memset (0, 0, 0);

Joern Rennecke
2003-04-04T14:54:09Z

On some processors, memset can be implemented more efficiently
when you always read - and possibly also write back - the first
memory word contained partially or in whole in the to-be-modified
object

This conflicts with gdb usage of memset (0, 0, 0); in some places.
There are three practical questions here:
- should gdb use this idiom?
- should all target-specific variants of newlib's memset implement it?
- should all target-specific variants of glibc's memset implement it?

My understanding of the standard is that memset with an unmapped
destination address always invokes undefined behavior. It says:

3.14: object:
... Except for bit-fields, objects are composed of contigous
sequences of one or more bytes, ...
7.1.7: Use of library functions
Each of the following statements apply unless explicitly stated
otherwise in the detailed descriptions that follow. If an argument
has an invalid value (such as a value outside the domain of the
function, or a pointer outside the address space of the program, or
a null pointer), the behaviour is undefined. If a function argument
is described as being an array, the pointer actually passed to the
function shall have a value such that all address computations and
accesses to objects (that would be valid if the pointer did point to
the first element of such an array) are in fact valid. ...
7.11.1 String function conventions
The header <string.h> declares one type and several functions, and
defines one macro useful for manipulating arrays of character type
and other objects treated as arrays of character type. ... in all
cases a char * or a void * argument points to the initial (lowest
addressed) character of the array. ...
7.11.6.1 The memset function
... void *memset (void *s, int c, size_t n); ...
The memset function copies the value of c (converted to unsigned
char) into each of the first n characters of the object pointed to
by s. ...

So, as i understand this, this means that the first argument of memset
must point to an object, which contains at least one (the first)
character. Passing a NULL pointer, or any other address which is
outside the address space of the program, invokes undefined behaviour.

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