Merge remote-tracking branch 'adambard/master' into adambard/master-cn

1 files changed, 541 insertions, 296 deletions

diff --git a/c.html.markdown b/c.html.markdown
index 132f75dc..db2ac930 100644
--- a/c.html.markdown
+++ b/c.html.markdown
@@ -3,289 +3,432 @@ language: c
 filename: learnc.c
 contributors:
     - ["Adam Bard", "http://adambard.com/"]
+    - ["Árpád Goretity", "http://twitter.com/H2CO3_iOS"]
+    - ["Jakub Trzebiatowski", "http://cbs.stgn.pl"]
+    - ["Marco Scannadinari", "https://marcoms.github.io"]
+
 ---
 
-Ah, C. Still the language of modern high-performance computing.
+Ah, C. Still **the** language of modern high-performance computing.
 
 C is the lowest-level language most programmers will ever use, but
 it more than makes up for it with raw speed. Just be aware of its manual
 memory management and C will take you as far as you need to go.
 
 ```c
-// Single-line comments start with //
+// Single-line comments start with // - only available in C99 and later.
 
 /*
-Multi-line comments look like this.
+Multi-line comments look like this. They work in C89 as well.
 */
 
+/*
+Multi-line comments don't nest /* Be careful */  // comment ends on this line...
+*/ // ...not this one!
+
+// Constants: #define <keyword>
+#define DAYS_IN_YEAR 365
+
+// Enumeration constants are also ways to declare constants.
+// All statements must end with a semicolon
+enum days {SUN = 1, MON, TUE, WED, THU, FRI, SAT};
+// MON gets 2 automatically, TUE gets 3, etc.
+
+
 // Import headers with #include
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>
 
+// (File names between <angle brackets> are headers from the C standard library.)
+// For your own headers, use double quotes instead of angle brackets:
+//#include "my_header.h"
+
 // Declare function signatures in advance in a .h file, or at the top of
 // your .c file.
 void function_1();
-void function_2();
+int function_2(void);
+
+// Must declare a 'function prototype' before main() when functions occur after
+// your main() function.
+int add_two_ints(int x1, int x2); // function prototype
 
 // Your program's entry point is a function called
 // main with an integer return type.
-int main() {
-
-// print output using printf, for "print formatted"
-// %d is an integer, \n is a newline
-printf("%d\n", 0); // => Prints 0
-// All statements must end with a semicolon
-
-///////////////////////////////////////
-// Types
-///////////////////////////////////////
-
-// You have to declare variables before using them. A variable declaration
-// requires you to specify its type; a variable's type determines its size
-// in bytes.
-
-// ints are usually 4 bytes 
-int x_int = 0;
-
-// shorts are usually 2 bytes
-short x_short = 0;
-
-// chars are guaranteed to be 1 byte
-char x_char = 0;
-char y_char = 'y'; // Char literals are quoted with ''
-
-// longs are often 4 to 8 bytes; long longs are guaranteed to be at least
-// 64 bits
-long x_long = 0;
-long long x_long_long = 0; 
-
-// floats are usually 32-bit floating point numbers
-float x_float = 0.0;
-
-// doubles are usually 64-bit floating-point numbers
-double x_double = 0.0;
-
-// Integral types may be unsigned. This means they can't be negative, but
-// the maximum value of an unsigned variable is greater than the maximum
-// value of the same size.
-unsigned char ux_char;
-unsigned short ux_short;
-unsigned int ux_int;
-unsigned long long ux_long_long;
-
-// Other than char, which is always 1 byte, these types vary in size depending
-// on your machine. sizeof(T) gives you the size of a variable with type T in 
-// bytes so you can express the size of these types in a portable way.
-// For example,
-printf("%lu\n", sizeof(int)); // => 4 (on machines with 4-byte words)
-
-// Arrays must be initialized with a concrete size.
-char my_char_array[20]; // This array occupies 1 * 20 = 20 bytes
-int my_int_array[20]; // This array occupies 4 * 20 = 80 bytes
-                      // (assuming 4-byte words)
-
-
-// You can initialize an array to 0 thusly:
-char my_array[20] = {0};
-
-// Indexing an array is like other languages -- or,
-// rather, other languages are like C
-my_array[0]; // => 0
-
-// Arrays are mutable; it's just memory!
-my_array[1] = 2;
-printf("%d\n", my_array[1]); // => 2
-
-// Strings are just arrays of chars terminated by a NUL (0x00) byte,
-// represented in strings as the special character '\0'.
