Question

I have a ‘runtime error‘ when I try to copy the value of the variable ‘b’ into the variable ‘a’.

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

typedef struct{
        unsigned short a;
}st1;

main()
{
        st1* myStruct;
        unsigned short b = 0xFFFF;

        memcpy(&myStruct->a, &b,sizeof(b));
}

I would like to know why it happens.
Any help would be appreciated.

Answer 1

Because you did not allocate memory for myStruct. You didn’t initialize it, so its value is some random value during memcpy(). Thus, &myStruct->a is accessing some random address, and writing to &myStruct->a is likely to lead to runtime error.