C string append

Question

I want to append two strings. I used the following command:

new_str = strcat(str1, str2); 

This command changes the value of str1. I want new_str to be the concatanation of str1 and str2 and at the same time str1 is not to be changed.

Answer 1

You need to allocate new space as well. Consider this code fragment:

char * new_str ; if((new_str = malloc(strlen(str1)+strlen(str2)+1)) != NULL){     new_str[0] = '\0';   // ensures the memory is an empty string     strcat(new_str,str1);     strcat(new_str,str2); } else {     fprintf(STDERR,"malloc failed!\n");     // exit? } 

You might want to consider strnlen(3) which is slightly safer.

Updated, see above. In some versions of the C runtime, the memory returned by malloc isn't initialized to 0. Setting the first byte of new_str to zero ensures that it looks like an empty string to strcat.