CODEWITHSUNDEEP
cstringgccalignmentclang
ScrimpyCat
ScrimpyCat

Reputation: 63

C Aligning string literals for a specific use case

I'm trying to align string literals in a specific way, as how I'm using it in my code is fairly specific. I don't want to have to assign it to a variable, for instance many of my functions are using it as a direct argument. And I want it to work both in local scope or global scope.

Usage example:

char *str = ALIGNED_STRING("blah"); //what I want
foo(ALIGNED_STRING("blah")); //what I want

_Alignas(16) char str[] = "blah"; //not what I want (but would correctly align the string)

The ideal solution would be (_Alignas(16) char[]){ "blah" } or a worser case using the GCC/Clang compiler extensions for alignment (__attribute__((alignment(16))) char[]){ "blah" }, but neither works (they're ignored and the default alignment for the type is used).

So my next thought was to align it myself, and then my functions that use the string could then fix it up correctly. e.g. #define ALIGNED_STRING(str) (char*)(((uintptr_t)(char[]){ "xxxxxxxxxxxxxxx" str } + 16 - 1) & ~(16 - 1)) (where the string containing 'x' would represent data needed to understand where the real string can be found, that's easy but just for the example assume the 'x' is fine). Now that works fine in local scope, but fails in the global scope. Since the compiler complains about it not being a compile-time constant (error: initializer element is not a compile-time constant); I would've thought it would work but it seems only addition and subtraction are valid operations on the pointer at compile-time.

So I'm wondering if there's anyway to achieve what I want to do? At the moment I'm just using the latter example (padding and manually aligning) and avoiding to use it in the global scope (but I would really want to). And the best solution would avoid needing to make runtime adjustments (like using the alignment qualifier would), but that doesn't seem possible unless I apply it to a variable (but as mentioned that's not what I want to do).

Upvotes: 6

Views: 670

Answers (1)

chux
chux

Reputation: 153338

Was able to get close to OP's need with a compound literal. (C99)

#include <stdio.h>
#include <stddef.h>

void bar(const char *s) {
  printf("%p %s\n", (void*)s, s);
}

//                         v-- compound literal --------------------------v
#define ALIGNED_STRING(S)  (struct { _Alignas(16) char s[sizeof S]; }){ S }.s

int main() {
  char s[] = "12";
  bar(s);
  char t[] = "34";
  bar(t);
  bar(ALIGNED_STRING("asdfas"));
  char *u = ALIGNED_STRING("agsdas");
  bar(u);
}

Output

0x28cc2d 12
0x28cc2a 34
0x28cc30 asdfas  // 16 Aligned
0x28cc20 agsdas  // 16 Aligned

Upvotes: 4

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