garbage value when using malloc

I have to compare few characters of string say from 2nd char till 4th character( counting starts from zero)

The string is stored in elements of structure say zoopla->real

for example zoopla ->real has '447889036' where real is of type char real[20];

Also please note I cant use function strnstr.

The code works as expected but just for curiousity, I have added printf statement and it shows me value till 4th cahr and then some garabe characters.

I want to know why the printf statment is showing 2 extra garabe values?

char * copyTemp;                   
char *strptr;           
copyTemp = (char *)malloc(sizeof(char)*6);           

strncpy(copyTemp, zoopla->real, 5);              
printf("the string copied is %s", copyTemp); // debug statemnt            
strptr = strstr(copyTemp, "666");           

if(strptr != NULL)              
{         
//some other function        
}        
else        
//some other function    开发者_运维问答         
free(copyTemp);               

All criticism and suggestions are welcome

It seems to me that copyTemp is not null terminated. That's why printf shows you garbage characters after the characters you put in there. It doesn't know where to stop so it continues iterate through memory.

Add

copyTemp[5] = '\0';

after strncpy.

See this example from the documentation of strncpy:

/* strncpy example */
#include <stdio.h>
#include <string.h>

int main ()
{
  char str1[]= "To be or not to be";
  char str2[6];
  strncpy (str2,str1,5);
  str2[5]='\0';
  puts (str2);
  return 0;
}

see http://www.cplusplus.com/reference/clibrary/cstring/strncpy/ No null-character is implicitly appended to the end of destination, so destination will only be null-terminated if the length of the C string in source is less than num.

you have to add null your self.

if you will be allocating memory of constant size then use array only.

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

int main ()
{
 char * copyTemp;                   
char *strptr;           
copyTemp = (char *)calloc(sizeof(char),6);           

strncpy(copyTemp, "88666782921", 5);              
printf("the string copied is %s", copyTemp); // debug statemnt            
strptr = strstr(copyTemp, "666");           

if(strptr != NULL)              
{         
//some other function        
}        
else        
//some other function             
free(copyTemp);        
return 0;  
}

According to my old K&R, strncpy will only implicitly add null bytes if the source string has fewer characters than the number to be copied.

In this case, zoopla->real has more than 5 characters, so the function is simply copying the first five characters. Since you haven't initialized the memory to zero, or explicitly added a null byte, the string is not terminated after the fifth character. So when you print the string, you get additional bytes with essentially random values, until one is hit that happens to be zero.