Standards-compliant* alloca()

The alloca() function in C is used to allocate a dynamic amount of memory on the stack. Despite its advantages in some situations, it is non-standard and will probably remain so forever.

There's a feature of C99 (made optional in C11) called variable-length arrays (VLAs for short) that works in many of the same situations. So instead of something like this:

char *path = alloca(strlen(dir) + strlen(file) + 2);
sprintf(path, "%s/%s", dir, file);

you can write this and have a conforming program:

char path[strlen(dir) + strlen(file) + 2];
sprintf(path, "%s/%s", dir, file);

But sometimes VLAs aren't quite as powerful as alloca(). The most recent example I encountered was trying to allocate a struct with a flexible array member on the stack. Here's a simpler example:

Imagine a library author has hidden the definition of a struct to ensure better forward-compatibility. To allow clients to allocate the struct on the stack, she makes the size of the structure available in a global variable, intended to be used like this:

// Library header
struct foo;
extern const size_t foo_size /* = sizeof(struct foo) */;

// Client code
void bar(void) {
    struct foo *foo = alloca(foo_size);
}

If we try to replace the alloca() with a VLA, we get this:

// Client code
void bar(void) {
    char buffer[foo_size];
    struct foo *foo = (struct foo *)buffer;
}

Unfortunately that's not quite standards-compliant, because buffer likely has a weaker alignment requirement than foo. Furthermore, char can't be replaced with any other type, because that would violate strict aliasing rules*.

C11 provides us with a solution: the alignas specifier. Using it, we can force the memory block to have the strictest possible alignment:

#include <stdalign.h>

void bar(void) {
    alignas(max_align_t) char buffer[foo_size];
    struct foo *foo = (struct foo *)buffer;
}

The ugliness can be hidden inside a macro that's almost as convenient as alloca():

#if __STDC_VERSION__ < 201112L || defined(__STDC_NO_VLA__)
#error "C11 with VLAs is required"
#endif

#include <stdalign.h>

#define ALLOCA(var, size)             \
    ALLOCA2(var, size, __LINE__)
#define ALLOCA2(var, size, unique_id) \
    ALLOCA3(var, size, unique_id)
#define ALLOCA3(var, size, unique_id)                         \
    alignas(max_align_t) char alloca_buffer##unique_id[size]; \
    var = (void *)alloca_buffer##unique_id

void bar(void) {
    ALLOCA(struct foo *foo, foo_size);
}

Aside from the slightly uglier syntax, the only difference is in the lifetime of the allocation. Traditionally, memory allocated with alloca() is freed at the end of the calling function, while VLAs are freed ad the end of the enclosing block.

*Reddit user nooneofnote points out that there is still an aliasing issue, because while char * can alias any other type, other types may not necessarily alias an array of char. This trick does work in practice but is sadly not as compliant as I thought.