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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? ?
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1
  • The return type of strlen() is size_t. You will find greater portability with size_t len1,len2,i,j Commented Nov 23, 2013 at 15:47

2 Answers 2

Reset to default
4

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.
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7 Comments

thanx...its working now....if not clrscr() and getch(), <conio.h> header,then how which statements must replace them?
if i declare string length as 10 instead of 20, i get output:enter string 1: i love ; enter string 2 : my india ; output string : i love m ....does not work
@user4815162342 Substituting gets() with fgets(), though a good idea has a side effect: fgets() retains the '\n'. gets() does not. An closer alternative is gets_s().
instead of gets i have usedfgets(stdin, string1, sizeof(string1)); gives error1:type mismatch in parameter '_n' in call to 'fgets'; error 2:type mismatch in parameter '_n' in call to 'fgets'` and using gets_s gives undefined symbol _gets_s
@chux Good point, I've now amended the answer to mention the trailing newline. gets_s only made it to C11, and is still not available on many systems - for example, my fairly up-to-date ArchLinux doesn't have a manpage for gets_s, nor is it present in the system include files.
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3

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).

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2 Comments

i know about strcat but i want to accomplish it this way.
Given OP's char string1[20],string2[20], "enter string 1 : this program does" and "enter string 2 : not give output", the suggested strcat(string1,string2) writes outside string1[].

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