access.2

<B>Digital的Unix操作系统VAX 4.2源码</B>

.\" SCCSID: @(#)access.2	2.4	8/10/87
.TH access 2
.SH Name
access \- determine the accessibility of file
.SH Syntax
.nf
.B #include <unistd.h>
.br
.B accessible = access(\fIpath, mode\fP)
.br
.B int accessible;
.br
.B char *\fIpath\fP;
.br
.B int \fImode;\fP
.fi
.SH Description
.NXR "access system call"
.NXR "file" "checking accessibility"
The system call,
.PN access ,
checks the given file
.I path
for accessibility according to
.IR mode .
The argument 
.I mode
is an inclusive OR of the bits R_OK, W_OK, and X_OK.
Specifying the argument
.I mode
as F_OK
tests whether the directories leading to the file can be
searched and whether the file exists.
.PP
The real user ID and the group access list
(including the real group ID) are used
to verify permissions.  This call is useful to set-UID programs.
.PP
Note that only access bits are checked.
The 
.PN access 
call may indicate that a directory is writeable, but an attempt to open
the directory fails, although files are present in the directory.
Additionally, a file may appear to be executable, but 
.PN execve
fails unless the file is in proper format.
.PP
If a
.I path
cannot be found, or if the desired access modes are
not granted, a \-1 value is returned; otherwise,
a 0 value is returned.
.SH Diagnostics
Access to the file is denied if any of the following is true:
.TP 15
[EACCES]
Permission bits of the file mode do not permit the requested
access or search permission is denied on a component of the
path prefix.  The owner of a file has permission checked with
respect to the owner's read, write, and execute mode bits.
Members of the file's group, other than the owner, have permission
checked with respect to the group's mode bits.  All others 
have permissions checked with respect to the other
mode bits.
.TP 15
[EFAULT]
The
.I path
points outside the process's allocated address space.
.TP 15
[EIO]
An I/O error occurred while reading from or writing to the
file system.
.TP 15
[ELOOP]
Too many symbolic links were encountered in translating the pathname.
.TP
[ENAMETOOLONG]
A 
.I path
component length exceeds 255 characters or the length of \fIpath\fP exceeds
1023 characters.
.TP 15
[ENOENT]
The file referred to by
.I path
does not exist or the 
.I path
points to an empty string and the environment defined is POSIX or SYSTEM_FIVE.
.TP 15
[ENOTDIR]
A component of the path prefix is not a directory.
.TP 15
[EROFS]
Write access is requested for a file on a read-only file system.
.TP 15
[ESTALE]
The file handle given in the argument was invalid.  The file
referred to by that file handle no longer exists or has been 
revoked.
.TP 15
[ETIMEDOUT]
A connect request or remote file operation fails
because the connected party
did not respond after a period
of time determined by the communications protocol.
.TP 15
[ETXTBSY]
Write access is requested for a pure procedure (shared text)
file that is being executed.
.SH See Also
chmod(2), stat(2)