ref: 75a8003a8afb22e8557da98ad5dae63efa365d5e
dir: /sys/man/2/open/
.TH OPEN 2 .SH NAME open, create, close \- open a file for reading or writing, create file .SH SYNOPSIS .B #include <u.h> .br .B #include <libc.h> .PP .B int open(char *file, int omode) .PP .B int create(char *file, int omode, ulong perm) .PP .B int close(int fd) .SH DESCRIPTION .I Open opens the .I file for I/O and returns an associated file descriptor. .I Omode is one of .BR OREAD , .BR OWRITE , .BR ORDWR , or .BR OEXEC , asking for permission to read, write, read and write, or execute, respectively. In addition, there are three values that can be ORed with the .IR omode : .B OTRUNC says to truncate the file to zero length before opening it; .B OCEXEC says to close the file when an .IR exec (2) or .I execl system call is made; and .B ORCLOSE says to remove the file when it is closed (by everyone who has a copy of the file descriptor). .I Open fails if the file does not exist or the user does not have permission to open it for the requested purpose (see .IR stat (2) for a description of permissions). The user must have write permission on the .I file if the .B OTRUNC bit is set. For the .I open system call (unlike the implicit .I open in .IR exec (2)), .B OEXEC is actually identical to .BR OREAD . .PP .I Create creates a new .I file or prepares to rewrite an existing .IR file , opens it according to .I omode (as described for .IR open ), and returns an associated file descriptor. If the file is new, the owner is set to the userid of the creating process group; the group to that of the containing directory; the permissions to .I perm ANDed with the permissions of the containing directory. If the file already exists, it is truncated to 0 length, and the permissions, owner, and group remain unchanged. The created file is a directory if the .B DMDIR bit is set in .IR perm , an exclusive-use file if the .B DMEXCL bit is set, and an append-only file if the .B DMAPPEND bit is set. Exclusive-use files may be open for I/O by only one client at a time, but the file descriptor may become invalid if no I/O is done for an extended period; see .IR open (5). .PP .I Create fails if the path up to the last element of .I file cannot be evaluated, if the user doesn't have write permission in the final directory, if the file already exists and does not permit the access defined by .IR omode , or if there are no free file descriptors. In the last case, the file may be created even when an error is returned. If the file is new and the directory in which it is created is a union directory (see .IR intro (2)) then the constituent directory where the file is created depends on the structure of the union: see .IR bind (2). .PP Since .I create may succeed even if the file exists, a special mechanism is necessary for those applications that require an atomic create operation. If the .B OEXCL .RB ( 0x1000 ) bit is set in the .I mode for a .IR create, the call succeeds only if the file does not already exist; see .IR open (5) for details. .PP .I Close closes the file associated with a file descriptor. Provided the file descriptor is a valid open descriptor, .I close is guaranteed to close it; there will be no error. Files are closed automatically upon termination of a process; .I close allows the file descriptor to be reused. .SH SOURCE .B /sys/src/libc/9syscall .SH SEE ALSO .IR intro (2), .IR bind (2), .IR stat (2) .SH DIAGNOSTICS These functions set .IR errstr .