-// (We don't have to include the NUL byte in string literals; the compiler
-//  inserts it at the end of the array for us.)
-char a_string[20] = "This is a string";
-printf("%s\n", a_string); // %s formats a string
-
-/*
-You may have noticed that a_string is only 16 chars long.
-Char #17 is the NUL byte. 
-Chars #18, 19 and 20 have undefined values.
-*/
-
-printf("%d\n", a_string[16]); // => 0
-
-///////////////////////////////////////
-// Operators
-///////////////////////////////////////
-
-int i1 = 1, i2 = 2; // Shorthand for multiple declaration
-float f1 = 1.0, f2 = 2.0;
-
-// Arithmetic is straightforward
-i1 + i2; // => 3
-i2 - i1; // => 1
-i2 * i1; // => 2
-i1 / i2; // => 0 (0.5, but truncated towards 0)
-
-f1 / f2; // => 0.5, plus or minus epsilon
-
-// Modulo is there as well
-11 % 3; // => 2
-
-// Comparison operators are probably familiar, but
-// there is no boolean type in c. We use ints instead.
-// 0 is false, anything else is true. (The comparison 
-// operators always return 0 or 1.)
-3 == 2; // => 0 (false)
-3 != 2; // => 1 (true)
-3 > 2; // => 1
-3 < 2; // => 0
-2 <= 2; // => 1
-2 >= 2; // => 1
-
-// Logic works on ints
-!3; // => 0 (Logical not)
-!0; // => 1
-1 && 1; // => 1 (Logical and)
-0 && 1; // => 0
-0 || 1; // => 1 (Logical or)
-0 || 0; // => 0
-
-// Bitwise operators!
-~0x0F; // => 0xF0 (bitwise negation)
-0x0F & 0xF0; // => 0x00 (bitwise AND)
-0x0F | 0xF0; // => 0xFF (bitwise OR)
-0x04 ^ 0x0F; // => 0x0B (bitwise XOR)
-0x01 << 1; // => 0x02 (bitwise left shift (by 1))
-0x02 >> 1; // => 0x01 (bitwise right shift (by 1))
-
-///////////////////////////////////////
-// Control Structures
-///////////////////////////////////////
-
-if (0) {
-  printf("I am never run\n");
-} else if (0) {
-  printf("I am also never run\n");
-} else {
-  printf("I print\n");
-}
-
-// While loops exist
-int ii = 0;
-while (ii < 10) {
-    printf("%d, ", ii++); // ii++ increments ii in-place, after using its value.
-} // => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "
-
-printf("\n");
-
-int kk = 0;
-do {
+int main(void) {
+  // print output using printf, for "print formatted"
+  // %d is an integer, \n is a newline
+  printf("%d\n", 0); // => Prints 0
+
+  ///////////////////////////////////////
+  // Types
+  ///////////////////////////////////////
+
+  // ints are usually 4 bytes
+  int x_int = 0;
+
+  // shorts are usually 2 bytes
+  short x_short = 0;
+
+  // chars are guaranteed to be 1 byte
+  char x_char = 0;
+  char y_char = 'y'; // Char literals are quoted with ''
+
+  // longs are often 4 to 8 bytes; long longs are guaranteed to be at least
+  // 64 bits
+  long x_long = 0;
+  long long x_long_long = 0;
+
+  // floats are usually 32-bit floating point numbers
+  float x_float = 0.0f; // 'f' suffix here denotes floating point literal
+
+  // doubles are usually 64-bit floating-point numbers
+  double x_double = 0.0; // real numbers without any suffix are doubles
+
+  // integer types may be unsigned (greater than or equal to zero)
+  unsigned short ux_short;
+  unsigned int ux_int;
+  unsigned long long ux_long_long;
+
+  // chars inside single quotes are integers in machine's character set.
+  '0'; // => 48 in the ASCII character set.
+  'A'; // => 65 in the ASCII character set.
+
+  // sizeof(T) gives you the size of a variable with type T in bytes
+  // sizeof(obj) yields the size of the expression (variable, literal, etc.).
+  printf("%zu\n", sizeof(int)); // => 4 (on most machines with 4-byte words)
+
+
+  // If the argument of the `sizeof` operator is an expression, then its argument
+  // is not evaluated (except VLAs (see below)).
+  // The value it yields in this case is a compile-time constant.
