alloca inside compound statement

Question

is it possible to use alloca inside compound statement? Example:

typedef struct
{
    size_t len;
    char* data;
} string_t;

#define str_to_cstr(str) \
({ \
    char* v = alloca(str.len + 1); \
    v[len] = 0; \
    memcpy(v, str.data, str.len); \
})

// ... and somewhere in deep space
int main()
{
    string_t s = {4, "test"};
    printf("%s\n", str_to_cstr(s));
    return 0;
}

From my experience it works well, but I am not sure it is safe. BTW, it compiled with gcc 4.8.4

Although what you suggest should work, why not just maintain a null char at the end of `.data` and use that (maybe cast to `const char *`)? — Dmitri, Jul 11 '15 at 18:22
While that is ok, you should think about using a [_variable length array_](http://port70.net/~nsz/c/c11/n1570.html#6.7.6.2p10) (VLA). They were introduced with C99. — too honest for this site, Jul 11 '15 at 18:24
Why do you use a macro instead of an `inline` function? The macro breaks your code and uses unnecessarily a gcc extension. — too honest for this site, Jul 11 '15 at 18:43
Yes, I know about VLA, but I tried to find "one line solution". Inline is not suitable here, since alloca uses stack of function from which alloca was called — user939407, Jul 11 '15 at 20:17

score 3 · Answer 1 · answered Jul 11 '15 at 18:26

3

Not safe in your example here:

 printf("%s\n", str_to_cstr(s));

From glibc documentation of alloca:

Do not use alloca inside the arguments of a function call—you will get unpredictable results, because the stack space for the alloca would appear on the stack in the middle of the space for the function arguments. An example of what to avoid is foo (x, alloca (4), y).

Note that ({}) is not a compound statement but a GNU C statement expression.

answered Jul 11 '15 at 18:26

ouah

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Hmm, I found quite weak point in "man alloca": `On many systems alloca() cannot be used inside the list of arguments of a function call, because the stack space reserved by alloca() would appear on the stack in the middle of the space for the function arguments` So, does it means, what some systems allow to use alloca inside the arguments? – user939407 Jul 11 '15 at 20:06
@user939407 it means you should not do it. – ouah Jul 12 '15 at 10:49

alloca inside compound statement

1 Answers1