Question

As you can see below I have created a little program to concatenate 2 strings using C, as you may imagine this code doesn’t work, I have already corrected it myself by using Array notation instead of pointers, and it works just fine, however I’m still not sure why is it that my code fails being almost a replica of my corrected code.

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

void concatena(char *str1, char *str2){
 char *strAux;
 int mover;
 mover = 0;
 strAux = (char *)(malloc(strlen(str1) + strlen(str2)+2));
 *(strAux) = '\0';
 if(str1 == '\0')
    *strAux = '\0';
 else
     while(str1 != '\0'){
         *(strAux+mover++)=*(str1++);
     }
 if(str2 == '\0')
         *strAux = '\0';
 else
     while(str2 != '\0'){
         *(strAux+mover++)=*(str2++);
     }
 strAux='\0';
 str1=strAux;
 printf("%s", str1);
 free(strAux);
 }

I´m still a C beginner (And yes, I’m aware that there are libraries like string.h, I’m asking this for academic reasons) and I have been told that char pointers and arrays are the same thing, something that confuses the heck out of me.

Any help is greatly appreciated.

Answer 1

The first problem I see is with this section:

if(str2 == '\0')
    *strAux = '\0';

Just before this code, you’ve filled up strAux with the string from str1.
Then, if str2 is empty, you suddenly put a null-terminator at the beginning of strAux, eliminating all the work you’ve done so far!

I think what you intend is:

if(*str2 == '\0')
     *(strAux+mover) = '\0';

Its the same thing again after your loop for str2, you have the code:

strAux='\0';

Again, this puts a null-terminator at the start of strAux, effectively ending the newly created string before it even gets started.

---

Here’s how I’d re-write your code:

void concatena(char *str1, char *str2){
    char *strAux;
    int mover = 0;

    strAux = (char *)(malloc(strlen(str1) + strlen(str2)+1));  // Changed to +1, NOT +2
    *(strAux) = '\0';   // Start the string as (empty)

    while(*str1 != '\0'){   // Copy the first string over.
         *(strAux+mover++)=*(str1++);
    }

    while(*str2 != '\0'){   // Copy the second string over.
         *(strAux+mover++)=*(str2++);
    }

    *(strAux+mover)='\0';  // End the new, combined string.

    printf("%s", strAux);  // Show the results.
    free(strAux);
 }