unable to initialize array in c | warning: conversion to long unsigned from long

Question

I'm trying to write a program which starts by creating a 2D array that will represent a graph (note that the dimensions of the graph will be given as input).

I want to initialize this array at 0 and after googling it I found out that you have to do it with memset(), cause the memory for the array will be dynamically allocated from the input file.

Below is a small part of my program so far:

long int N;

int main(int argc, char *argv[]) {
    long int i;

    FILE * txt;
    txt = fopen(argv[1], "r");
    if (txt == NULL) {
        printf("There was an error trying to open the file.\n");
        return 0;
    }

    fscanf(txt, "%li", &N);

    long int graph[N][N];
    memset(graph, 0, N*N*sizeof(long int));

    return 1;
}

When I'm compiling it with gcc -Wconversion i'm getting the following warning:

warning: conversion to ‘long unsigned int’ from ‘long int’ may change the sign of the result [-Wsign-conversion] memset(graph, 0, N*N *sizeof(long int));

I can't understand at which point of the code the compiler is trying to convert a long int to an unsigned one. I searched it and people had the same problem only when they had variables of different types mixed.

Any help would be appreciated!

---EDIT---

With the help of 'Vlad from Moscow' I was able to solve the above warning (I had to change N to size_t type in order to call memset() correctly), but now I want to initialize another long int array to LONG_MAX and I get a different one:

memset(dist, LONG_MAX, N*sizeof(long int));

warning: overflow in implicit constant conversion [-Woverflow]

Answer 1

The function memset expects the third argument of the type size_t. So declare the variable N as

size_t N;

and write

fscanf(txt, "%zu", &N);

The second warning is related to the second argument of the call of memset

memset(dist, LONG_MAX, N*sizeof(long int));

that expects it of the type int that is internally within the function is converted to the type unsigned char.

Here is the function declaration.

void *memset(void *s, int c, size_t n);

Usually the only applying of the function is to zero-initialize an array.