[libc] Tracking progress for `std::cout ... std::endl` on baremetal

[saturn691]

Downstream (https://github.com/arm/arm-toolchain), this is a very highly requested feature for a lot of embedded codebases, in order for it to be compatible with previous libraries. In particular, we need the following to compile.

// Note we need std::endl
std::cout << "Hello, World!" << std::endl;

Link to Discourse post: https://discourse.llvm.org/t/rfc-implementation-of-stdio-on-baremetal/86944

The implementation of this will involve:

• fprintf ([libc] Add putc, fputc, and fprintf to stdio/baremetal #144567)
• vfprintf (easily implementable after fprintf)
• ungetc (requires full FILE API)
• getc (requires full FILE API)
• fwrite (requires full FILE API)
• fflush (requires full FILE API + buffering)

It should follow the philosophy of "don't pay for what you're not using". Ideally, this should be written in a way that allows non-semihosted platforms to throw away code.

However, these functions still face the challenges:

• fflush requires buffering, which is difficult (stated at the last Embedded WG meeting)
• FILE will need to be defined somewhere.
• stdout/stderr need to be defined somewhere. Probably in the semihosting library.

As more of the library gets designed, this will be filled in over time. However, at this moment in time, this still requires discussion. See the Discourse post for more details.

[llvmbot]

@llvm/issue-subscribers-libc

Author: William Huynh (saturn691)

Downstream (https://github.com/arm/arm-toolchain), this is a very highly requested feature for a lot of embedded codebases, in order for it to be compatible with previous libraries. In particular, we need the following to compile.

// Note we need std::endl
std::cout << "Hello, World!" << std::endl;

Link to Discourse post: https://discourse.llvm.org/t/rfc-implementation-of-stdio-on-baremetal/86944

The implementation of this will involve:

• fprintf (#144567)
• vfprintf (easily implementable after fprintf)
• ungetc (requires full FILE API)
• getc (requires full FILE API)
• fwrite (requires full FILE API)
• fflush (requires full FILE API + buffering)

It should follow the philosophy of "don't pay for what you're not using". Ideally, this should be written in a way that allows non-semihosted platforms to throw away code.

However, these functions still face the challenges:

• fflush requires buffering, which is difficult (stated at the last Embedded WG meeting)
• FILE will need to be defined somewhere.
• stdout/stderr need to be defined somewhere. Probably in the semihosting library.

As more of the library gets designed, this will be filled in over time. However, at this moment in time, this still requires discussion. See the Discourse post for more details.