GCC - How to stop malloc being linked?

问题:

I am struggling to get my code down to minimal bare bones size! I am using a STM32F0 with only 32k flash and need a good part of the flash for data storage. My code is already at approx 20k flash size! Some of this is due to use of the STM32 HAL functions which I can account for and optimise later if needed.

However, my biggest consumer of flash is all the implicitly included library routines. I can not seem to remove these functions. They are not called anywhere in my code or any HAL code. Functions such as _malloc_r (1.3k Bytes), and __vfiprintf_r (3kB) and many others are using a large part of my flash. I think these are all the libc functions. I do not use these and would like them gone!

Does anybody know how to remove these?

I have tried different optimisation levels and linker options but no luck so far. I have tried -nostdlib and --specs=nosys.specs with no change. If I remove my file with definitions for functions such as _exit I get a linker error suggesting that the library is still included and needs these. Also linker map confirms presence of a lot of unwanted functions!

Any suggestions?

回答1:

When you are wondering about what takes space, or why functions and libraries have been linked in, generate a map file with cross references - something like "-Wl,-Map=program.map,--cref". Look at the file with a text editor, and you can see why a function like malloc has been included.

回答2:

Solved... Some of my code included and called assert. The moment I removed assert calls my code size more than halved! I instead used the STM32 HALs assert_param macro that is a light weight assert that just redirects to a user defined function.

It would be helpful if someone could explain to me how gcc decides to include library functions when assert is called? I see that assert.h declares an external function __assert_func. How does the linker know to reference it from a library rather than just say "undefined reference to __asert_func"?