remaking strcat, changes not reflecting to pointer - c

Question

Hello guys so I am learning C and I am creating the strcat function and when I print out the values of dest at the index i concatenate a char at I get that char but when I return dest and print it out back in my main function the changes aren't reflected. Can someone please help me out? thanks.
#include <stdio.h>
#include <stdlib.h>
int size_s(char *str) {
int size = 0;
int index = 0;
while (str[index] != '\0') {
size += 1;
index += 1;
}
return (size + 1);
}
/*
* #function: strcat
* #desc: Takes in two char pointers and concatenates them. provided the destination has enough size otherwise undefined behavior can occur. Overwrites the null terminator
*/
char *strcat_s(char *dest, char *source)
{
int index_of_src = 0;
int index_of_dest = size_s(dest);
while (source[index_of_src] != '\0') {
*(dest + index_of_dest) = source[index_of_src];
index_of_src += 1;
index_of_dest += 1;
}
// Add Null terminator
*(dest + (index_of_dest + 1)) = '\0';
return dest;
}
int main(int argc, char **argv) {
char firstname[100];
scanf("%s", firstname);
char lastname[100];
scanf("%s", lastname);
int sizeofFirst = size_s(firstname);
printf("Sizeof first: %d\n", sizeofFirst);
int sizeofSecond = size_s(lastname);
printf("Sizeof second: %d\n", sizeofSecond);
char *concatinated = strcat_s(firstname, lastname);
printf("%s\n", concatinated);
}

Answer 1

The function size_s returns the index of the character after the zero-terminating character '\0' due to this return statement
return (size + 1);
So in this while loop
int index_of_src = 0;
int index_of_dest = size_s(dest);
while(source[index_of_src] != '\0')
{
*(dest + index_of_dest) = source[index_of_src];
index_of_src += 1;
index_of_dest += 1;
}
the array pointed to by the pointer dest is filled after the terminating zero character '\0'.
As a result this call of printf
printf("%s\n", concatinated);
outputs the initially stored string in the array firstname.
Rewrite the function size_s the following way
size_t size_s( const char *s )
{
size_t n = 0;
while ( s[n] != '\0' ) ++n;
return n;
}
In turn the function strcat_s that should be renamed because there is standard function strcat_s can look for example the following way
char * strcat_str( char *dest, const char *source )
{
size_t n = size_s( dest );
while ( ( *( dest + n++ ) = *source++ ) != '\0' );
return dest;
}

Answer 2

There are multiple issues in your code:
the size_s function really computes the size of the string, including the null terminator, but counting the null terminator is not helping for the task at hand, you should instead compute the length of the string, ie: the number of bytes before the null terminator, which is exactly the offset where to copy the second string at the end of the first.
*(dest + (index_of_dest + 1)) = '\0'; does not store the null terminator at the correct place: it places it one step too far. You should write *(dest + index_of_dest) = '\0'; or simply dest[ndex_of_dest] = '\0';
the name strcat_s may conflict with a library function of the same name defined in the infamous Annex K of the C Standard. A different name is preferable.
scanf("%s", firstname); is a security flaw: sufficient long input will cause a buffer overflow and carefully crafted input may allow the user to execute arbitrary code. Use scanf("%99s", firstname); to avoid this.
Here is a modified version:
#include <stdio.h>
#include <stdlib.h>
int my_strlen(const char *str) {
int index = 0;
while (str[index] != '\0') {
index += 1;
}
return index;
}
/*
* #function: strcat
* #desc: Takes in two char pointers and concatenates them. provided the destination has enough size otherwise undefined behavior can occur. Overwrites the null terminator
*/
char *my_strcat(char *dest, const char *source) {
int index_of_src = 0;
int index_of_dest = my_strlen(dest);
while (source[index_of_src] != '\0') {
dest[index_of_dest] = source[index_of_src];
index_of_src += 1;
index_of_dest += 1;
}
// Add the null terminator
dest[index_of_dest] = '\0';
return dest;
}
int main(int argc, char **argv) {
char firstname[200];
char lastname[100];
if (scanf("%99s %99s", firstname, lastname) != 2)
return 1;
printf("length of first: %d\n", my_strlen(firstname));
printf("length of second: %d\n", my_strlen(lastname));
char *concatenated = my_strcat(firstname, lastname);
printf("%s\n", concatenated);
printf("length of concatenation: %d\n", my_strlen(concatenated));
return 0;
}