using ANSI C libraries for STM32 programs
Yes you can. You should definitely be aware of their possibilities.

Check the documentation for when you are using the ARM Toolchain.
https://developer.arm.com/docs/dui0475/l/the-arm-c-and-c-libraries
https://static.docs.arm.com/dui0475/l/DUI0475L_libraries_user_guide.pdf

Or for GNU:
https://gcc.gnu.org/onlinedocs/gcc-9.2.0/gcc/
https://gcc.gnu.org/onlinedocs/gcc-9.2.0/gcc.pdf

You can also find info in the POSIX docs, such as stdint.h.