C - 用指针连接字符串

Question

why does the following string concatenation does not work?

main()
{
char *str1 = "United";
char *str2= "Front";
char *str3;
str3 = strcat(str1, str2 ) ;
printf("\n%s",str3 );

}

I got this problem in exercise questions in one of a book on pointers. The question mentions

[Q] Is the code correct if not why and also correct the code.

Answer 1

See my answer to the question concatenation of character arrays in c

You may not change string literals.

This statement

str3 = strcat(str1, str2 ) ;

tries to change the string literal str1 and moreover tries to write beyond the string literal.

To make a concatenated string you have to allocate a memory large enough to contain the both strings.

What you need is the following

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

int main(void) 
{
    const char *str1 = "United";
    const char *str2 = "Front";

    char *str3 = malloc( strlen( str1 ) + strlen( str2 ) + 1 );

    strcpy( str3, str1 );
    puts( strcat( str3, str2 ) );

    free( str3 );

    return 0;
}

The program output is

UnitedFront

Answer 2

When you write char* s = "something" a piece of read-only memory is allocated. More on this here .

The declaration of strcat looks like this:

char *strcat( char *dest, const char *src );

Basically what it will do is append src to dest , but since your destination, str1 does not have enough memory to hold both strings.

So what I would do is, either use snprintf with a pre-allocated buffer or:

char *str1 = "United";
char *str2 = "Front";

char *buf = calloc(strlen(str1) + strlen(str2) + 1, sizeof(char));

strncpy(buf, str1, strlen(str1));
strncat(buf, str2, strlen(str2));

printf("%s", buf);

Or with snprintf :

char *str1 = "United";
char *str2 = "Front";

int buf_len = strlen(str1) + strlen(str2) + 1;
char *buf = calloc(buf_len, sizeof(char));

snprintf(buf, buf_len, "%s%s", str1, str2);

Answer 3

The first parameter of strcat ( str1 in your case) needs to have enough allocated memory to hold the concatenated string. You can either create it with malloc or declare it as an array with a big enough size.