C99: simplify size_t printing with printf

[gilles-peskine-arm]

Since Mbed TLS 2.23, we require a C99 platform, with some pragmatic restrictions to support platforms that are mostly but not fully compliant to C99. In particular, we do not require printf and friends (snprintf, vsnprintf, fprintf) to support C99 format specifiers.

In Mbed TLS 4.0, consider requiring printf and friends to support the z modifier to print size_t values with %zu. This only affects the TLS debugging code (gated by MBEDTLS_DEBUG_C). That code could be simplified and it would be easier to ensure that we use the right modifiers.

We would lose support for older C libraries that don't provide the z modifier. Currently, integrators can define MBEDTLS_PRINTF_SIZET to e.g. u or lu if their platform does not support %zu. If we remove this possibility, integrators on a platform that does not support %zu would need to provide a whole printf alternative by defining mbedtls_printf and friends.

Mailing list discussion: https://lists.trustedfirmware.org/archives/list/mbed-tls@lists.trustedfirmware.org/thread/RAV67RL6GF5CX6CFDH5LZAR7RPAY2AEK/

[mschulz-at-hilscher]

Could we just rely on format modifier definitions in inttypes.h?
If a compiler does not provide that header, an integrator could just provide one that works for the used compiler?

[gilles-peskine-arm]

@mschulz-at-hilscher Unfortunately, there's no definition in inttypes.h for a size_t format. But other projects e.g. Chromium have defined a macro using the same naming pattern as inttypes.h:

#define PRIuS "zu"

I would suggest that we rename MBEDTLS_PRINTF_SIZET to PRIuS, but we'd continue to not use %zu directly.