Assembly Language for Beginners

1.7. STACK

Local variable storage

A function could allocate space in the stack for its local variables just by decreasing thestack pointer

towards the stack bottom.

Hence, it’s very fast, no matter how many local variables are defined. It is also not a requirement to store

local variables in the stack. You could store local variables wherever you like, but traditionally this is how

it’s done.

x86: alloca() function

It is worth noting thealloca()function^63. This function works likemalloc(), but allocates memory

directly on the stack. The allocated memory chunk does not have to be freed via afree()function call,

since the function epilogue (1.6 on page 29) returnsESPback to its initial state and the allocated memory

is justdropped. It is worth noting howalloca()is implemented. In simple terms, this function just shifts

ESPdownwards toward the stack bottom by the number of bytes you need and setsESPas a pointer to

theallocatedblock.

Let’s try:

#ifdef __GNUC__

#include <alloca.h> // GCC

#else

#include <malloc.h> // MSVC

#endif

#include <stdio.h>

void f()

{

char *buf=(char*)alloca (600);

#ifdef __GNUC__

snprintf (buf, 600, "hi! %d, %d, %d\n", 1, 2, 3); // GCC

#else

_snprintf (buf, 600, "hi! %d, %d, %d\n", 1, 2, 3); // MSVC

#endif

puts (buf);

};

_snprintf()function works just likeprintf(), but instead of dumping the result intostdout(e.g., to
terminal or console), it writes it to thebufbuffer. Functionputs()copies the contents ofbuftostdout.
Of course, these two function calls might be replaced by oneprintf()call, but we have to illustrate small
buffer usage.

MSVC

Let’s compile (MSVC 2010):

Listing 1.38: MSVC 2010

...

mov eax, 600 ; 00000258H

call __alloca_probe_16

mov esi, esp

push 3

push 2

push 1

push OFFSET $SG2672

push 600 ; 00000258H

push esi

call __snprintf

(^63) In MSVC, the function implementation can be found inalloca16.asmandchkstk.asmin
C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\crt\src\intel