Accessing values from user-defined functions - c

Question

I've got a (I think) pretty easy question that I need help with. I'm working on learning pointers with called user-defined functions in C.
I've got two functions, my main() and another which pulls certain data from a file, read_file(). Read_file() pulls data from a specified file - an integer, (coeff_num) and a series of values (coeff_values) - and prints out the data. Then main() is meant to reprint this data as a check, to ensure that it's being called correctly in order to perform further calculations.
My problem is this: everything is printing out fine in the read_file() function. While checking in main(), data_num is printing out fine, but data_values isn't.
The code (with some irrelevant bits edited out, mainly error checking stuff) is as follows:
int read_file(int *data_num, double **data_values, char *filename);
int main()
{
int data_num;
double **data_values;
char *filename[] = {/*FILES*/};
read_fule(&data_num, data_values, *filename);
printf("Check Number:\n%u\n", data_num);
printf("Check Values:\n);
for(int i = 0; i<data_num; i++)
{
printf("%u\t", data_values[i]);
}
return(0);
}
and:
int read_file(int *data_num, double **data_values, char *filename)
{
FILE * fp = fopen(filename, "rb"); //Opening file
/* Error checking code here - see if file opens, find length
return if not long enough, etc */
//Find number of values
char buffer_n[4];
fread(buffer_n, 4, 1, fp);
int result = buffer_n[0]|(buffer_n[1] << 8)|(buffer_n[2] << 16)|(buffer_n[3] << 24); //Change endian
*data_num = result;
/* Instructed to allocate memory */
data_values = malloc(8*result); //Allocating memory
//Reads data values
fread(data_values, 8, result, fp);
//Prints results as float
printf("Number of data values: %d \n", result);
for(int i=0; i<result; i++)
{
printf("%0.3f\t", data_values[i]);
}
fclose(fp);
return 1;
}
I'm fairly certain that this is an issue with my pointers. For the program, I have to use malloc() when reading data_values, and read_file has to have the prototype of: int read_file(int *data_num, double **data_values, char *filename). Can someone give me some guidance about where I've messed up?
Cheers for any help, it'd be greatly appreciated.

Answer 1

I set result = 30 as you told me, changed a few places, and I think now it works fine.
I think you don't need double** pointer, what you need is double* pointer.
And you need cast the void* to double*:
data_values = (double*)malloc(8*result); //Allocating memory
This code works fine:
//data_values should be defined as double*
int read_file(int *data_num, double *data_values, char *filename)
{
FILE * fp = fopen(filename, "rb"); //Opening file
/* Error checking code here - see if file opens, find length
return if not long enough, etc */
//Find number of values
char buffer_n[4];
size_t i;//added
fread(buffer_n, 4, 1, fp);
//int result = buffer_n[0]|(buffer_n[1] << 8)|(buffer_n[2] << 16)|(buffer_n[3] << 24); //Change endian
int result = 30;
*data_num = result;
/* Instructed to allocate memory */
data_values = (double*)malloc(8*result); //Allocating memory
//Reads data values
fread(data_values, 8, result, fp);
//Prints results as float
printf("Number of data values: %d \n", result);
for(int i=0; i<result; i++)
{
printf("%0.3f\t", data_values[i]);
}
fclose(fp);
return 1;
}
Edit:
If you want to use double**, you can try this solution:
/* Instructed to allocate memory */
data_values[0] = (double*)malloc(8 * result); //Allocating memory
//Reads data values
fread(data_values[0], 8, result, fp);
//Prints results as float
printf("Number of data values: %d \n", result);
for(int i=0; i<result; i++)
{
printf("%0.3f\t", data_values[0][i]);
}
But you have to notice that this is not a form, this is still used as an array, so you may change it later according to your situation.