passing a pointer from a function help needed

[needchelp]

Hi,
I'm very new to C and need help with what I'm doing wrong here and why this piece of code doesn't work (the total column prints rubbish):

/* Total 2 arrays into a 3rd array in a function and return the pointer for the first element of the 3rd array. Display the 1st, 2nd and 3rd array in columns */

#include <stdio.h>
#define MAX34 5
 
long array3[MAX34] = {2,3,2,3,2};
long array4[MAX34] = {3,4,3,4,3};
long *point_a, *point_b; 
long ctr = 0;
 
long * addarrays(long d[], long e[], long len);
 
main()
{
 
point_a = addarrays(array3, array4, MAX34);
 
point_b = point_a;
 
for (ctr = 0; ctr < MAX34; ctr++)
{
printf("\n\t%ld\t\%ld\t%ld\n", array3[ctr], array4[ctr], *(point_b++));
}
 
 
return 0;
}
 
 
long * addarrays(long d[], long e[], long len)
{
long ctr = 0;
long array5[] = {0,0,0,0,0};
long *newpoint;
 
newpoint = array5;
 
for (ctr = 0; ctr < len; ctr++)
{
array5[ctr] = array3[ctr] + array4[ctr];
}
 
return newpoint;
}
 

[Banfa]

long * addarrays(long d[], long e[], long len)
{
    long ctr = 0;
    long array5[] = {0,0,0,0,0};
    long *newpoint;
 
/* newpoint points to array5, array5 is on the stack */
    newpoint = array5;
 
    for (ctr = 0; ctr < len; ctr++)
    {
/* You are using your global variables in stead of the function parameters */
        array5[ctr] = array3[ctr] + array4[ctr];
    }
 
/* You return newpoint, but newpoint points to array5 and array5 is on the stack.  As soon as the function returns array5 is released so you are returning a pointer to data that has just been relesed and is almost certainly used for something else by printf.  When returning a pointer it is imperative to make sure that the data you are returning a pointer to still exists after the function has ended.  In this particular case I would say the best thing would be to pass as a 4th parameter a pointer to the location to store the result of the function, however another option is to malloc data for the function to return but the the calling function has to remember to free it. */
    return newpoint;
}
 

[needchelp]

Thanks for the reply, but I'm a bit lost. This was an exercise from "Teach yourself C in 21 days" - day 9, before malloc has been discussed. Unfortunately, it's an ON YOUR OWN exercise, so no answer is given at the back of the book, and the chapter doesn't have any examples of how to do this. I don't really understand your first option - would it be rude to ask for an example?

[Banfa]

Instead of

#include <stdio.h>
#include <string.h>
 
static char *ReturnHelloWorld(void);
 
int main()
{
    printf("%s\n", ReturnHelloWorld());
 
    return 0;
}
 
char *ReturnHelloWorld(void)
{
    char string[50];
 
    strcpy(string, "Hello World");
 
    return string;
}
 

which would have the same problem as your code, returning a pointer to an object that no longer exists do this

#include <stdio.h>
#include <string.h>
 
static void ReturnHelloWorld(char *buffer);
 
int main()
{
    char string[50];
 
    ReturnHelloWorld(string)
 
    printf("%s\n", string);
 
    return 0;
}
 
void ReturnHelloWorld(char *buffer)
{
    strcpy(buffer, "Hello World");
}
 

Now the result buffer is passed to the function by the caller. The caller has the responsibility for the data. This would be even better if ReturnHelloWorld also took a buffer length and ensured that the end of the buffer was not overwritten thus ensuring the function could not cause memory corruption.

Try both examples (there may be some compilation errors I have not been able to do a test compile as I have just switched from Windows to Linux and I have yet to get a development environment setup again).

[needchelp]

thanks, I'll try those.

[needchelp]

Thanks for your help - got it working at last.