Solution to the bus toy problem Khety created for me

bus.asm

global busString

extern sprintf

section .data
    busStringFormat: db "%ld more people are allowed onto the bus.", 10, 0
    busOutOfSpace: db "No more people can fit on the bus.", 10, 0

section .bss
    output_buff: resb 5000

section .text
busCount:
    xor rax, rax
    cmp rdi, rsi
    jg no_more_room

    sub rsi, rdi
    mov rax, rsi
    jmp return

no_more_room:
    xor rax, rax
return:
    ret

busString:
    sub rsp, 8
    call busCount
    cmp rax, 0
    je no_more_room_return
    ; diff now in rax
    mov r8, rax
    xor eax, eax
    mov rdi, output_buff
    mov rsi, busStringFormat
    mov rdx, r8
precall:
    call sprintf
postcall:
    mov rax, output_buff
    add rsp, 8
    ret
no_more_room_return:
    mov rax, busOutOfSpace
    add rsp, 8
    ret

bus.c

#include <stdio.h>
#include <stdint.h>

char* busString(int64_t, int64_t);

int main(void) {
    printf("%s", busString(50, 60));
    printf("%s", busString(50, 49));
    return 0;
}

Finally works!

alex@home:~/Code/Khety$ nasm -felf64 bus.s && gcc bus.c bus.o -no-pie && ./a.out 
10 more people are allowed onto the bus.
No more people can fit on the bus.

I had gotten quite stuck with a segfault because I was not realigning the stack to 16 bits, so I asked this question on StackOverflow:
https://stackoverflow.com/questions/65435315/calling-sprintf-in-nasm-x86-64-gives-segmentation-fault

Peter Cordes (the legend himself) helped me and now it works. Tbh I still don't deeply understand stack alignment or why it matters, so I guess that's something I'll have to look into.