使用指针连接两个字符串但无法运行

Question

This code wants to concatenate 2 strings using pointers but it cannot run. It only runs for 2 lines and then it stops.

#include <stdio.h>

int main()
{
    int i = 0, j = a0;
    char *str1, *str2, *str3;

    puts("Enter first string");
    gets(str1);

    puts("Enter second string");
    gets(str2);

    printf("Before concatenation the strings are\n");
    puts(str1);
    puts(str2);

    while(*str1)
    {
      str3[i++] = *str1++;
    }
    while(*str2)
    {
      str3[i++] = *str2++;
    }
    str3[i] = '\0';
    printf("After concatenation the strings are\n");
    puts(str3);

    return 0;
}

Answer 1

gets() needs a buffer to put the string into, but you are passing it an uninitialized pointer instead. Also, as mentioned, you should not use it in new code. fgets(str, len, stdin) will do the same but not overrun the buffer.

example:

#include <stdio.h>
#include <string.h>
int main()
{
    char str1[100], str2[100], str3[200];
    puts("Enter first string: ");
    fgets(str1, 100, stdin);
    puts("Enter second string: ");
    fgets(str2, 100, stdin);
    printf("Before concatenation the strings are\n%s\n%s\n", str1, str2);
    memset(str3, 0, 200);
    strncat(str3, str1, 200);
    strncat(str3, str2, 200);
    printf("After concatenation the strings are\n%s\n", str3);
    return 0;
}

Answer 2

#include<stdio.h>
#include<stdlib.h>
    int main(){
      int i=0,j=0;
      char *str1,*str2,*str3;
      puts("Enter first string");
      str1=malloc(50*sizeof(char));
      str2=malloc(50*sizeof(char));
      str3=malloc(50*sizeof(char));
      gets(str1);
      puts("Enter second string");
      gets(str2);
      printf("Before concatenation the strings are\n");
      printf("%s\n",str1);
      printf("%s",str2);
      while(*str1){
          str3[i++]=*str1++;
      }
      while(*str2){
          str3[i++]=*str2++;
      }
      str3[i]='\0';
      printf("\nAfter concatenation the strings are\n");
      puts(str3);
      return 0;
    }

here i have allocated memmory for the pointers so that your code works fine.

Answer 3

#include <stdio.h>

void concatStr(char* str1,char* str2,char* str3){
    while(*str3++ = *str1++);
    str3--;
     while(*str3++ = *str2++);
}
int main()
{
 char str1[100],str2[100],str3[200];
 char *strp1,*strp2,*strp3;

 printf("enter str 1\t");
 scanf("%s",str1);
printf("enter str 2\t");
 scanf("%s",str2);

 printf("str1\t%s\n",str1);
 printf("str2\t%s\n",str2);

 concatStr(str1,str2,str3);
 printf("%s",str3);
    return 0;
}

you can try this...