support picolibc

[abobija]

With current HAVE_OPEN_FUNOPEN and HAVE_OPEN_FOPENCOOKIE defines i can not configure src/open_memstream.c to work with picolibc's version of funopen.

This PR is just POC (i'm sure there is a cleaner way) how to configure it to work.

picolibc is default libc for espressif's esp-idf from version 6 so it's going to be used there a lot so it would be cool if tinycbor could support picolibc somehow as well.

[thiagomacieira]

Thank you for the POC, but we're going to need to work on this. The patch cannot be right.

Please start with what error messages you're getting.

[thiagomacieira]

This is the same thing, so you didn't do anything.

[abobija]

Thank you for response.

Error that I'm facing:

tinycbor/src/open_memstream.c:103:29: error: passing argument 3 of 'funopen' from incompatible pointer type [-Wincompatible-pointer-types]
  103 |     return funopen(b, NULL, write_to_buffer, NULL, close_buffer);
      |                             ^~~~~~~~~~~~~~~
      |                             |
      |                             RetType (*)(void *, const char *, LenType) {aka int (*)(void *, const char *, int)}

In file included from tinycbor/src/open_memstream.c:31:
picolibc/include/stdio.h:429:28: note: expected '_ssize_t (*)(void *, const void *, size_t)' {aka 'int (*)(void *, const void *, unsigned int)'} but argument is of type 'RetType (*)(void *, const char *, LenType)' {aka 'int (*)(void *, const char *, int)'}
  429 |                 _ssize_t (*writefn)(void *cookie, const void *buf, size_t n),
      |                 ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The goal is to align signature of tinycbor's write_to_buffer function with picolibc's writefn function signature:

_ssize_t (*writefn)(void *cookie, const void *buf, size_t n)

In my POC, for __PICOLIBC__, I've set:

• RetType to be ssize_t
• LenType to be size_t

With that I'm still getting error of signature mismatch since second parameter (data) of tinycbor's function write_to_buffer is type of const char* but picolibc expects that param to be type ofconst void*. I've fixed that with this:

+ #ifdef __PICOLIBC__
+ static RetType write_to_buffer(void *cookie, const void *data, LenType len)
+ #else
static RetType write_to_buffer(void *cookie, const char *data, LenType len)
+ #endif

^ you can see there that type of second parameter is changed from const char* to const void* for picolibc.

Maybe we need new DataType beside RetType and LenType to make open_memstream.c more scalable?

Hope I've explain it better this time.

[thiagomacieira]

Ok, I see the problem: for BSD systems (including Apple's macOS), funopen's callback functions take a const char * second parameter, but in picolibc, it's const void *. I suppose that makes sense, as we could be writing binary data through fwrite and such. However, this is very stupid of picolibc to mismatch the API: if you're trying to implement the API that 4.4 BSD introduced, it should match the signature.

To fix this, we need to update the CMakeLists.txt test to do more than check_symbol_exists:

tinycbor/CMakeLists.txt

Lines 133 to 135 in 6442e74

	check_symbol_exists(open_memstream stdio.h HAVE_OPEN_MEMSTREAM)
	check_symbol_exists(funopen stdio.h HAVE_OPEN_FUNOPEN)
	check_symbol_exists(fopencookie stdio.h HAVE_OPEN_FOPENCOOKIE)

We need a compile check that we are matching the arguments correctly, to be able to match the two variants.