Lecture 019

The C Pre-processor Language

C program has statements written: - C preprocessor language - C proper

Header File Inclusion

#include <stdio.h> // for io
#include <stdbool.h> // for boolean
#include "lib/xalloc.h"
// you can include other things, but not a good idea (never include a .c file)
// notice that inclution have different symbol

header file:

• library interface: type definition, function prototype

• other preprocessor commands

Macro Definition

#define INT_MIN 0x80000000
// preprocessor replaces INT_MIN with 0x80000000
// compiler never sees INT_MIN

#define INT_MAX INT_MIN^-1
// then when you write INT_MAX / 2
// it will replace string as 0x80000000^-1/2
// which C understand as 0x80000000^(-1/2)
#define INT_MAX (INT_MIN^-1)
// this is better

#define MULT(x, y)((x)*(y))
// you can also define function
// observe how many prentheses you need

Conditional Compilation

Compile based on whether a variable is defined

#ifdef DEBUG
  printf("Reached this point\n")
#endif

// use else
#ifdef X86_ARCH
  #include "arch/x86_optimizations.h"
  x86_optimize(code);
#else
  generic_optimize(code);
#endif

// we can also check if not defined
#ifndef INT_MIN
  #define INT_MIN 0x80000000
#endif

Contracts Emulation

C has no contracts contracts.h:

• REQUIRES

• ENSURES

• ASSERT

Translation Example

• Translating contracts

• Translating library, variables

• TODO: what is the first line

• Translating

• Translating assertion

• Translating all print statements to printf()

• Translating alloc

  • malloc takes number of bytes need to allocate memory for
  • size of a data structure can be accessed by sizeof()
  • they are in
  • malloc return NULL if no more memory (next deference will be unsafe)
  • xmalloc will fail if no enough memory in "lib/xalloc.h"
  • xmalloc will not clean the memory allocated, to initialize to default value, use xcmalloc

  • 

Printing

int32_t: %d, %i

printf("%d", 50);            // Prints 50
printf("%d", -213);          // Prints -213
printf("%i", -213);          // Prints -213
int64_t x = 4294967296;      // 4,294,967,296 is 2^32
unsigned int y = 4294967295; // 4,294,967,295 is 2^32 - 1
printf("%d and %d", x, y);   // Prints 0 and -1

uint32_t: %u

printf("%u", 500);       // Prints 500
printf("%u", -500);      // Prints 4294966796
uint64_t x = 4294967296; // 4,294,967,296 is 2^32
printf("%u", x);         // Prints 0

uint32_t: %x, %X

printf("%x", 31);    // Prints 1f
printf("%X", 31);    // Prints 1F
printf("%X", -2);    // Prints FFFFFFFE
long a = 4294967296; // 4,294,967,296 is 2^32
printf("%x", a);     // Prints 0

Unsigned Octal Integer

printf("%o", 31);    // Prints 37
printf("%o", -2);    // Prints 37777777776
long a = 4294967296; // 4,294,967,296 is 2^32
printf("%o", a);     // Prints 0

More on Printing: Here

Compiling a C Program

• so include everything else

• also if you have multiple include of the same thing in different file within one compile, bad bad.

• 

More on Valgrind: Here

Memory and Garbage Collection

When Allocation

1. allocate the size of bytes
2. using xmalloc or xcalloc instead
3. clean the memory allocated

Garbage Collection

Without Garbage Collection, memory will only un-allocate upon program exit.

• free(P) whenever you do xalloc(), make sure client know who to free data stored in data structure if needed

• 

• 

Freeing Data Structure:

• we pass typedef void entry_free_fn(entry e)

• to data structure function

• void dict_free(dict_t D, entry_free_fn* Fr)

Lost	Gained
contracts	preprocessor
safety	whimsical execution
garbege collection	explicit memory management
memory initialization	separate compilation

String in C: https://www.youtube.com/watch?v=90gFFdzuZMw&ab_channel=EngineerMan You should not free the function by passing free(fn_pointer) because functions are not in heap Array Pointer: https://www.youtube.com/watch?v=y2BAaD3fW8A&ab_channel=LuisCeze