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Re: doc access?
Re: doc access?
Sun, 21 Sep 2003 11:18:27 +1000
Gnus/5.1003 (Gnus v5.10.3) Emacs/21.3 (gnu/linux)
> * posix.texi (File System): In access?, reword a bit, clarify real
> versus effective ID handling, add an example of that, and recommend
> against access tests in library functions.
I toned it down a bit and checked it in,
- Scheme Procedure: access? path how
- C Function: scm_access (path, how)
Test accessibility of a file under the real UID and GID of the
calling process. The return is `#t' if PATH exists and the
permissions requested by HOW are all allowed, or `#f' if not.
HOW is an integer which is one of the following values, or a
bitwise-OR (`logior') of multiple values.
- Variable: R_OK
Test for read permission.
- Variable: W_OK
Test for write permission.
- Variable: X_OK
Test for execute permission.
- Variable: F_OK
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.
It's important to note that `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 `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 `access?' to predict the result of an open or execute,
instead simply attempt that and catch any exception.
The main use for `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 *Note Testing File Access:
(libc)Testing File Access.
- doc access?, Kevin Ryde, 2003/09/08
- Re: doc access?,
Kevin Ryde <=