+  int a = 1;
+  // size_t is an unsigned integer type of at least 2 bytes used to represent
+  // the size of an object.
+  size_t size = sizeof(a++); // a++ is not evaluated
+  printf("sizeof(a++) = %zu where a = %d\n", size, a);
+  // prints "sizeof(a++) = 4 where a = 1" (on a 32-bit architecture)
+
+  // Arrays must be initialized with a concrete size.
+  char my_char_array[20]; // This array occupies 1 * 20 = 20 bytes
+  int my_int_array[20]; // This array occupies 4 * 20 = 80 bytes
+  // (assuming 4-byte words)
+
+
+  // You can initialize an array to 0 thusly:
+  char my_array[20] = {0};
+
+  // Indexing an array is like other languages -- or,
+  // rather, other languages are like C
+  my_array[0]; // => 0
+
+  // Arrays are mutable; it's just memory!
+  my_array[1] = 2;
+  printf("%d\n", my_array[1]); // => 2
+
+  // In C99 (and as an optional feature in C11), variable-length arrays (VLAs)
+  // can be declared as well. The size of such an array need not be a compile
+  // time constant:
+  printf("Enter the array size: "); // ask the user for an array size
+  int size;
+  fscanf(stdin, "%d", &size);
+  char buf[size];
+  fgets(buf, sizeof buf, stdin);
+
+  // strtoul parses a string to an unsigned integer
+  size_t size2 = strtoul(buf, NULL, 10);
+  int var_length_array[size2]; // declare the VLA
+  printf("sizeof array = %zu\n", sizeof var_length_array);
+
+  // A possible outcome of this program may be:
+  // > Enter the array size: 10
+  // > sizeof array = 40
+
+  // Strings are just arrays of chars terminated by a NULL (0x00) byte,
+  // represented in strings as the special character '\0'.
+  // (We don't have to include the NULL byte in string literals; the compiler
+  //  inserts it at the end of the array for us.)
+  char a_string[20] = "This is a string";
+  printf("%s\n", a_string); // %s formats a string
+
+  printf("%d\n", a_string[16]); // => 0
+  // i.e., byte #17 is 0 (as are 18, 19, and 20)
+
+  // If we have characters between single quotes, that's a character literal.
+  // It's of type `int`, and *not* `char` (for historical reasons).
+  int cha = 'a'; // fine
+  char chb = 'a'; // fine too (implicit conversion from int to char)
+
+  // Multi-dimensional arrays:
+  int multi_array[2][5] = {
+    {1, 2, 3, 4, 5},
+    {6, 7, 8, 9, 0}
+  };
+  // access elements:
+  int array_int = multi_array[0][2]; // => 3
+
+  ///////////////////////////////////////
+  // Operators
+  ///////////////////////////////////////
+
+  // Shorthands for multiple declarations:
+  int i1 = 1, i2 = 2;
+  float f1 = 1.0, f2 = 2.0;
+
+  int b, c;
+  b = c = 0;
+
+  // Arithmetic is straightforward
+  i1 + i2; // => 3
+  i2 - i1; // => 1
+  i2 * i1; // => 2
+  i1 / i2; // => 0 (0.5, but truncated towards 0)
+
+  // You need to cast at least one integer to float to get a floating-point result
+  (float)i1 / i2; // => 0.5f
+  i1 / (double)i2; // => 0.5 // Same with double
+  f1 / f2; // => 0.5, plus or minus epsilon
+  // Floating-point numbers and calculations are not exact
+
+  // Modulo is there as well
+  11 % 3; // => 2
+
+  // Comparison operators are probably familiar, but
+  // there is no Boolean type in c. We use ints instead.
+  // (Or _Bool or bool in C99.)
+  // 0 is false, anything else is true. (The comparison
+  // operators always yield 0 or 1.)
+  3 == 2; // => 0 (false)
+  3 != 2; // => 1 (true)
+  3 > 2; // => 1
+  3 < 2; // => 0
+  2 <= 2; // => 1
+  2 >= 2; // => 1
+
+  // C is not Python - comparisons don't chain.
+  // Warning: The line below will compile, but it means `(0 < a) < 2`.
+  // This expression is always true, because (0 < a) could be either 1 or 0.
+  // In this case it's 1, because (0 < 1).
