learnxinyminutes-docs/c.html.markdown

---
language: c
filename: learnc.c
contributors:
    - ["Adam Bard", "http://adambard.com/"]
    - ["Árpád Goretity", "http://twitter.com/H2CO3_iOS"]

---

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 // - only available in C99 and later.

  /*
Multi-line comments look like this. They work in C89 as well.
  */

  // Constants: #define <keyword>
#define DAYS_IN_YEAR 365

  // Enumeration constants are also ways to declare constants. 
  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();
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
  ///////////////////////////////////////

  // 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.
  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
  char buf[0x100];
  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)

  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.
  // WRONG:
  //int between_0_and_2 = 0 < a < 2;
  // Correct:
  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 = (a > b) ? a : b; // => 10 "if a > b return a, else return b." 

  //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; // => 0xF0 (bitwise negation, "1's complement")
  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 << 32)
  // - 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 BEFORE using its current value.
  // => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "

  printf("\n");

  // 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");

  // *****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
    //do_stuff();
    break; // if you don't break, control flow falls over labels
  case 1:
    //do_something_else();
    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 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

///////////////////////////////////////
// Functions
///////////////////////////////////////

// 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
}

/*
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;
  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;
  }
}

/*
char c[] = "This is a test.";
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 testFunc() cannot access variable i
void testFunc2() {
  extern int j;
}
//**You may also declare functions as static to make them private**



///////////////////////////////////////
// User-defined types and structs
///////////////////////////////////////

// Typedefs can be used to create type aliases
typedef int my_type;
my_type my_type_var = 0;

// Structs are just collections of data, the members are allocated sequentially,
// in the order they are written:
struct rectangle {
  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()
{
  struct rectangle my_rec;

  // 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;

  // Use dereferencing to set struct pointer members...
  (*my_rec_ptr).width = 30;

  // ... 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;
}

// 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 
///////////////////////////////////////
/*
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 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.
}

/*
As long as function signatures match, you can assign any function to the same pointer.
Function pointers are usually typedef'd for simplicity and readability, as follows:
*/

typedef void (*my_fnp_type)(char *);

// Then used when declaring the actual pointer variable:
// ...
// 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
'\ooo'; // 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 the creators of C. 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/).

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