main_x86_64.c

#include <stdio.h>

__asm__(".globl func\n\t"   // optional, makes the function visible outside of main_x86_64.c
        "func:\n\t"         // for macOS (Mach-O), change func to _func
        "movl $7,%eax\n\t"
        "ret"
        );

// the definition of func is written in assembly language above
extern int func(void);

int func2(int x, int y)
{
    int z;
    __asm__("movl %1,%%eax\n\t"
            "imull $3,%%eax\n\t"
            "addl $4,%%eax\n\t"
            "addl %2,%%eax\n\t"
            "movl %%eax,%0"     // if we returned here, a ret would be automatically generated
            : "=r" (z)          // output registers (register index starts here)
            : "r" (x), "r" (y)  // input registers
            : "eax"             // clobbered registers
            );                  // stores 3x + 4 + y in z
    return z + 5;
}

int main(void)
{
    int n = func();
    __asm__("leal (%0,%0,4),%0" // stores n + 4n = 5n in n
            : "=r" (n)          // r means load into any available register
            : "0" (n)           // a means %rax, D means %rdi, S means %rsi
            :                   // d means %rdx, c means %rcx
            );                  // 0 means that input register must be the same as the 0th output register
    printf("7*5 = %d\n", n);
    printf("(3*%d + 4 + %d) + 5 = %d\n", 5, 8, func2(5, 8));
    fflush(stdout); // flush is intentional

    return 0;
}

This uses AT&T syntax