+  int between_0_and_2 = 0 < a < 2;
+  // Instead use:
+  int between_0_and_2 = 0 < a && a < 2;
+
+  // Logic works on ints
+  !3; // => 0 (Logical not)
+  !0; // => 1
+  1 && 1; // => 1 (Logical and)
+  0 && 1; // => 0
+  0 || 1; // => 1 (Logical or)
+  0 || 0; // => 0
+
+  // Conditional expression ( ? : )
+  int e = 5;
+  int f = 10;
+  int z;
+  z = (e > f) ? e : f; // => 10 "if e > f return e, else return f."
+
+  // Increment and decrement operators:
+  char *s = "iLoveC";
+  int j = 0;
+  s[j++]; // => "i". Returns the j-th item of s THEN increments value of j.
+  j = 0;
+  s[++j]; // => "L". Increments value of j THEN returns j-th value of s.
+  // same with j-- and --j
+
+  // Bitwise operators!
+  ~0x0F; // => 0xFFFFFFF0 (bitwise negation, "1's complement", example result for 32-bit int)
+  0x0F & 0xF0; // => 0x00 (bitwise AND)
+  0x0F | 0xF0; // => 0xFF (bitwise OR)
+  0x04 ^ 0x0F; // => 0x0B (bitwise XOR)
+  0x01 << 1; // => 0x02 (bitwise left shift (by 1))
+  0x02 >> 1; // => 0x01 (bitwise right shift (by 1))
+
+  // Be careful when shifting signed integers - the following are undefined:
+  // - shifting into the sign bit of a signed integer (int a = 1 << 31)
+  // - left-shifting a negative number (int a = -1 << 2)
+  // - shifting by an offset which is >= the width of the type of the LHS:
+  //   int a = 1 << 32; // UB if int is 32 bits wide
+
+  ///////////////////////////////////////
+  // Control Structures
+  ///////////////////////////////////////
+
+  if (0) {
+    printf("I am never run\n");
+  } else if (0) {
+    printf("I am also never run\n");
+  } else {
+    printf("I print\n");
+  }
+
+  // While loops exist
+  int ii = 0;
+  while (ii < 10) { //ANY value not zero is true.
+    printf("%d, ", ii++); // ii++ increments ii AFTER using its current value.
+  } // => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "
+
+  printf("\n");
+
+  int kk = 0;
+  do {
     printf("%d, ", kk);
-} while (++kk < 10); // ++kk increments kk in-place, before using its value
-// => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "
+  } while (++kk < 10); // ++kk increments kk BEFORE using its current value.
+  // => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "
 
-printf("\n");
+  printf("\n");
 
-// For loops too
-int jj;
-for (jj=0; jj < 10; jj++) {
+  // For loops too
+  int jj;
+  for (jj=0; jj < 10; jj++) {
     printf("%d, ", jj);
-} // => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "
-
-printf("\n");
-
-///////////////////////////////////////
-// Typecasting
-///////////////////////////////////////
-
-// Every value in C has a type, but you can cast one value into another type
-// if you want.
-
-int x_hex = 0x01; // You can assign vars with hex literals
-
-// Casting between types will attempt to preserve their numeric values
-printf("%d\n", x_hex); // => Prints 1
-printf("%d\n", (short) x_hex); // => Prints 1
-printf("%d\n", (char) x_hex); // => Prints 1
-
-// Types will overflow without warning
-printf("%d\n", (char) 257); // => 1 (Max char = 255)
-
-// Integral types can be cast to floating-point types, and vice-versa.
-printf("%f\n", (float)100); // %f formats a float
-printf("%lf\n", (double)100); // %lf formats a double
-printf("%d\n", (char)100.0);
-
-///////////////////////////////////////
-// Pointers
-///////////////////////////////////////
-
-// A pointer is a variable declared to store a memory address. Its declaration will
-// also tell you the type of data it points to. You can retrieve the memory address 
-// of your variables, then mess with them.
-
-int x = 0;
-printf("%p\n", &x); // Use & to retrieve the address of a variable
-// (%p formats a pointer)
-// => Prints some address in memory;
-
-// Pointer types end with * in their declaration
-int* px; // px is a pointer to an int
-px = &x; // Stores the address of x in px
-printf("%p\n", px); // => Prints some address in memory
-
-// To retreive the value at the address a pointer is pointing to,
-// put * in front to de-reference it.
-printf("%d\n", *px); // => Prints 0, the value of x, which is what px is pointing to the address of
-
-// You can also change the value the pointer is pointing to.
