send strings into function

hello everyone this is my first post here :)

how i correctly make dynamic allocation to string and then make input from the user ?
lets say that i dont know the length of the string.

here is the code i wrote , when i input the strings i dont get an error when i allocate small amount of memory , how i make that correctly

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


void stringMerge(char*, char*, int n);


int main()
{
char *strA, *strB;
int insert_pos;


strA = (char *)malloc(3);
strB = (char *)malloc(3);
printf("enter the first string\n");
gets(strA);
printf("enter the second string\n");
gets(strB);
printf("enter the insert pos of the string\n"); //
scanf("%d", &insert_pos);


stringMerge(strA,strB,insert_pos);
}


void stringMerge(char *a,char *b ,int n)
{

int newString_lenth = strlen(a) + strlen(b) + 1;
char* newString = (char*)malloc(newString_lenth);



strncpy(newString, a, n);
newString[n] = '\0';
strcat(newString, b);
strcat(newString, a + n);
printf("%s\n", newString);


getch();


}

**orginal problem sorted, i updated the qestion.

Be sure to use code tags, and that your code is neatly formatted and indented.

Code:

---

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


void stringMerge(char*, char*, int n);


int main()
{
    char *strA, *strB;
    int insert_pos;

    strA = (char *)malloc(3);
    strB = (char *)malloc(3);

    printf("enter the first string\n");
    gets(strA);

    printf("enter the second string\n");
    gets(strB);

    printf("enter the insert pos of the string\n"); //
    scanf("%d", &insert_pos);

    stringMerge(strA,strB,insert_pos);
}

void stringMerge(char *a,char *b ,int n)
{
    int newString_lenth = strlen(a) + strlen(b) + 1;
    char* newString = (char*)malloc(newString_lenth);

    strncpy(newString, a, n);
    newString[n] = '\0';

    strcat(newString, b);
    strcat(newString, a + n);
    printf("%s\n", newString);

    getch();
}

---

You're only allocating enough space for 3 characters, one of which must be the string-terminating null character '\0'. You should use a reasonably large size if you don't know what the length of the input would be.

Additional comments:

• Do not cast malloc(): FAQ > Casting malloc - Cprogramming.com
• Do not use gets(): FAQ > Why gets() is bad / Buffer Overflows - Cprogramming.com
• Be sure to free() any memory you allocate
• conio is an obsolete and system-specific header that you don't need to use in this program

Another potential problem is that the function could be called in such a way that the expected result is a concatenation, A + B, which would make doing the third step incorrect. It's a simple check.

Code:

---

// Assuming result is a zero'd out buffer of sufficient length
    if (result != NULL)
    {
        strncpy(result, a, n);
        strncat(result, b, lenB);
        if (n < lenA)
        {
            strncat(result, a + n, lenA - n);
        }
    }

---

thank you for your answers .
on my code example , it works even with space for 3 chars , i inputing a large string and the code works fine , why is that ?

Quote:

---

Originally Posted by Vladi Pikovski

thank you for your answers .
on my code example , it works even with space for 3 chars , i inputing a large string and the code works fine , why is that ?

---

Pure dumb luck.

If you overrun the bounds of where you know you can access, then the program is broken.
When the problem shows up can be anywhere between microseconds or years.
Where the problem shows up is usually in some unrelated part of the program (which makes debugging malloc related memory issues notoriously difficult).

Quote:

---

Originally Posted by Vladi Pikovski

thank you for your answers .
on my code example , it works even with space for 3 chars , i inputing a large string and the code works fine , why is that ?

---

Windows (Especially older versions) is notorious for not trapping for buffer overruns!

You are probably using a compiler that uses the C89/90 Standard. The use of gets() would have flagged a warning in C99, (Depreciated) and would be an error in a C11 Standard compliant compiler. gets() has been removed completely from the C11 Standard Library!!!

Please think about installing a newer compiler that is C99 / C11 compliant.

Also, please turn on and turn up your warning level to the highest setting to see any errors and warnings the compiler can detect.