed even if the file exists, a special mechanism is necessary for applications that require an atomic create operation. If the Sys->OEXCL bit is set in the mode for a create, the call succeeds only if the file does not already exist; see open(5) for details.

There is no explicit ``close'' routine: when the last reference to the file descriptor is released, the system closes the associated file. For devices and network protocols where shutdown must be guaranteed, write a hangup message to the associated control file and use the return value of the write to verify closure.