西邮Linux兴趣小组2020纳新面试题题解

#include <stdio.h>
int main(int argc, char *argv[]) {
    unsigned char a = 255;
    char ch = 128;
    // char overflows to -128
    a -= ch;
    printf("a = %d ch = %d\n", a, ch);
    // a = 127, ch = -128
}

#include <stdio.h>
int main(int argc, char *argv[]) {
    char *str = "Xi You Linux Group 20";
    printf("%d\n", printf(str));
    // printf(str) returns the length of the string, 21
    // So the result is "Xi You Linux Group 2021"
    return 0;
}

#include <stdio.h>
int main(int argc, char * argv[]) {
    char ch = 'A';
    int i = 65;
    unsigned int f = 33554433;
    // f = 0000 0010 0000 0000 0000 0000 0000 0001
    *(int *)&f >>= 24;
    // f = 0000 0000 0000 0000 0000 0000 0000 0010
    *(int *)&f = *(int *)&f + '?';
    // *(int *)&f = 2 + 63 = 65
    // ch = 'A', i = 'A', f = 'A'
    printf("ch = %c i = %c f = %c\n", ch, i, *(int *)&f);
    return 0;
}

#include <stdio.h>
int main(int argc, char *argv[]) {
    int a[2][2];
    // &a, &a[0] and &a[0][0] all refer to the address of a[0][0]
    printf("&a   = %p \t &a[0]   = %p \t &a[0][0]   = %p \n",
           &a, &a[0], &a[0][0]);
    // &a + 1 actually returns &a + sizeof(a)
    // &a[0] + 1 actually returns &a + sizeof(a[0])
    // &a[0][0] + 1 actually returns &a + sizeof(a[0][0])
    printf("&a+1 = %p \t &a[0]+1 = %p \t &a[0][0]+1 = %p \n",
           &a+1, &a[0] + 1, &a[0][0] + 1);
    return 0;
}

#include <stdio.h>
int* get_array() {
    int array[1121];
    for (int i = 0; i < sizeof(array) / sizeof(int); i++) {
        array[i] = i;
    }
    // How dare you return this!?
    return array;
}
int main(int argc, char *argv[]) {
    // get_array() returns the unusable address of a local variable,
    // the content of whom was discarded, but you can declare
    // a static variable in get_array() and return it to avoid so
    int *p = get_array();
}

#include <stdio.h>
int main(int argc, char *argv[]) {
    char str[] = "XiyouLinuxGroup";
    char *p = str;
    char x[] = "XiyouLinuxGroup\t\106F\bamily";
    // XiyouLinuxGroup Family
    // sizeof(str) = 16, containing '\0' at the end
    // sizeof(p) = 8, 'cuz p is a char pointer,
    // only storing the address of variable str
    // sizeof(x) = 24, containing '\0' at the end
    // strlen(x) = 24, 'cuz of '\b', 'F' is covered by 'a'
    printf("%zu %zu %zu %zu\n", sizeof(str), sizeof(p),
                                sizeof(x), strlen(x));
    return 0;
}

// It's totally about the basic concept of pointers
#include <stdio.h>
void func() {
    int a = 2020;
    unsigned long c;
    // We get the address of variable a
    printf("%p\n", &a);
    printf("我们想要修改的地址：");
    // We want to modify the value on address entered
    scanf("%lx", &c);
    printf("请随便输入一个数字：");
    // And then we modified it. All done.
    scanf("%d", (int *)c);
    printf("a = %d\n", a);
}

#include <stdio.h>
int main() {
    // It outputs a anoymous int array as a char array
    // using little-endian storing methods
    puts((char *)(int const[]){
        // o y i X     n i L u     r G x u
        0X6F796958, 0X6E694C75, 0X72477875,
        // 2 p u o (\0)2 1 0
        0X3270756F, 0X313230, 0X00000A});
}

#include <stdio.h>
long long fibo(int n) {
    if (n == 1 || n == 2) return 1;
    return fibo(n - 1) + fibo(n - 2);
}
int main() {
    int n;
    scanf("%d", &n);
    printf("%lld\n", fibo(n));
}