-// We'll have to wrap the de-reference in parenthesis because
-// ++ has a higher precedence than *.
-(*px)++; // Increment the value px is pointing to by 1
-printf("%d\n", *px); // => Prints 1
-printf("%d\n", x); // => Prints 1
-
-int x_array[20]; // Arrays are a good way to allocate a contiguous block of memory
-int xx;
-for (xx=0; xx<20; xx++) {
+  } // => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "
+
+  printf("\n");
+
+  // *****NOTES*****:
+  // Loops and Functions MUST have a body. If no body is needed:
+  int i;
+  for (i = 0; i <= 5; i++) {
+    ; // use semicolon to act as the body (null statement)
+  }
+
+  // branching with multiple choices: switch()
+  switch (a) {
+  case 0: // labels need to be integral *constant* expressions
+    printf("Hey, 'a' equals 0!\n");
+    break; // if you don't break, control flow falls over labels
+  case 1:
+    printf("Huh, 'a' equals 1!\n");
+    break;
+  default:
+    // if `some_integral_expression` didn't match any of the labels
+    fputs("error!\n", stderr);
+    exit(-1);
+    break;
+  }
+
+  ///////////////////////////////////////
+  // Typecasting
+  ///////////////////////////////////////
+
+  // Every value in C has a type, but you can cast one value into another type
+  // if you want (with some constraints).
+
+  int x_hex = 0x01; // You can assign vars with hex literals
+
+  // Casting between types will attempt to preserve their numeric values
+  printf("%d\n", x_hex); // => Prints 1
+  printf("%d\n", (short) x_hex); // => Prints 1
+  printf("%d\n", (char) x_hex); // => Prints 1
+
+  // Types will overflow without warning
+  printf("%d\n", (unsigned char) 257); // => 1 (Max char = 255 if char is 8 bits long)
+
+  // For determining the max value of a `char`, a `signed char` and an `unsigned char`,
+  // respectively, use the CHAR_MAX, SCHAR_MAX and UCHAR_MAX macros from <limits.h>
+
+  // Integral types can be cast to floating-point types, and vice-versa.
+  printf("%f\n", (float)100); // %f formats a float
+  printf("%lf\n", (double)100); // %lf formats a double
+  printf("%d\n", (char)100.0);
+
+  ///////////////////////////////////////
+  // Pointers
+  ///////////////////////////////////////
+
+  // A pointer is a variable declared to store a memory address. Its declaration will
+  // also tell you the type of data it points to. You can retrieve the memory address
+  // of your variables, then mess with them.
+
+  int x = 0;
+  printf("%p\n", (void *)&x); // Use & to retrieve the address of a variable
+  // (%p formats an object pointer of type void *)
+  // => Prints some address in memory;
+
+
+  // Pointers start with * in their declaration
+  int *px, not_a_pointer; // px is a pointer to an int
+  px = &x; // Stores the address of x in px
+  printf("%p\n", (void *)px); // => Prints some address in memory
+  printf("%zu, %zu\n", sizeof(px), sizeof(not_a_pointer));
+  // => Prints "8, 4" on a typical 64-bit system
+
+  // To retrieve the value at the address a pointer is pointing to,
+  // put * in front to dereference it.
+  // Note: yes, it may be confusing that '*' is used for _both_ declaring a
+  // pointer and dereferencing it.
+  printf("%d\n", *px); // => Prints 0, the value of x
+
+  // You can also change the value the pointer is pointing to.
+  // We'll have to wrap the dereference in parenthesis because
+  // ++ has a higher precedence than *.
+  (*px)++; // Increment the value px is pointing to by 1
+  printf("%d\n", *px); // => Prints 1
+  printf("%d\n", x); // => Prints 1
+
+  // Arrays are a good way to allocate a contiguous block of memory
+  int x_array[20]; //declares array of size 20 (cannot change size)
+  int xx;
+  for (xx = 0; xx < 20; xx++) {
     x_array[xx] = 20 - xx;
-} // Initialize x_array to 20, 19, 18,... 2, 1
-
-// Declare a pointer of type int and initialize it to point to x_array
-int* x_ptr = x_array;
-// x_ptr now points to the first element in the array (the integer 20). 
-// This works because arrays are actually just pointers to their first element.
