bext.md

B extensions

Here we document all the things that deviate or extend the original description of the B programming language from kbman.

Top Level extrn declarations

main() {
    printf("Hello, World\n");
}

extrn printf;

printf is now visible to all the functions in the global scope.

Inline assembly

main() {
    // for gas-x86_64-linux
    __asm__(
    "movq $60, %rax", // exit syscall number
    "movq $69, %rdi", // exit code
    "syscall"
    );
}

__asm__ is a function-like statement that takes a list of string literals as arguments and passes them directly to the assembler.

Naked functions

// for gas-x86_64-linux
main __asm__(
    "movq $69, %rax",
    "ret"
);

These are a special kind of function for which the compiler does not generate a prologue or an epilogue.
the syntax is name __asm__(...); (this is different from name() __asm__(...);).

__variadic__

extrn printf;
__variadic__(printf, 1);

main() {
    printf("Hello, world!\n");
}

Some targets like gas-aarch64-darwin have a different calling convention for variadic functions from normal ones,
this is needed to make the compiler use the correct calling convention.
the syntax is __variadic__(function_name, number_of_fixed_args);