参考musl libc 库：

#ifndef _RISCV_SYSCALL_IMPL_
#define _RISCV_SYSCALL_IMPL_

#define __SYSCALL_LL_E(x) (x)
#define __SYSCALL_LL_O(x) (x)

#define __asm_syscall(...) do { \
        __asm__ __volatile__ (  \
                                "ecall"              \
                                : "=r"(a0) : __VA_ARGS__ :  \
                                "memory", "ra", "cc");      \
        return a0;              \
    } while (0)

static inline long __syscall0(long n)
{
    register long a7 __asm__("a7") = n;
    register long a0 __asm__("a0");
    __asm_syscall("r"(a7));
}

static inline long __syscall1(long n, long a)
{
    register long a7 __asm__("a7") = n;
    register long a0 __asm__("a0") = a;
    __asm_syscall("r"(a7), "0"(a0));
}

static inline long __syscall2(long n, long a, long b)
{
    register long a7 __asm__("a7") = n;
    register long a0 __asm__("a0") = a;
    register long a1 __asm__("a1") = b;
    __asm_syscall("r"(a7), "0"(a0), "r"(a1));
}

static inline long __syscall3(long n, long a, long b, long c)
{
    register long a7 __asm__("a7") = n;
    register long a0 __asm__("a0") = a;
    register long a1 __asm__("a1") = b;
    register long a2 __asm__("a2") = c;
    __asm_syscall("r"(a7), "0"(a0), "r"(a1), "r"(a2));
}

static inline long __syscall4(long n, long a, long b, long c, long d)
{
    register long a7 __asm__("a7") = n;
    register long a0 __asm__("a0") = a;
    register long a1 __asm__("a1") = b;
    register long a2 __asm__("a2") = c;
    register long a3 __asm__("a3") = d;
    __asm_syscall("r"(a7), "0"(a0), "r"(a1), "r"(a2), "r"(a3));
}

static inline long __syscall5(long n, long a, long b, long c, long d, long e)
{
    register long a7 __asm__("a7") = n;
    register long a0 __asm__("a0") = a;
    register long a1 __asm__("a1") = b;
    register long a2 __asm__("a2") = c;
    register long a3 __asm__("a3") = d;
    register long a4 __asm__("a4") = e;
    __asm_syscall("r"(a7), "0"(a0), "r"(a1), "r"(a2), "r"(a3), "r"(a4));
}

static inline long __syscall6(long n, long a, long b, long c, long d, long e, long f)
{
    register long a7 __asm__("a7") = n;
    register long a0 __asm__("a0") = a;
    register long a1 __asm__("a1") = b;
    register long a2 __asm__("a2") = c;
    register long a3 __asm__("a3") = d;
    register long a4 __asm__("a4") = e;
    register long a5 __asm__("a5") = f;
    __asm_syscall("r"(a7), "0"(a0), "r"(a1), "r"(a2), "r"(a3), "r"(a4), "r"(a5));
}

#endif