[cfe-dev] new alloc_size_ex attribute

Nuno Lopes nunoplopes at sapo.pt
Mon Jun 18 13:32:57 PDT 2012


Right now we have support for parsing the alloc_size attribute  
(introduced in GCC 4.3). This attribute specifies that a given  
function allocates x bytes, where x is given by the multiplication of  
the specified parameters.
However, this syntax is still too restrictive for many custom  
allocation functions I've seen in the wild. Therefore I propose a new  
attribute: alloc_size_ex (or whatever name we come up with). For the  
best of my knowledge, GCC doesn't have an equivalent attribute.

char *my_strdup(char *str) __attribute__((alloc_size_ex(strlen(str)+1)));
void *my_complex_alloc(int n, int size, int add)  
__attribute__((alloc_size_ex(n * size + add)));
char *middle(int size) __attribute__((alloc_size_ex(size, size/2)));

So the idea is that the first parameter of the attribute is an  
expression that computes the size of the allocated buffer, and the  
second (optional) argument gives the offset from the beginning of the  
buffer. (The remaining bytes that can be read are given by size-offset  
if offset <= size).
I would suggest the expression's variables to be restricted to the  
variables used in the call expression.

The second point is how to implement this thing. Right now, clang  
doesn't allow expressions as attribute's parameters. Can the parser be  
extended accordingly?
For the codegen side, LLVM already has metadata to support this  
advanced buffer size information. However, one needs to synthesize a  
function from the expressions given for the size and offset of the  

Any comments/suggestions/etc..?


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