learnxinyminutes-docs/c.html.markdown

---
language: c
author: Adam Bard
author_url: http://adambard.com/
filename: learnc.c
---

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
// 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;

// 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++) {
    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"
*/

/* 
Functions are located in known memory addresses, so they can be called 
through function pointers. 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 alternate but equally valid syntax for calling it.
}

/*
As long as function signatures match, you can assign any function to the same pointer.
Useful for passing handlers (or callback functions) around.
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; 


///////////////////////////////////////
// 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;
}

```

## 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.