C - Integer Input Validation Code

I have the following code in C:

#include "stdafx.h"
#include <stdlib.h>
#include <cstring>
#include <ctype.h>

int main()
{
    char buffer[20];
    int num;
    bool valid = true;

    printf("Please enter a number\n");
    fgets(buffer, sizeof(buffer), stdin);
    printf("\n\n");

    if(!isdigit(buffer[0])) //Checking if the first character is -
    {
        if(buffer[0] != '-')
        {
            valid = false;
        }

        else
        {
            if(!isdigit(buffer[1]))
            {
                valid = false;
            }
        }
    }

    char *pend = strrchr(buffer, '\n'); //Replacing the newline character with '\0'

    if (pend != NULL)
    {
        *pend = '\0';
    }

    for (int i = 1; i < strlen(buffer); i++) //Checking that each character of the string is numeric
    {
        if (!isdigit(buffer[i]))
        {
            valid = false;
            break;
        }
    }

    if(valid == false)
    {
        printf("Invalid input!");
    }

    else
    {
        num = atoi(buffer);
        printf("The number entered is %d", num);
    }
    getchar();
}

Basically, the code ensures that the user input is a positive or negative whole number. No letters, floating point numbers etc. are allowed.

The code works perfectly and does its job well.

However, the code is too long and I have to implement it in a number of programs. Is there a simple way to perform all of the above in C? Maybe a shorter alternative that ensures that the input is:

i) not a letter ii) a positive or negative WHOLE number