Why is the use of alloca() not considered good practice?

Question

alloca() allocates memory on the stack rather than on the heap, as in the case of malloc(). So, when I return from the routine the memory is freed.

Answer 1

alloca() is very useful if you can't use a standard local variable because its size would need to be determined at runtime and you can absolutely guarantee that the pointer you get from alloca() will NEVER be used after this function returns.

You can be fairly safe if you

• do not return the pointer, or anything that contains it.
• do not store the pointer in any structure allocated on the heap
• do not let any other thread use the pointer

The real danger comes from the chance that someone else will violate these conditions sometime later. With that in mind it's great for passing buffers to functions that format text into them :)