-
-// Arrays are pointers to their first element
-printf("%d\n", *(x_ptr)); // => Prints 20
-printf("%d\n", x_array[0]); // => Prints 20
-
-// Pointers are incremented and decremented based on their type
-printf("%d\n", *(x_ptr + 1)); // => Prints 19
-printf("%d\n", x_array[1]); // => Prints 19
-
-// You can also dynamically allocate contiguous blocks of memory with the
-// standard library function malloc, which takes one integer argument 
-// representing the number of bytes to allocate from the heap.
-int* my_ptr = (int*) malloc(sizeof(int) * 20);
-for (xx=0; xx<20; xx++) {
-    *(my_ptr + xx) = 20 - xx; // my_ptr[xx] = 20-xx would also work here
-} // Initialize memory to 20, 19, 18, 17... 2, 1 (as ints)
-
-// Dereferencing memory that you haven't allocated gives
-// unpredictable results
-printf("%d\n", *(my_ptr + 21)); // => Prints who-knows-what?
-
-// When you're done with a malloc'd block of memory, you need to free it, 
-// or else no one else can use it until your program terminates
-free(my_ptr);
-
-// Strings can be char arrays, but are usually represented as char
-// pointers:
-char* my_str = "This is my very own string";
-
-printf("%c\n", *my_str); // => 'T'
-
-function_1();
+  } // Initialize x_array to 20, 19, 18,... 2, 1
+
+  // Declare a pointer of type int and initialize it to point to x_array
+  int* x_ptr = x_array;
+  // x_ptr now points to the first element in the array (the integer 20).
+  // This works because arrays often decay into pointers to their first element.
+  // For example, when an array is passed to a function or is assigned to a pointer,
+  // it decays into (implicitly converted to) a pointer.
+  // Exceptions: when the array is the argument of the `&` (address-of) operator:
+  int arr[10];
+  int (*ptr_to_arr)[10] = &arr; // &arr is NOT of type `int *`!
+  // It's of type "pointer to array" (of ten `int`s).
+  // or when the array is a string literal used for initializing a char array:
+  char otherarr[] = "foobarbazquirk";
+  // or when it's the argument of the `sizeof` or `alignof` operator:
+  int arraythethird[10];
+  int *ptr = arraythethird; // equivalent with int *ptr = &arr[0];
+  printf("%zu, %zu\n", sizeof arraythethird, sizeof ptr);
+  // probably prints "40, 4" or "40, 8"
+
+
+  // Pointers are incremented and decremented based on their type
+  // (this is called pointer arithmetic)
+  printf("%d\n", *(x_ptr + 1)); // => Prints 19
+  printf("%d\n", x_array[1]); // => Prints 19
+
+  // You can also dynamically allocate contiguous blocks of memory with the
+  // standard library function malloc, which takes one argument of type size_t
+  // representing the number of bytes to allocate (usually from the heap, although this
+  // may not be true on e.g. embedded systems - the C standard says nothing about it).
+  int *my_ptr = malloc(sizeof(*my_ptr) * 20);
+  for (xx = 0; xx < 20; xx++) {
+    *(my_ptr + xx) = 20 - xx; // my_ptr[xx] = 20-xx
+  } // Initialize memory to 20, 19, 18, 17... 2, 1 (as ints)
+
+  // Dereferencing memory that you haven't allocated gives
+  // "unpredictable results" - the program is said to invoke "undefined behavior"
+  printf("%d\n", *(my_ptr + 21)); // => Prints who-knows-what? It may even crash.
+
+  // When you're done with a malloc'd block of memory, you need to free it,
+  // or else no one else can use it until your program terminates
+  // (this is called a "memory leak"):
+  free(my_ptr);
+
+  // Strings are arrays of char, but they are usually represented as a
+  // pointer-to-char (which is a pointer to the first element of the array).
+  // It's good practice to use `const char *' when referring to a string literal,
+  // since string literals shall not be modified (i.e. "foo"[0] = 'a' is ILLEGAL.)
+  const char *my_str = "This is my very own string literal";
+  printf("%c\n", *my_str); // => 'T'
+
+  // This is not the case if the string is an array
+  // (potentially initialized with a string literal)
+  // that resides in writable memory, as in:
+  char foo[] = "foo";
+  foo[0] = 'a'; // this is legal, foo now contains "aoo"
+
+  function_1();
 } // end main function
 
 ///////////////////////////////////////
@@ -295,26 +438,33 @@ function_1();
 // Function declaration syntax:
 // <return type> <function name>(<args>)
 
-int add_two_ints(int x1, int x2){
-    return x1 + x2; // Use return to return a value
+int add_two_ints(int x1, int x2)
+{
+  return x1 + x2; // Use return to return a value
 }
 
 /*
-Functions are pass-by-value, but you can make your own references
-with pointers so functions can mutate their values.
