A C program can declare a Rust function and still fail to link: the linker looks for `solve`, but Rust may give that function a mangled symbol name. `#[no_mangle]` tells the compiler to keep the function’s name available as `solve` instead. section 3.2.2
A function C can call by name
`#[no_mangle] pub extern "C" fn solve(line: *const c_char, solution: *mut c_int) -> c_int { 0 }` combines three distinct requirements: `#[no_mangle]` preserves the symbol name, `pub` makes the function public, and `extern "C"` selects C calling conventions. Compiling a `cdylib` alone does not make every function C-compatible. section 3.2.2
Listing 3.6 Rust solve function that can be exported as compatible with C
#[no_mangle]
pub extern "C" fn solve(
line: *const c_char, solution: *mut c_int) -> c_int {
0
}
Compare the attribute, visibility keyword, and calling convention in the complete `solve` signature. section 3.2.2
Mangling normally helps keep symbol names unique. Without it, a named C-facing function needs a name unlikely to clash with other functions. An NGINX handler therefore uses the `ngx_http_calculator` prefix. section 3.2.2 section 4.4
Use the attribute according to how C reaches the function:
- For a handler C calls by name, use `#[no_mangle]` with `pub` and `extern "C"`. section 4.4
- For a callback passed to C by its address, use `extern "C"`; its name need not be exposed, so `#[no_mangle]` and `pub` are unnecessary. section 4.4
