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(File System): In access?, reword a bit, clarify real

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versus effective ID handling, cross reference glibc on that, and
recommend against access tests in library functions.
This commit is contained in:
Kevin Ryde 2003-09-21 01:18:26 +00:00
parent 69fc37da29
commit ad1c1f1808
1 changed files with 29 additions and 13 deletions
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42
doc/ref/posix.texi
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@ -538,29 +538,45 @@ contents; syncing the file system and creating special files.
@deffn {Scheme Procedure} access? path how
@deffnx {C Function} scm_access (path, how)
Return @code{#t} if @var{path} corresponds to an existing file
and the current process has the type of access specified by
@var{how}, otherwise @code{#f}. @var{how} should be specified
using the values of the variables listed below. Multiple
values can be combined using a bitwise or, in which case
@code{#t} will only be returned if all accesses are granted.
Test accessibility of a file under the real UID and GID of the calling
process. The return is @code{#t} if @var{path} exists and the
permissions requested by @var{how} are all allowed, or @code{#f} if
not.
Permissions are checked using the real id of the current
process, not the effective id, although it's the effective id
which determines whether the access would actually be granted.
@var{how} is an integer which is one of the following values, or a
bitwise-OR (@code{logior}) of multiple values.
@defvar R_OK
test for read permission.
Test for read permission.
@end defvar
@defvar W_OK
test for write permission.
Test for write permission.
@end defvar
@defvar X_OK
test for execute permission.
Test for execute permission.
@end defvar
@defvar F_OK
test for existence of the file.
Test for existence of the file. This is implied by each of the other
tests, so there's no need to combine it with them.
@end defvar
It's important to note that @code{access?} does not simply indicate
what will happen on attempting to read or write a file. In normal
circumstances it does, but in a set-UID or set-GID program it doesn't
because @code{access?} tests the real ID, whereas an open or execute
attempt uses the effective ID.
A program which will never run set-UID/GID can ignore the difference
between real and effective IDs, but for maximum generality, especially
in library functions, it's generally best not to use @code{access?} to
predict the result of an open or execute, instead simply attempt that
and catch any exception.
The main use for @code{access?} is to let a set-UID/GID program
determine what the invoking user would have been allowed to do,
without the greater (or perhaps lesser) privileges afforded by the
effective ID. For more on this, see @ref{Testing File Access,,, libc,
The GNU C Library Reference Manual}.
@end deffn
@findex fstat