c - c 中的 fgets 和 strlen

Question

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

main()
{
    char str[5]={'\0'};
    printf("Initial length before passing = %ld\n",strlen(str));
    input(str);
    printf("Received string = %s\n",str);
    printf("Length after getting input(calling function) = %ld\n",sizeof(str));
}

input(char * buffer)
{
    puts("Enter something: ");
    printf("Initial length after passing = %ld\n",strlen(buffer));

    if ( fgets(buffer, sizeof(buffer), stdin) == NULL )
        return -1;
    else
    {
        printf("Length after getting input(called function)= %ld\n",strlen(buffer));
        return 0;
    }
}

输出1

Initial length before passing = 0
Enter something: 
Initial length after passing = 0
hello
Length after getting input(called function)= 6
Received string = hello

Length after getting input(calling function) = 5

输出2

Initial length before passing = 0
Enter something: 
Initial length after passing = 0
helloooooo
Length after getting input(called function)= 7
Received string = hellooo
Length after getting input(calling function) = 5

为什么输入不同时打印的长度不同？

1. 在输出 1 和 2 中，当我只为 5 个字符分配空间时，为什么初始长度为 6？
2. 为什么输出1和输出2传入前后字符串长度不一样？
3. 在输出 2 中，当我只分配了更少的空间时，为什么“获取输入后的长度(调用函数)= 7”？

Answer 1

strlen 应该使用 C 字符串，即以 \0 结尾的字符数组。当你定义:

char str[5];

str 包含垃圾，您很幸运调用 strlen 没有导致段错误。