c - 为什么我在 CS50 Pset 中总是出现段错误？

Question

我目前正在尝试CS50的Pset2，在caesar.c中，用户应该通过argv输入k；如果他不这样做，我就应该“大喊大叫”用户告诉他使用命令行参数。但如果我尝试以下操作，我总是会遇到段错误。

#include <cs50.h>
#include <stdio.h>
#include <stdlib.h>

int main(int argc, const string argv[])
{
    unsigned int k = atoi(argv[1]);

    if(k == NULL)
    {
        printf("YOU DID NOT INPUT ANYTHING AS COMMAND LINE ARGUMENT! ");
        return 1;
    }

    return 0;
}

Answer 1

当你这样做时

unsigned int k = atoi(argv[1]);

argv[1] 可能为 NULL。您应该在调用 atoi 之前检查它，因为将 NULL 传递给 atoi 会导致未定义的行为。改变

if(k == NULL)
{
printf("YOU DID NOT INPUT ANYTHING AS COMMAND LINE ARGUMENT! ");
return 1;
}

至

if(argv[1] == NULL)
{
    printf("YOU DID NOT INPUT ANYTHING AS COMMAND LINE ARGUMENT! ");
    return 1;
}

或

if(argc < 2)
{
    printf("YOU DID NOT INPUT ANYTHING AS COMMAND LINE ARGUMENT! ");
    return 1;
}

并将其移到前面

unsigned int k = atoi(argv[1]);