C strcat-警告：传递'strcat'的arg 2会使指针从整数转换而无需强制转换

Question

我下面的程序有问题。我正在尝试浏览用户输入的某些单词的字符串命令。我现在的主要问题是，当我运行以下命令时，我得到一个警告，即“传递'strcat'的arg 2会使指针从整数转换而无需强制转换”。我的意图是遍历字符串“ s”的前三个字符，将它们连接到字符串“ firstthree”，然后检查字符串“ firstthree”的值。任何帮助表示赞赏。

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <readline/readline.h>
#include <readline/history.h>

/* Simple example of using gnu readline to get lines of input from a user.
Needs to be linked with -lreadline -lcurses
add_history tells the readline library to add the line to it's
internal histiry, so that using up-arrow (or ^p) will allows the user
to see/edit previous lines.
*/

int main(int argc, char **argv) {
    char *s;
    while (s=readline("Enter Name: ")) {
            add_history(s); /* adds the line to the readline history buffer */
            printf("Hello %s\n",s);/*output message to the user*/
            char *firstthree;
            int i;
            for(i = 0; i < 3; i++){
                    strcat(firstthree, s[i]);
                    printf("Hello %s\n",firstthree);//checking to see the character added to the end of the string
            }
            printf("Hey %s\n",firstthree);/*prints out the first three characters*/
            free(s); /* clean up! */
            free(firstthree);
    }
    return(0);
}

Answer 1

您的程序有很多问题；firstthree例如，您永远不会初始化。

您收到所看到的特定错误的原因是由于此调用：

strcat(firstthree, s[i]);

s是一个char *，s[i]一个也是char，但是strcat希望两个参数都是指向以null终止的字符串的指针。您似乎想要的是这样的：

char firstthree[4] = { 0 };
for (int i = 0; i < 3; i++)
{
    firstthree[i] = s[i];
    printf("Hello %s\n", firstthree);
}