Question

Hello and thanks in advance,

In short, what I do is create a pointer which creates a local variable on the Stack and then I request memory by malloc which allocates space in the heap.

Then I create a local int on the stack.

Then I request users input through scanf.

But upon entering the last digit, the console vanishes and does not print me the result even if I try to hold it through getchar()

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX_LINE 80

int main()
{
    char *student = malloc(MAX_LINE * sizeof(char));
    int grade;

    printf("Enter students name ");
    scanf("%s", student);

    printf("Enter students grade ");
    scanf("%d", grade);// i would say it has to be grade and not &grade
    // it vanishes right after here. Cant get to see any result
     printf("%s received a %d\n", student, grade);

    free(student);
    getchar();



}

Answer 1

scanf("%d", grade); should be scanf("%d", &grade);

You should pass the address of the variable/buffer where you want to store the input element. And grade refers to the content, there the & addressof operator operating on grade &grade gives us the address of the variable.

In case of scanf("%s", student); the student itself refers to the address , ie the contents of the pointer variable student which contains the base address of the memory block you just allocated. Therefore this is correct.

The arguments should always point to the address of the location of the variable/buffer where you want scanf to put that particular section of that input.

Say initially you have the garbage 0x5964 stored in grade, and the memory address of grade is 0x1234abcd. In scanf ("%d", grade); , scanf would try to write the read in integer into the memory location 0x5964, which is invalid.

Whereas in scanf ("%d", &grade); , scanf would store the read in integer into the memory location 0x1234abcd. After this referring to grade as r-value would get you the contents on the memory location 0x1234abcd , this is what you want.

   +----------+                                   
   | 0x5964   |  <-------(uses this as address)---+-------------------+
   +----------+  <------+                         |                   |
   |  grade   |     (writes)       scanf ("%d", grade);               |  
   +----------+      (here)        scanf ("%d", &grade);              |
   |0x1234abcd|         |                         |                   |
   +----------+  <------+(uses this as address)---+                   |
                                                                      |
                                                                      |
   +----------+                                                       |
   |  ??????  | <----------(tries to write here)----------------------+
   +----------+
   |   ???    |
   +----------+
   |  0x5964  | 
   +----------+