+Functions are call by value. When a function is called, the arguments passed to
+≈the function are copies of the original arguments (except arrays). Anything you
+do to the arguments in the function do not change the value of the original
+argument where the function was called.
+
+Use pointers if you need to edit the original argument values.
 
 Example: in-place string reversal
 */
 
 // A void function returns no value
-void str_reverse(char* str_in){
-    char tmp;
-    int ii=0, len = strlen(str_in); // Strlen is part of the c standard library
-    for(ii=0; ii<len/2; ii++){
-        tmp = str_in[ii];
-        str_in[ii] = str_in[len - ii - 1]; // ii-th char from end
-        str_in[len - ii - 1] = tmp;
-    }
+void str_reverse(char *str_in)
+{
+  char tmp;
+  int ii = 0;
+  size_t len = strlen(str_in); // `strlen()` is part of the c standard library
+  for (ii = 0; ii < len / 2; ii++) {
+    tmp = str_in[ii];
+    str_in[ii] = str_in[len - ii - 1]; // ii-th char from end
+    str_in[len - ii - 1] = tmp;
+  }
 }
 
 /*
@@ -323,6 +473,21 @@ str_reverse(c);
 printf("%s\n", c); // => ".tset a si sihT"
 */
 
+// if referring to external variables outside function, must use extern keyword.
+int i = 0;
+void testFunc() {
+  extern int i; //i here is now using external variable i
+}
+
+// make external variables private to source file with static:
+static int j = 0; //other files using testFunc2() cannot access variable j
+void testFunc2() {
+  extern int j;
+}
+//**You may also declare functions as static to make them private**
+
+
+
 ///////////////////////////////////////
 // User-defined types and structs
 ///////////////////////////////////////
@@ -331,55 +496,69 @@ printf("%s\n", c); // => ".tset a si sihT"
 typedef int my_type;
 my_type my_type_var = 0;
 
-// Structs are just collections of data
+// Structs are just collections of data, the members are allocated sequentially,
+// in the order they are written:
 struct rectangle {
-    int width;
-    int height;
+  int width;
+  int height;
 };
 
+// It's not generally true that
+// sizeof(struct rectangle) == sizeof(int) + sizeof(int)
+// due to potential padding between the structure members (this is for alignment
+// reasons). [1]
 
-void function_1(){
+void function_1()
+{
+  struct rectangle my_rec;
 
-    struct rectangle my_rec;
+  // Access struct members with .
+  my_rec.width = 10;
+  my_rec.height = 20;
 
-    // Access struct members with .
-    my_rec.width = 10;
-    my_rec.height = 20;
+  // You can declare pointers to structs
+  struct rectangle *my_rec_ptr = &my_rec;
 
-    // You can declare pointers to structs
-    struct rectangle* my_rec_ptr = &my_rec;
+  // Use dereferencing to set struct pointer members...
+  (*my_rec_ptr).width = 30;
 
-    // Use dereferencing to set struct pointer members...
-    (*my_rec_ptr).width = 30;
-
-    // ... or use the -> shorthand
-    my_rec_ptr->height = 10; // Same as (*my_rec_ptr).height = 10;
+  // ... or even better: prefer the -> shorthand for the sake of readability
+  my_rec_ptr->height = 10; // Same as (*my_rec_ptr).height = 10;
 }
 
 // You can apply a typedef to a struct for convenience
 typedef struct rectangle rect;
 
-int area(rect r){
-    return r.width * r.height;
+int area(rect r)
+{
+  return r.width * r.height;
+}
+
+// if you have large structs, you can pass them "by pointer" to avoid copying
+// the whole struct:
+int areaptr(const rect *r)
+{
+  return r->width * r->height;
 }
 
 ///////////////////////////////////////
-// Function pointers 
+// Function pointers
 ///////////////////////////////////////
 /*
-At runtime, functions are located at known memory addresses. Function pointers are
-much likely any other pointer (they just store a memory address), but can be used 
+At run time, functions are located at known memory addresses. Function pointers are
+much like any other pointer (they just store a memory address), but can be used
 to invoke functions directly, and to pass handlers (or callback functions) around.
