使用指针的字符串连接

Question

How do I achieve concatenation using only pointers since the code below gives (a segmentation fault) error (at runtime)?

#include <stdio.h>
#include <string.h>
int main()
{
    char *s1="Front";
    char *s2="Back";
    char *s3=strcat(s1,s2);
    puts(s3);
    return 0;
}

Answer 1

Because you are trying to write to a string literal. The line char *s1 = "Front"; points to a string constant, which can't be written to.

Change it to char s1[20] = "Front"; and it should work out like you expect - as long as you are adding no more than 14 characters.

Answer 2

阅读有关strcat ，并记住文字字符串（例如"Front"和"Back" ）是不可修改的。

Answer 3

You need space for the result, like this:

#include <stdio.h>
#include <string.h>
int main()
{
    char *s1="Front";
    char *s2="Back";
    char s3[80];
    s3[0] = '\0';
    strcat(s3,s1);
    strcat(s3,s2);
    puts(s3);
    return 0;
}

Answer 4

The code gives an error because concatenating s1 with s2 will require the allocated memory for s1 to be >= the combined lengths of the two byte arrays. strcat will attempt to iterate over memory that wasn't assigned to s1 , thus causing a runtime error. The only way this will work through pointers is if you make str3 a char array where the size is known at compile time (or a pointer to a dynamically-allocated one for runtime):

char s1[10] = "Front";
char *s2 = "Back";

char *s3 = strcat(s1, s2);

Answer 5

You can do it this way too. Though it is not better than strcat() but it is implemented using pointers only.

#include<stdio.h>
#define SIZE 20
void concat(char *,char *);

int main()
{
    char string1[SIZE]="\0",string2[SIZE]="\0";

    printf("Enter String 1:\n");
    gets(string1);
    printf("Enter String 2:\n");
    gets(string2);

    concat(string1,string2);

    return 0;
}
void concat(char *str1,char *str2)
{
    char *conc=str1;

    while(*str1!='\0')
        str1++;

    *str1=' ';
    ++str1;

    while(*str2!='\0')
    {
        *str1=*str2;
        str1++,str2++;
    }

    *str1='\0';
    printf("Concatenated String:\n");
    puts(conc);

}