Initializing a heap-allocated structure in C

2015-05-01

A pretty common mistake that happens when programming things in C is to allocate less memory than necessary to hold a structure:

struct foobar * foobar = malloc ( sizeof ( struct foobaz ));

Note that struct foobaz is passed instead of struct foobar . We might get lucky, and sizeof(struct foobaz) might be larger or equal than sizeof(struct foobar) , but we might not.

There are lots of tools out there that will catch these mistakes: static analyzers such as the one from Clang, and Memcheck from Valgrind are just two examples that should be in any C programmer's toolbelt.

Even then, people often resort to a a nicer idiom: sizeof(*foobar) , which not only avoids these problems, but also is somewhat future-proof, should the type of foobar change:

struct foobar * foobar = malloc ( sizeof ( * foobar ));

However, structures often have members that, if someone forgets to initialize, will inflict some undefined behavior pains on the user. The things in the toolbelt might help here, as well as the calloc() function, that, in addition to allocating memory, also zero-out the memory block:

struct foobar * foobar = calloc ( 1 , sizeof ( * foobar ));

Not always one would want to zero out the whole memory chunk just to fill out important fields afterwards, though.

Here's a trick that's being used in a yet-to-be-released project I've been working on and off for the past few months. It starts by defining the generic-chunk-of-memory equivalent of strdup() , memdup() :

void * memdup ( const void * src , size_t sz ) { void * mem = malloc ( sz ); return mem ? memcpy ( mem , src , sz ) : NULL ; }

Then a macro is defined:

#define ALLOC_INIT(type, ...) \ (type *)memdup((type[]){ __VA_ARGS__ }, sizeof(type))

Then it is used like so:

struct foobar * foobar = ALLOC_INIT ( struct foobar , { . field = value , . other_field = other_value , . yet_another_field = yet_another_value });

The compiler will check if field , other_field , and yet_another_field are actually part of struct foobar , and will abort compilation of a field isn't there or is of the wrong type.

The cast prevents the allocated memory block from being assigned to the wrong type. (C will happily cast any void* to any other pointer.)

The amount of memory allocated will be exactly what's needed by the structure, and all fields that not mentioned will be initialized with their default values as per designated initializer rules.

If memdup() is inlined, a good compiler will generate pretty good code, that's often byte-by-byte equivalent to allocating directly with malloc() , initializing all the fields by hand, etc.

If GCC is being used, the __auto_type extension can be used, to avoid having to type struct foobar twice. This has been suggested by Thiago Macieira. I'd use this sparingly, though.

__auto_type foobar = ALLOC_INIT ( struct foobar , { . field = value , . other_field = other_value , . yet_another_field = yet_another_value });

It's a pretty nice idiom that I haven't seen anywhere else, and I'm blogging here as the project I'm working on might not ever see the light of day and it would be a shame if at least this didn't become public.