learnxinyminutes-docs/c.html.markdown

---
name: c
category: language
language: c
filename: learnc.c
contributors:
    - ["Adam Bard", "http://adambard.com/"]
---

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 //

/*
Multi-line comments look like this.
*/

// Import headers with #include
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

// Declare function signatures in advance in a .h file, or at the top of
// your .c file.
void function_1();
void function_2();

// 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
// signed 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 {
    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, "

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");

///////////////////////////////////////
// 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;


// 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", px); // => Prints some address in memory
printf("%d, %d\n", (int)sizeof(px), (int)sizeof(not_a_pointer));
// => Prints "8, 4" on 64-bit system

// 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++) {
    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();
} // 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 pass-by-value, but you can make your own references
with pointers so functions can mutate their 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;
    }
}

/*
char c[] = "This is a test.";
str_reverse(c);
printf("%s\n", c); // => ".tset a si sihT"
*/

///////////////////////////////////////
// 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
struct rectangle {
    int width;
    int height;
};


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 use the -> shorthand
    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;
}

///////////////////////////////////////
// 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 
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.
}

/*
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 *);

// The used when declaring the actual pointer variable:
// ...
// my_fnp_type f; 

```

## 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)

Another good resource is [Learn C the hard way](http://c.learncodethehardway.org/book/)

Other than that, Google is your friend.