concatenating strings using malloc

Question

This is a program to concatenate strings using malloc

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

char *sconcat(char *ptr1,char *ptr2);

void main()
{
char string1[20],string2[20],*ptr;
clrscr();

printf("enter string 1: ");
gets(string1);

printf("enter string 2: ");
gets(string2);

ptr=sconcat(string1,string2);

printf("output string : %s",ptr);
getch();
}

char *sconcat(char *ptr1,char *ptr2)
{
int len1,len2,i,j;
char *ptr3;

len1=strlen(ptr1);
len2=strlen(ptr2);

ptr3=(char *)malloc((len1+len2+1)*sizeof(char));

for(i=0;ptr1[i]!='\0';i++)
ptr3[i]=ptr1[i];

j=i;i=0;
for(;ptr2[j]!='\0';j++,i++)
ptr3[j]=ptr2[i];

ptr3[j]='\0';
return(ptr3);
}

output:
enter string 1 : this program does
enter string 2 : not give output
output string : this program does 

What correction is needed to concatenate strings. When I use char string1[20],string2[20],*ptr; after void main() ,

output:
enter string 1 : is this 
enter string 2 : correct ?
output string : correct? ?

Answer 1

The test in your second for loop is incorrect; it should be ptr2[i] != '\\0' , not ptr2[j] != '\\0' .

Several remarks on the code:

• Don't cast the return value of malloc .
• sizeof(char) is 1 by definition, so multiplying by sizeof(char) only makes the code harder to read.
• Declare the parameters of sconcat as const char * , since they're not modifying the strings they receive.
• malloc can return NULL; you must handle that case in your program, for example by displaying an error message and exiting.
• gets is unsafe and will crash your program if the user enters more characters than were allocated. Replace gets(string) with fgets(string, sizeof(string), stdin) , and getting rid of the trailing newline.
• clrscr() and getch() , as well as the infamous <conio.h> header, are not standard C and are non-portable; avoid them in simple programs like this one.

Answer 2

You can more simply use strcat

printf("enter string 1: ");
gets(string1);

printf("enter string 2: ");
gets(string2);

strcat(string1,string2);

It would, however, change string1 so you might want to use strcpy too (to copy string1 to another string and then return it).