C program to concatenate two strings

Question

I'm trying to concatenate two strings together with pointers without using any of the functions.When I enter the two strings for example first hello and then world the output is

hello

world

and not helloworld.Any help would be appreciated.

#include<stdio.h>
#include<stdlib.h>
int main(){
char *s=(char *)malloc(sizeof(char *));
char *s2=(char *)malloc(sizeof(char *));
fgets(s,10,stdin);
fgets(s2,10,stdin);
int i=0;
while(*s){
    s++;
    i++;

}
while(*s2!='\0'){
    *s=*s2;
    s++;
    s2++;
    i++;
}
*s='\0';
printf("%s",s-i);
}

Answer 1

The program has undefined behavior because you did not allocate memory for entered strings.

char *s=(char *)malloc(sizeof(char *));
char *s2=(char *)malloc(sizeof(char *));
fgets(s,10,stdin);
fgets(s2,10,stdin);

You only allocated memory for two pointers ( sizeof(char *) ).

You need to allocate memory large enough that can contains entered strings and their concatenation in the first character array.

The function fgets can append the new line character '\n' to an entered string. You need to overwrite it.

Also you should not change the original pointers because you need to use them to free the allocated memory.

And take into account that the result string will contain at least 11 characters including the terminating zero character '\0' instead of 10 characters if you are going to enter "hello" and "world" and concatenate them. Though in general it is better to reserve 13 characters if the entered strings will not contain the new line character.

The program can look for example the following way

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

int main( void )
{
    enum { N = 7 };
    char *s1 = malloc( 2 * N - 1 );
    char *s2 = malloc( N );
    s1[0] = '\0';
    s2[0] = '\0';

    fgets( s1, N, stdin );
    fgets( s2, N, stdin );

    char *p1 = s1;

    while (*p1 != '\n' && *p1 != '\0') ++p1;

    for (char *p2 = s2; *p2 != '\n' && *p2 != '\0'; ++p2)
    {
        *p1++ = *p2;
    }

    *p1 = '\0';

    puts( s1 );

    free( s1 );
    free( s2 );
}

The program output might be

hello
world
helloworld

Instead of these lines

char *s1 = malloc( 2 * N - 1 );
char *s2 = malloc( N );
s1[0] = '\0';
s2[0] = '\0';

you could write

char *s1 = calloc( 2 * N - 1, sizeof( char ) );
char *s2 = calloc( N, sizeof( char ) );

The arrays are zero initialized to keep empty strings in case when calls of fgets will be interrupted.