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Problem 1. Basic knowledge

1. Read the following code. Which of the following statements is/are true?

#ifndef NULL
#ifdef __cplusplus
#ifndef _WIN64
#define NULL 0
#else
#define NULL 0LL
#endif  /* W64 */
#else
#define NULL ((void *)0)
#endif
#endif

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• The value of NULL is undetermined until running time.
• If we defined NULL somewhere else, this code would report a Compile Error.
• NULL is a pointer.
• NULL will be defined multiple times in this code.
• In C++, NULL is 0, which is not a pointer.

2. Read the following header file. Suppose this file's name is cs100_hw3.h under current work directory.

#ifndef CS100_HW3_H
#define CS100_HW3_H

#include <stdio.h>

...

#endif //CS100_HW3_H

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• If we include stdio.h twice, we would have Compile Error.
• To include this header, write #include <cs100_hw3.h> in our main C file.
• Before we include cs100_hw3.h, we could define CS100_HW3_H in our main C file (with no affects to including header file).
• After we include cs100_hw3.h, we have defined CS100_HW3_H in our main C file.

3. Read the following code. Fill the blank with output corresponding to printfs, omitting the newline \n character.

   #include <stdio.h>
   
   int main(void) {
       char str[] = "Hello_CS100";
       printf("%s\n", str);      // (1)
       printf("%s\n", str + 7);  // (2)
       str[5] = 0;          
       printf("%s\n", str);      // (3)
       const char *fstr = "%s/\\\n";
       printf(fstr, str+1);      // (4)
   }
   

4. Read the following code. Assume memory address is 64-bit. Which of the following statements is/are true?

#include <stdlib.h>

char *str = "Hello_World";
char arr[] = "abcdef";
int main(void) {
    int *ptr = malloc(10*sizeof(int));
    static int b[] = {1, 2, 3};
    free(ptr);
}

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• The value of sizeof(str) is 11.
• The value of sizeof(arr) is 6.
• Before free, the memory ptr points to is in heap memory.
• &ptr is a memory address located in stack memory.
• The element of static array b is located in stack memory.
• The type of "abcdef" is char [7].
• We can use code such as str[2]='b'; to change the content of str.

5. Read the following code. Which of the following statements is/are true?

int main(void) {
    int x = 2, y = 2;
    const int N = 100;
    const int *p = &x;
    int* const q = &y;
}

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• If we write *p = 10, this would lead to Compile-Error.
• If we write p = &y, this would lead to Compile-Error.
• If we write *q = 10, this would lead to Compile-Error.
• If we write q = &x, this would lead to Compile-Error.
• const int *p = &x; is equivalent to int const *p = &x;.
• int *ptr = &N; would generate warning. Furthermore, *ptr = 100 would lead to undefined behavior.

6. Which of the following statements is/are true?

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• malloc() would return an address where value initialized to 0.
• If we want to free half memory of int *ptr = malloc(n * sizeof(int));, we could write free(ptr + n/2);
• If we forget to free memory of malloc, this memory would be freed by C Memory Manager after pointer lifetime ends.
• The freed memory may be allocated by next malloc() call.

7. Read the following code. Which of the following statements is/are true?

struct Account{
    char username[10];
    int ID;
};

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• sizeof(struct Account) is equal to 10*sizeof(char) + sizeof(int).
• We could use struct Account A = {.username = "Alice", .ID = 0xbadbeef}; to initialize a struct variable.
• If A and B are both struct Account, we could write b=a; to copy a's value to b.
• C's struct is equivalent to C++'s struct.

8. Which of the following statements is/are true?

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• #define TABLE(a, b, n) a*n + b, the value of TABLE(1, 2, 3) is 5.
• #define TABLE(a, b, n) a*n + b, the value of TABLE(1, 2, 3) * 3 is 9.
• #define TABLE(a, b, n) (a*n + b), the value of TABLE(1 + 1, 2, 3) is 8.
• #define TABLE(a, b, n) ((a)*(n) + (b), the value of TABLE(1 + 1, 2, 3) is 8.

9. Suppose we have allocated some memory as follows.

int *mem = malloc(sizeof(int) * 10);

Which of the following code snippets correctly deallocate(s) the memory?

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• for (int i = 0; i < 10; ++i)
    free(mem[i]);
  

• for (int i = 0; i < 10; ++i)
    free(mem + i);
  

• free(mem);
  

• free(mem + 5);
  free(mem);
  

• free(mem, mem + 10);
  

• free(mem, mem + 10 * sizeof(int));
  

10. The standard library function malloc is declared as follows.

void *malloc(size_t size);

Which of the following statements is/are true?

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• The asterisk (*) here is the dereference operator.
• The return type of malloc is void *.
• The return type of malloc is determined at runtime. For example, the return type of malloc(sizeof(int) * n) is int *.
• If size > 0, it will try to allocate at least size bytes of memory.
• If size is too large, malloc will fail and the program will crash.