stb_ds: build error with gcc -std=c99 up to c18 (works fine with gnu99 and c2x variants)

[sroccaserra]

Hi, thank you for stb_ds.

I have trouble building with gcc without gnu standards when using hmput() and hmget(), did I miss to set some configuration?

Problem

It works fine with -std=gnu99, up to gnu18, but I get the following error with -std=c99, up to c18:

$ gcc -std=c18 test_stb_ds.c
In file included from test_stb_ds.c:4:
test_stb_ds.c: In function ‘main’:
lib/stb_ds.h:526:49: warning: implicit declaration of function ‘typeof’ [-Wimplicit-function-declaration]
  526 | fine STBDS_ADDRESSOF(typevar, value)     ((typeof(typevar)[1]){value}) // literal array decays to pointer to value

Note: it works without error with -std=c2x with gcc 13, and -std=c23 with gcc 14 (tested with gcc 14.2.0).

Code

I use:

• stb_ds.h v0.67 from commit 40adb99
• gcc 13.3.0 (Ubuntu 13.3.0-6ubuntu2~24.04)

The code of test_stb_ds.c is from the documentation:

#include <stdio.h>

#define STB_DS_IMPLEMENTATION
#include "lib/stb_ds.h"

int main(void) {
    float f;
    struct { float key; char value; } *hash = NULL;
    f=10.5; hmput(hash, f, 'h');
    f=20.4; hmput(hash, f, 'e');
    f=50.3; hmput(hash, f, 'l');
    f=40.6; hmput(hash, f, 'X');
    f=30.9; hmput(hash, f, 'o');

    f=40.6; printf("%c - ", hmget(hash, f));

    f=40.6; hmput(hash, f, 'l');
    for (int i=0; i < hmlen(hash); ++i)
        printf("%c ", hash[i].value);
}

Ideas / what I tried

I noted that I have no issue with clang with c99+ standards, so I tried this diff and it seems to work:

diff --git a/stb_ds.h b/stb_ds.h
index d07bddb..1321ad4 100644
--- a/stb_ds.h
+++ b/stb_ds.h
@@ -520,7 +520,7 @@ extern void * stbds_shmode_func(size_t elemsize, int mode);

 // this macro takes the address of the argument, but on gcc/clang can accept rvalues
 #if defined(STBDS_HAS_LITERAL_ARRAY) && defined(STBDS_HAS_TYPEOF)
-  #if __clang__
+  #if defined(__GNUC__) || defined(__clang__)
   #define STBDS_ADDRESSOF(typevar, value)     ((__typeof__(typevar)[1]){value}) // literal array decays to pointer to value
   #else
   #define STBDS_ADDRESSOF(typevar, value)     ((typeof(typevar)[1]){value}) // literal array decays to pointer to value

[nothings]

Yes, that is the correct substitution, but I don't know enough about it to know if that's the right predefined macro to test.

[sroccaserra]

This is not my field of expertise, so if someone has a better view on this please help. What I found could work for GCC (I tested versions 13 and 14) and clang (I tested version 18.1.3).

Either this small change, that should not impact clang users (because the condition was always true and is still true in all cases for clang):

  #if __clang__ || __STDC_VERSION__ <= 201710L

Or this bigger change, tested on the same clang and gcc versions, that is more logical but this one could have an effect on clang users (because now for stdc > 18 clang users will use the new c23 standard typeof):

  #if __STDC_VERSION__ <= 201710L

About __STDC_VERSION__ values, I noted that:

• clang 18 sets it to 202311L for c2x and c23 standards
• GCC 13.3 sets it to 202000L for c2x and does not support c23
• GCC 14.2 sets it to 202000L for c2x and c23

I also noted that clang 18 still sets __GNUC__, to value 4, and gcc 13 and 14 without surprise set it to respectively 13 and 14.

Also: I tested for g++ and clang++, it works before and after the change for standards c++11 and c++20 (C++ should not use this code anyway, as __STDC_VERSION__ is not defined by C++ preprocessors AFAIK).

[sroccaserra]

(Also, please feel free to close the subject if irrelevant or not aligned with current priorities 🙏)

[ket395]

#1734 (reply in thread)

Thanks for testing the weird and poplar C compilers. I think any test will need to be sufficiently complex and simple.

The solution for the problem you mention is very clear and there is only one, but you have not just written it down here.

Manish
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