 However, definition syntax may be initially confusing.
 
 Example: use str_reverse from a pointer
 */
-void str_reverse_through_pointer(char * str_in) {
-    // Define a function pointer variable, named f. 
-    void (*f)(char *); // Signature should exactly match the target function.
-    f = &str_reverse; // Assign the address for the actual function (determined at runtime)
-    (*f)(str_in); // Just calling the function through the pointer
-    // f(str_in); // That's an alternative but equally valid syntax for calling it.
+void str_reverse_through_pointer(char *str_in) {
+  // Define a function pointer variable, named f.
+  void (*f)(char *); // Signature should exactly match the target function.
+  f = &str_reverse; // Assign the address for the actual function (determined at run time)
+  // f = str_reverse; would work as well - functions decay into pointers, similar to arrays
+  (*f)(str_in); // Just calling the function through the pointer
+  // f(str_in); // That's an alternative but equally valid syntax for calling it.
 }
 
 /*
@@ -389,16 +568,82 @@ Function pointers are usually typedef'd for simplicity and readability, as follo
 
 typedef void (*my_fnp_type)(char *);
 
-// The used when declaring the actual pointer variable:
+// Then used when declaring the actual pointer variable:
 // ...
-// my_fnp_type f; 
+// my_fnp_type f;
+
+//Special characters:
+/*
+'\a'; // alert (bell) character
+'\n'; // newline character
+'\t'; // tab character (left justifies text)
+'\v'; // vertical tab
+'\f'; // new page (form feed)
+'\r'; // carriage return
+'\b'; // backspace character
+'\0'; // NULL character. Usually put at end of strings in C.
+//   hello\n\0. \0 used by convention to mark end of string.
+'\\'; // backslash
+'\?'; // question mark
+'\''; // single quote
+'\"'; // double quote
+'\xhh'; // hexadecimal number. Example: '\xb' = vertical tab character
+'\0oo'; // octal number. Example: '\013' = vertical tab character
+
+//print formatting:
+"%d";    // integer
+"%3d";   // integer with minimum of length 3 digits (right justifies text)
+"%s";    // string
+"%f";    // float
+"%ld";   // long
+"%3.2f"; // minimum 3 digits left and 2 digits right decimal float
+"%7.4s"; // (can do with strings too)
+"%c";    // char
+"%p";    // pointer
+"%x";    // hexadecimal
+"%o";    // octal
+"%%";    // prints %
+*/
+///////////////////////////////////////
+// Order of Evaluation
+///////////////////////////////////////
+
+//---------------------------------------------------//
+//        Operators                  | Associativity //
+//---------------------------------------------------//
+// () [] -> .                        | left to right //
+// ! ~ ++ -- + = *(type)sizeof       | right to left //
+// * / %                             | left to right //
+// + -                               | left to right //
+// << >>                             | left to right //
+// < <= > >=                         | left to right //
+// == !=                             | left to right //
+// &                                 | left to right //
+// ^                                 | left to right //
+// |                                 | left to right //
+// &&                                | left to right //
+// ||                                | left to right //
+// ?:                                | right to left //
+// = += -= *= /= %= &= ^= |= <<= >>= | right to left //
+// ,                                 | left to right //
+//---------------------------------------------------//
 
 ```
 
 ## Further Reading
 
 Best to find yourself a copy of [K&R, aka "The C Programming Language"](https://en.wikipedia.org/wiki/The_C_Programming_Language)
+It is *the* book about C, written by Dennis Ritchie, the creator of C, and Brian Kernighan. Be careful, though - it's ancient and it contains some
+inaccuracies (well, ideas that are not considered good anymore) or now-changed practices.
+
+Another good resource is [Learn C The Hard Way](http://c.learncodethehardway.org/book/).
 
-Another good resource is [Learn C the hard way](http://c.learncodethehardway.org/book/)
+If you have a question, read the [compl.lang.c Frequently Asked Questions](http://c-faq.com).
+
+It's very important to use proper spacing, indentation and to be consistent with your coding style in general.
+Readable code is better than clever code and fast code. For a good, sane coding style to adopt, see the
+[Linux kernel coding style](https://www.kernel.org/doc/Documentation/CodingStyle).
 
 Other than that, Google is your friend.
+
+[1] http://stackoverflow.com/questions/119123/why-isnt-sizeof-for-a-struct-equal-to-the-sum-of-sizeof-of-each-member