ref: 390c71f0c79b2b8b94997cd9ae03037272075f8b
dir: /sys/man/5/open/
.TH OPEN 5 .SH NAME open, create \- prepare a fid for I/O on an existing or new file .SH SYNOPSIS .ta \w'\fLTcreate 'u .IR size [4] .B Topen .IR tag [2] .IR fid [4] .IR mode [1] .br .IR size [4] .B Ropen .IR tag [2] .IR qid [13] .IR iounit [4] .PP .IR size [4] .B Tcreate .IR tag [2] .IR fid [4] .IR name [ s ] .IR perm [4] .IR mode [1] .br .IR size [4] .B Rcreate .IR tag [2] .IR qid [13] .IR iounit [4] .SH DESCRIPTION The .B open request asks the file server to check permissions and prepare a fid for I/O with subsequent .B read and .B write messages. The .I mode field determines the type of I/O: 0 (called .BR OREAD in .BR <libc.h> ), 1 .RB ( OWRITE ), 2 .RB ( ORDWR ), and 3 .RB ( OEXEC ) mean .I read access, write access, read and write access, and .I execute access, to be checked against the permissions for the file. In addition, if .I mode has the .B OTRUNC .RB ( 0x10 ) bit set, the file is to be truncated, which requires write permission (if the file is append-only, and permission is granted, the .B open succeeds but the file will not be truncated); if the .I mode has the .B ORCLOSE .RB ( 0x40 ) bit set, the file is to be removed when the fid is clunked, which requires permission to remove the file from its directory. All other bits in .I mode should be zero. It is illegal to write a directory, truncate it, or attempt to remove it on close. If the file is marked for exclusive use (see .IR stat (5)), only one client can have the file open at any time. That is, after such a file has been opened, further opens will fail until .I fid has been clunked. All these permissions are checked at the time of the .B open request; subsequent changes to the permissions of files do not affect the ability to read, write, or remove an open file. .PP The .B create request asks the file server to create a new file with the .I name supplied, in the directory .RI ( dir ) represented by .IR 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 .IR dir , and the permissions are the value of .ce .B "perm & (~0666 | (dir.perm & 0666))" if a regular file is being created and .ce .B "perm & (~0777 | (dir.perm & 0777))" if a directory is being created. This means, for example, that if the .B create allows read permission to others, but the containing directory does not, then the created file will not allow others to read the file. .PP Finally, the newly created file is opened according to .IR mode , and .I fid will represent the newly opened file. .I Mode is not checked against the permissions in .IR perm . The .I qid for the new file is returned with the .B create reply message. .PP Directories are created by setting the .B DMDIR bit .RB ( 0x80000000 ) in the .IR perm . .PP The names .B . and .B .. are special; it is illegal to create files with these names. .PP It is an error for either of these messages if the fid is already the product of a successful .B open or .B create message. .PP An attempt to .B create a file in a directory where the given .I name already exists will be rejected; in this case, the .I create system call (see .IR open (2)) uses .B open with truncation. The algorithm used by the .IR create system call is: first walk to the directory to contain the file. If that fails, return an error. Next .B walk to the specified file. If the .B walk succeeds, send a request to .B open and truncate the file and return the result, successful or not. If the .B 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 .B walk and .B open again. .PP For the behavior of .I create on a union directory, see .IR bind (2). .PP The .B iounit field returned by .B open and .B create may be zero. If it is not, it is the maximum number of bytes that are guaranteed to be read from or written to the file without breaking the I/O transfer into multiple 9P messages; see .IR read (5). .SH ENTRY POINTS .I Open and .I create both generate .B open messages; only .I create generates a .B create message. The .B iounit associated with an open file may be discovered by calling .IR iounit (2). .PP For programs that need atomic file creation, without the race that exists in the .B open-create sequence described above, the kernel does the following. If the .B OEXCL .RB ( 0x1000 ) bit is set in the .I mode for a .B create system call, the .B open message is not sent; the kernel issues only the .BR create . Thus, if the file exists, .B create will draw an error, but if it doesn't and the .B create system call succeeds, the process issuing the .B create is guaranteed to be the one that created the file.