NAME

open, create − prepare a fid for I/O on an existing or new file

SYNOPSIS

Topen tag[2] fid[2] mode[1]
Ropen tag[2] fid[2] qid[8]

Tcreate tag[2] fid[2] name[28] perm[4] mode[1]
Rcreate tag[2] fid[2] qid[8]

DESCRIPTION

The ­open request asks the file server to check permissions and prepare a fid for I/O with subsequent ­read and ­write messages.  The ­mode field determines the type of I/O: 0, 1, 2, and 3 (OREAD, OWRITE, ORDWR, OEXEC) mean read access, write access, read and write access, and execute access, to be checked against the permissions for the file.  In addition, if ­mode has the ­OTRUNC (16r10) bit set, the file is to be truncated, which requires write permission; if the ­mode has the ­ORCLOSE (16r40) bit set, the file is to be removed when the fid is clunked, which requires permission to remove the file from its directory.  If other bits are set in ­mode they will be ignored.  It is illegal to write a directory, truncate it, or attempt to remove it on close.  All these permissions are checked at the time of the ­open request; subsequent changes to the permissions of files do not affect the ability to read, write, or remove an open file. 

The ­create request asks the file server to create a new file with the ­name supplied, in the directory (dir) represented by fid, and requires write permission in the directory. The owner of the file is the implied user id of the request, the group of the file is the same as dir, and the permissions are the value of

­(perm&(~0777|0111)) | (dir.perm&perm&0666)

if a regular file is being created and

­(perm&~0777) | (dir.perm&perm&0777)

if a directory is being created.  This means, for example, that if the ­create allows read permission to others, but the containing directory does not, then the created file will not allow others to read the file. 

Finally, the newly created file is opened according to mode, and ­fid will represent the newly opened file.  ­Mode is not checked against the permissions in perm. The ­qid for the new file is returned with the ­create reply message. 

Directories are created by setting the ­CHDIR bit (16r80000000) in perm.

The names ­.  and ­..  are special; it is illegal to create files with these names. 

It is an error for either of these messages if the fid is already the product of a successful ­open or ­create message. 

An attempt to ­create a file in a directory where the given ­name already exists will be rejected; in this case, the ­create system call (see sys-open(2)) uses ­open with truncation.  The algorithm used by the ­create system call is: first walk to the directory to contain the file.  If that fails, return an error.  Next ­walk to the specified file.  If the ­walk succeeds, send a request to ­open and truncate the file and return the result, successful or not.  If the ­walk fails, send a ­create message.  If that fails, it may be because the file was created by another process after the previous ­walk failed, so (once) try the ­walk and ­open again.  For the behaviour of ­create on a union directory, see sys-bind(2).

ENTRY POINTS

The ­open and ­create calls both generate ­open messages; only ­create generates a ­create message. 

SEE ALSO

sys-bind(2), sys-open(2), stat(5)

Plan 9  —  June 29, 2000