util: make STATIC_ASSERT usable at file scope #1952

pull real-or-random wants to merge 2 commits into bitcoin-core:master from real-or-random:worktree-better-static-assert changing 3 files +55 −63

real-or-random commented at 3:41 PM on October 1, 2026: contributor

Replace the switch-based STATIC_ASSERT, which is a statement and thus only usable inside functions, with a declaration-like macro. It uses _Static_assert in C11 mode and, as an extension, with GCC >= 4.6 and Clang in pre-C11 modes. Other compilers fall back to declaring an extern array whose size is negative if the assertion fails.

Since STATIC_ASSERT now expands to a declaration, move its use in secp256k1_ge_impl_to_bytes before VERIFY_CHECK to avoid mixing declarations and code, which C89 forbids.

util: make STATIC_ASSERT usable at file scope

Replace the switch-based STATIC_ASSERT, which is a statement and thus
only usable inside functions, with a declaration-like macro. It uses
_Static_assert in C11 mode and, as an extension, with GCC >= 4.6 and
Clang in pre-C11 modes. Other compilers fall back to declaring an
extern array whose size is negative if the assertion fails.

Since STATIC_ASSERT now expands to a declaration, move its use in
secp256k1_ge_impl_to_bytes before VERIFY_CHECK to avoid mixing
declarations and code, which C89 forbids.

7d53109b85

real-or-random added the label tweak/refactor on Oct 1, 2026
real-or-random added the label meta/development on Oct 1, 2026

assumptions: move static assertions to file scope

Now that STATIC_ASSERT can be used at file scope, the
secp256k1_assumption_checker function, which existed only to provide a
function context for the assertions, is no longer needed.

20fcbae219

theStack commented at 5:10 PM on October 2, 2026: contributor

Concept ACK
in src/util.h:76 in 7d53109b85
```
  80 | -        case /* ERROR: static assertion failed */ (expr): \
  81 | -        ; \
  82 | -    } \
  83 | -} while(0)
  84 | +#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L
  85 | +#  define STATIC_ASSERT(expr) _Static_assert(expr, #expr)
```
theStack commented at 2:43 PM on October 5, 2026:

potential follow-up nit: seems like currently only the clang-cl Windows CI job is hitting this conditional compilation block (verified by putting garbage inside, see https://github.com/theStack/secp256k1/commit/1d8e8948e0e2e06616f73c1a729cff1e31bd5740), maybe long-term it would make sense to add an explicit C11 mode CI job

real-or-random commented at 9:21 AM on October 7, 2026:

Agreed... Or maybe we want to reconsider upgrading to --std=c99 at least.

in src/util.h:82 in 7d53109b85

  86 | +#elif !defined(__cplusplus) && (defined(__clang__) || (defined(__GNUC__) && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 6))))
  87 | +/* GCC >= 4.6 and Clang support _Static_assert as an extension in pre-C11 modes,
  88 | + * but g++ does not support it in C++. */
  89 | +#  define STATIC_ASSERT(expr) __extension__ _Static_assert(expr, #expr)
  90 | +#else
  91 | +#  define STATIC_ASSERT(expr) extern int secp256k1_static_assert_dummy[(expr) ? 1 : -1]

theStack commented at 2:57 PM on October 5, 2026:

heh, i've only seen the dummy[1 - 2 * !!(expr)] approach to achieve the same and found that always a bit weird, using a ternary seems more straight-forward

theStack approved
theStack commented at 3:04 PM on October 5, 2026: contributor

ACK 20fcbae2197486b5dce996eb246677c2c7b73595
theStack merged this on Oct 19, 2026
theStack closed this on Oct 19, 2026

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theStack

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tweak/refactor meta/development