strlen and free memory

Question

I allocated memory to a pointer to the maximum size of characters it could have. Then I had to write code that will change its values depending on the value that was read from the file and I needed to know what is the length of the value in the pointer, so I used strlen() function.

I got what I needed. The problem occured when I tried to free the memory of that pointer. The program crashed, I'm assuming im doing something "ilegal" and would like to know why and how to fix it.

here is part of the code:

char *StudID = (char*)malloc(sizeof(char)*15);
char *StudIDcpy = (char*)malloc(sizeof(char) * 15);
fread(stud[i].ID, sizeof(char), 4, in);
stud[i].ID[4] = '\0';
IDtemp = atoi(stud[i].ID);//convert ID string to integer and store value in IDtemp
StudIDcpy = itoba(IDtemp);//convert integer to binary number as a string
strcpy(StudID, StudIDcpy);
IDtemp = strlen(StudIDcpy);
free(StudIDcpy); // <---- I BELIEVE THIS IS WHERE IT CRASHES

Here is my itoba() function:

char *itoba(int a){
    int i = 0, j;
    char temp[15];
    while(a){
        if (a % 2)temp[i] = '1';
        else temp[i] = '0';
        i++;
        a = a / 2;
    }
    temp[i] = '\0';
    for (j = 0; j < i / 2; j++)swapc(&temp[j], &temp[i - j-1]);
    return temp;
}

By the way I know I don't have to write sizeof(char) because it is equal to 1, but I write it anyways so I remember what value should be put there.

Answer 1

In your itoba() function, temp, a local array, which decays to a pointer to local variables, is returned.

After a function returns, its local variables are "free"ed immediately, allowing these memory space to be reused by someone else. Consequently, values held by them will soon be overridden by other values on the stack.

You can rewrite itoba() like this:

char *itoba(int a)
{
    int i = 0, j;
    char *temp = malloc(15); // <--- This line is different
    while(a){
        if (a % 2)
            temp[i] = '1';
        else
            temp[i] = '0';
        i++;
        a = a / 2;
    }
    temp[i] = '\0';
    for (j = 0; j < i / 2; j++)
        swapc(&temp[j], &temp[i - j -1]);
    return temp;
}

BTW: You should remove char *StudIDcpy = (char*)malloc(sizeof(char) * 15);, because the pointer value returned by malloc() is later discarded by itoba(IDtemp);. As a result, the memory allocated to StudIDcpy by this malloc() will never be freed, causing memory leak.