Question

I am back to C from Java and C#. I am stuck with the following simple program attempting to read two arrays from a file with a function. Can anyone point out where am I messing up?

Compiler says:
error: invalid operands to binary * (have ‘int *’ and ‘int *’)

The file format is

    4
    1   2   3   4
    23  23  14  11

My ReadFromFile function needs to fill buffers A and B from the file.

#include<stdio.h>

void ReadFromFile (const char* file_name, int *A, int *B, int *length)
{
    FILE* file = fopen (file_name, "r");
    fscanf (file, "%d", length);            
    int i;
    for(i = 0; i < length; i++)
    {
        fscanf (file, "%d", A+i); 
    }  
    for(i = 0; i < length; i++)
    {
        fscanf (file, "%d", B+i); 
    }   
    fclose (file);        
}

int main()
{
    int *A; int *B; int length;
    ReadFromFile("input.txt", A, B, &length);

    return 0;
}

Answer 1

void ReadFromFile (const char* file_name, int *A, int *B, int *length)
/* ... */
    for(i = 0; i < length; i++)
    {
        fscanf (file, "%d", A+i); 
    }

You have passed in a single integer A from main(), but here you are trying to access completely unrelated memory. Same goes for B. Did you mean to allocate A and B as arrays?

e.g. change:

int *A; int *B; int length;

to

int A[100], B[100], length;

or something similar. (Perhaps dynamically allocate the arrays once you know how many you need — or allocate them with malloc(3) and grow them with realloc(3) if you need to.)