C17 to C23: what changed (nullptr, constexpr, bool, typeof, #embed)

C23, the 2023 revision of ISO C (published in 2024), adds nullptr, constexpr objects, bool/true/false as keywords, typeof, auto type inference, #embed, binary literals and digit separators. C17 itself only corrected and clarified C11, so the real migration is C17 to C23. Most new features need GCC 13 or later, and C23 became the default dialect in GCC 15.

Language changes

The versions come from the GCC C23 status table. __VA_OPT__ (GCC 8) and __has_include (GCC 5) were also standardised, but compilers had them long before C23.

See the difference

The block below compiles one file that uses constexpr, nullptr, bool without stdbool.h and a binary literal. It fails as C17 and builds as C23. GCC 13 spells the C23 mode -std=c2x; GCC 14 and later accept -std=c23.

mkdir -p /tmp/work && cd /tmp/work
cat > demo.c <<'EOF'
int main(void) {
    constexpr int n = 4;
    int *p = nullptr;
    bool ok = true;
    int x = 0b1010;
    return (p == nullptr && ok && n == 4 && x == 10) ? 0 : 1;
}
EOF
gcc -std=c17 -c demo.c 2>&1 | grep -m1 "undeclared"
gcc -std=c2x -c demo.c && echo "c2x: ok"

demo.c:2:5: error: ‘constexpr’ undeclared (first use in this function)
c2x: ok

Migration checklist

1. Find clashes first. Search for your own bool, true, false, nullptr or constexpr names. In C23 they are reserved, so a typedef int bool; is now a compile error.
2. Pin the standard. Set -std=c17 or -std=c23 in the build. Without a flag the default depends on the compiler version (gnu17 before GCC 15, gnu23 after).
3. Replace NULL gradually. nullptr has its own type, so it avoids the integer-or-pointer ambiguity of NULL in variadic calls. Both keep working.
4. Turn on warnings. Build with -Wall -Wextra (see gcc -Wall -Wextra) so the new diagnostics are visible, and run the sanitizer described in undefined behavior.
5. Check the library side. Some C23 additions, such as the %b printf conversion, also depend on the C library version, not only on the compiler.

A name that was not declared at all gives the error in use of undeclared identifier; under -std=c17 that is what constexpr produces in the example above.

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