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ccurtis@ee.fit.edu (Christopher W. Curtis) writes:

>Hi.

>I was wondering if this is a Linux thing (kernel 1.2.1) or a
>more general 'problem'.  If you call strncpy as such:
>	strncpy( dest, NULL, x );
>it will seg fault.  Is this common among all platforms, or
>just with Linux?

>The man page says nothing about it, but I would have presumed
>that if str[n]cpy were passed NULL as a source, it would
>merely catenate the destination.  Did I presume wrong?

If the strncpy() manual does not report any special cases, you should
assume that there are none.
For example, free() supports NULL as a special no-op case, but
strncpy() just tries to copy a string from address 0.
As this is generally not correct, Linux will trap accesses to address
0, so you can immediately see your program has an error.

Frank
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Christopher W. Curtis (ccurtis@ee.fit.edu) wrote:
: I was wondering if this is a Linux thing (kernel 1.2.1) or a
: more general 'problem'.  If you call strncpy as such:
: 	strncpy( dest, NULL, x );
: it will seg fault.  Is this common among all platforms, or
: just with Linux?
:
: The man page says nothing about it, but I would have presumed
: that if str[n]cpy were passed NULL as a source, it would
: merely catenate the destination.  Did I presume wrong?

My man page says:

       The  strcpy() function copies the string pointed to be src
       (including the terminating `\0' character)  to  the  array
       pointed  to by dest.  The strings may not overlap, and the
       destination string dest must be large  enough  to  receive
       the copy.

       The  strncpy()  function  is similar, except that only the
       first n bytes of src are copied.

When passing NULL as the src pointer, it definetly does not reference
a string. Therefore NULL is not a legal parameter to strcpy() and
strncpy(). Your expectation of what should happen is true for the
empty string (i.e. ""), but this is completely different from NULL.

- Jochen

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In article <ccurtis.826776589@ee.fit.edu>
ccurtis@ee.fit.edu (Christopher W. Curtis) writes:

<snip>
CWC: I was wondering if this is a Linux thing (kernel 1.2.1) or a
CWC: more general 'problem'.  If you call strncpy as such:
CWC: 	strncpy( dest, NULL, x );
CWC: it will seg fault.  Is this common among all platforms, or
CWC: just with Linux?
CWC: 
CWC: The man page says nothing about it, but I would have presumed
CWC: that if str[n]cpy were passed NULL as a source, it would
CWC: merely catenate the destination.  Did I presume wrong?

Yes. The standard library functions should usually always be passed
pointers which actually point to objects of appropriate types. A null
pointer points nowhere: and is therefore invalid in most of these
contexts. In this context, you may pass the constant string ""
instead. 

Some library functions specifically allow you to pass in null
pointers: that is obviously different.

Cheers
Tanmoy
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