c- Strange behavior after getting inputs

Question

I have to make a program that gets as an input a and b, gets as input a number "a" of lines of the following form: "studentId studentName studentPhone" and b lines of input in the form "stId mark1 mark2 mark3". The program then outputs all the stid from the first input and if the same id exists in input b the program outputs the students marks besides its id.

I've gone through hell to getting the input properly and I think this is close but I get a strange behavior: after I input the marks in the second input it seems like some of the student ids in the first input are changed.

This is my code: (here i try to input just the student IDs. http://ideone.com/dBYzwe )

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <malloc.h>


void chomp(char *s);

struct listA{
    int stId;
    char stName[19];
    char stPhone[12];

    };

struct listB{
    int stId;
    char marks[11];

};


int main(void) {
    int i, j, m, n;

    scanf("%d%d", &m, &n);

struct listA *a = malloc(sizeof(m * sizeof(struct listA)));
    struct listB *b = malloc(sizeof(n * sizeof(struct listB)));

    for(i = 0; i < m; i++)
    {
        scanf("%d", &a[i].stId);
        fgets(a[i].stName, 19, stdin);
        fgets(a[i].stPhone, 11, stdin);
        chomp(a[i].stName);
        chomp(a[i].stPhone);

    }
    for(i = 0; i < m; i++)
            printf("%d ", a[i].stId);


    for(i = 0; i < n; i++)
    {
        scanf("%d ", &b[i].stId);
        fgets(b[i].marks, 12, stdin);
        fflush(stdin);
    }

    printf("\n");


        for(i = 0; i < n; i++)
    {
        printf("%d ", b[i].stId);
    }


    printf("\n");

    for(i = 0; i < m; i++)
            printf("%d ", a[i].stId);




    return 0;









}
void chomp(char *s) {
    while(*s && *s != '\n' && *s != '\r') s++;

    *s = 0;
}

Answer 1

The problem is in the

struct listA *a = malloc(sizeof(m * sizeof(struct listA)));
struct listB *b = malloc(sizeof(n * sizeof(struct listB)));

The result of m * sizeof(struct listA) is an integer, so when you put that into sizeof you get the size of the integer, not the number you want. You should change this to:

struct listA *a = malloc(m * sizeof(struct listA));
struct listB *b = malloc(n * sizeof(struct listB));