I try to sanitize an custom allocator (basically it will nuke the contents
on an memory block on free, and therefore needs to prefix an size header).
So far I get an exception when the pointer returned from the allocator is
casted to any struct and then data assigned.
How can I rewrite the code compliant with -fcatch-undefined-behavior ?
typedef struct { int foo, bar; } MyStruct_t;
int main( int argc, char argv[] ) {
MyStruct_t *p = (MyStruct_t *) My_MemAlloc( sizeof( MyStruct_t ) );
p->foo = 0; // crash here
return 0;
}
Old code:
void *My_MemAlloc(unsigned int size ) {
unsigned int *rawData;
MY_ASSERT(size);
rawData = (unsigned int *) malloc(size + sizeof(unsigned int));
if( rawData ) {
rawData[0] = size;
// tried also to keep the void pointer and return rawVoidPtr + sizeof(
unsigned int )
return rawData + 1;
}
return NULL;
}
Modified variant:
typedef struct {
unsigned size;
char mem[1];
} My_MemoryBlockHeader_t;
void *My_MemAlloc(unsigned int size )
{
void *rawData;
void *result = NULL;
size_t memOffset = offsetof( Tal_MemoryBlockHeader_t, mem );
MY_ASSERT(size);
size_t mySize = size;
if( (SIZE_MAX - memOffset) < mySize ) {
return result;
}
mySize += memOffset;
rawData = malloc( mySize );
if( rawData ) {
My_MemoryBlockHeader_t *hdr = rawData;
hdr->size = size;
result = rawData + memOffset; // or hdr->mem
}
return result;
}
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