How does strcat() fucntion actually works and its alternate

Question

Let's take an example.

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

int main() {
    char str1[7] = "hello ";
    printf("Initial size of str1 is: %d\n", strlen(str1));
    char str2[] = "buddy";
    printf("%s\n", strcat(str1, str2));
    printf("Final size: %d\n", strlen(str1));
}

The output of the above program will be

Initial size of str1 is: 6
hello buddy
Final size: 11

--------------------------------
Process exited after 0.835 seconds with return value 0
Press any key to continue . . .

See? how the size of str1 changed from 7 to 11 (including null variable), regarding that what I think would have happened is :

• Some function I do not know may have reallocated contiguous memory for str1 starting from same address as before i.e str1 with size strlen(str1)+strlen(str2)+1 +1 for null value, and then redefined it to get hello buddy.

If I am wrong please tell, if not then, what function is it and how does it work?

One more question: how can I write a code to do the above task without the use of strcat function.

I tried doing it using realloc() but didn't quite succeed may be that's because realloc() can only reallocate dynamically allocated memory, is it so?

Answer 1

Buffer overflow

OP's code fails as strcat(str1,str2) attempts to write past the end of str1[] - result: undefined behavior (UB). @dimich

Instead use a larger destination buffer.

// char str1[7]="hello ";
char str1[7 + 5]="hello ";
char str2[]="buddy";
printf("%s\n",strcat(str1,str2));

Use correct print specifier

strlen() returns a size_t, not an int.

// printf("Initial size of str1 is: %d\n",strlen(str1));
printf("Initial size of str1 is: %zu\n",strlen(str1));

Tip: enable all warnings.

Alternative

One of many alternatives: copy str2 to the end of str1.

// printf("%s\n",strcat(str1,str2));
strcpy(str1 + strlen(str1), strt2);
printf("%s\n",str1);

realloc()

realloc() can only reallocate dynamically allocated memory, is it so?

realloc() should not be used on pointers to non-allocated, non-NULL pointers.
In addition to re-allocating dynamically allocated memory, realloc() can start with no prior allocation.

char *p = realloc(NULL, size);
// just like
char *p = malloc(size);

Moral of the story

• Be mindful of memory usage with string functions.
• Enable all warnings.