纯底层代码

纯底层，无bits/stdc++.h，无iostream，纯底层实现（基于Linux）

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// a+b.cpp  严格单整数输出，无换行
extern "C" {
    long read(int, void *, unsigned long);
    long write(int, const void *, unsigned long);
    void exit(int);
}
typedef unsigned int u32;

/* 把 u32 v 变 decimal 字符串，返回长度 */
int u32toa(char *buf, u32 v) {
    char tmp[16];
    int i = 0, j = 0;
    do { tmp[i++] = char('0' + v % 10); } while (v /= 10);
    while (i--) buf[j++] = tmp[i];
    return j;
}

int main() {
    char ibuf[32], obuf[16];
    long n = read(0, ibuf, sizeof ibuf);
    if (n <= 0) exit(0);

    /* 过滤空白 */
    const char *p = ibuf, *q;
    while (p < ibuf + n && (*p == ' ' || *p == '\t' || *p == '\n' || *p == '\r')) ++p;
    q = p;
    while (q < ibuf + n && *q >= '0' && *q <= '9') ++q;
    u32 a = 0, b = 0;
    for (const char *t = p; t < q; ++t) a = a * 10 + (*t - '0');
    p = q;
    while (p < ibuf + n && (*p == ' ' || *p == '\t' || *p == '\n' || *p == '\r')) ++p;
    q = p;
    while (q < ibuf + n && *q >= '0' && *q <= '9') ++q;
    for (const char *t = p; t < q; ++t) b = b * 10 + (*t - '0');

    u32 c = a + b;
    int len = u32toa(obuf, c);
    write(1, obuf, len);   // 不写 '\n'
    return 0;
}