Charlie Palmer Charlie Palmer - 3 months ago 9
C Question

Passing list to a function to fill the list with variable number of members in C

I have written code to generate a list of (x, y, z) coordinates of a given length. The code functions if I generate this list in a main function, but passing a pointer to the list into a fill_xyz_list function produces a runtime error. Presumably there is an issue with dynamic memory allocation or pointers, but I can't find it!

What is the correct way to do this?

Background:
I am trying to generate a walk cycle trajectory for a robotic leg, consisting of a finite list of (x, y, z) coordinates. As the trajectory resolution is dynamic, the list is of unknown length.

The following code produces a runtime error:

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

// core data structure
struct Point
{
float x;
float y;
float z;
};

// function that allocates space for our array and fills it with dummy values
void fill_xyz_list(struct Point **xyz_list_ptr, int list_size)
{
// allocate memory for 100 Point structs
*xyz_list_ptr = realloc(*xyz_list_ptr, sizeof(struct Point)*list_size);

// set values for each member
int i;
for (i=0; i<list_size; i++)
{
xyz_list_ptr[i]->x = i + 0.1;
xyz_list_ptr[i]->y = i + 0.2;
xyz_list_ptr[i]->z = i + 0.3;
}
}

int main()
{
struct Point *xyz_list = NULL; // our array of (x, y, z)
int list_size; // our array size
int i;

// set list size
list_size = 10;

// fill xyz_list array with dummy values
fill_xyz_list(&xyz_list, list_size);

// print all members
for (i=0; i<list_size; i++)
{
printf("xyz_list[%d]: x=%.2f, y=%.2f, z=%.2f\n", i, xyz_list[i].x, xyz_list[i].y, xyz_list[i].z);
}

return 0;
}

Answer

This line

xyz_list_ptr[i]->x = i + 0.1;

should be

(*xyz_list_ptr)[i].x = i + 0.1;

Otherwise you are interpreting xyz_list_ptr as an array of pointers instead of interpreting it as a pointer to an array, which it really is.

Demo.

Note: Assigning realloc back to the pointer being reallocated could lead to memory leaks. You should assign it to a temporary, check it for NULL, and only then assign the temporary to the original pointer:

struct Point *tmp = realloc(*xyz_list_ptr, sizeof(struct Point)*list_size);
if (!tmp) {
    ... // deal with allocation failure
    ... // exit the function
}
*xyz_list_ptr = tmp;
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