linker - use own stdlib implementation

Question

I have a problem. Requirement for the project is that we cannot link our app with standard library ( so -nostdlib is on in gcc).

my_stdlib.c contains implementation of all functions my_memset, my_memcpy ... but linker needs memcpy to copy structs

MyStruct struct = my_struct;

and is complaining about "undefined reference to `memcpy'", which is of course correct.

Is it possible to remap memcpy to my_memcpy using linker script, parameters passed to ld or other way, so linker can use our implementation to copy structs?

Probably -wrap,function could help but I cannot change my_memcpy to __wrap_memcpy.

Anyway, beyond the obvious question why your are restricted in this way you can create a linker script that uses `PROVIDE` to create aliases for symbols (including functions). So you can add lines like `PROVIDE(memset = my_memset);` , `PROVIDE(memcpy = my_memcpy);` etc. — Michael Petch, Jul 15 '17 at 16:49
the other thing you can do is create a wrapper function in _C_ called `memcpy` that calls `my_memcpy`. — Michael Petch, Jul 15 '17 at 17:00
What version of gcc are you using that invokes `memcpy` internally to copy structs? Please post a [mcve] showing this. — Mike Kinghan, Jul 18 '17 at 18:17
@MikeKinghan : gcc 4.5 with Ubuntu 11.04 will produce these results. — Michael Petch, Jul 26 '17 at 16:46

score 1 · Answer 1 · answered Jul 17 '17 at 08:45

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At the GCC level, you can redirect the memcpy symbol to a different symbol using:

void *memcpy (void *, const void *, size_t) __asm__ ("my_memcpy");

This will apply to internally-generated memcpy calls, too. (With GCC. I think it does not change the internal call sites with Clang.)

answered Jul 17 '17 at 08:45

Florian Weimer

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Other interesting things beyond OPs scope: complie your own glibc: https://stackoverflow.com/questions/10763394/how-to-build-a-c-program-using-a-custom-version-of-glibc-and-static-linking and Newlib https://electronics.stackexchange.com/questions/223929/c-standard-libraries-on-bare-metal/223931 – Ciro Santilli OurBigBook.com Dec 06 '18 at 14:46

score 0 · Answer 2 · answered Jul 26 '17 at 16:11

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compile with -fno-builtin. This should avoid it.

answered Jul 26 '17 at 16:11

mfro

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If you compile with `-fno-builtin` that will not solve the issue. No builtin will still allow the compiler to generate the non builtin form which may be a call to an external `memcpy`. The OP would still have to map `memcpy` to `my_memcpy` routine. – Michael Petch Jul 26 '17 at 16:59
Don't think so. If this is indeed the case, I would consider it a bug (which might be avoided with `-ffreestanding`, but this might cause other issues). – mfro Jul 26 '17 at 17:15
At a glance, I didn't find any `libgcc` (for a host of usual and unusual platforms I have available) that would provide it's own `memcpy()` function. – mfro Jul 26 '17 at 17:24

linker - use own stdlib implementation

2 Answers2