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----
-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.
-*/
-
-//Special characters:
-'\a' // alert (bell) character
-'\n' // newline character
-'\t' // tab character (left justifies text)
-'\v' // vertical tab
-'\f' // new page (formfeed)
-'\r' // carriage return
-'\b' // backspace character
-'\0' // null character. Usually put at end of strings in C lang.
- // hello\n\0. \0 used by convention to mark end of string.
-'\\' // backspace
-'\?' // 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" // hexidecimal
-"%o" // octal
-"%%" // prints %
-
-// Constants: #define <keyword> (no semicolon at end)
-#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();
-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
- ///////////////////////////////////////
-
- // 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 on the ASCII character set.
- 'A' //==> 65 on 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 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 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 size = strtoul(buf, NULL, 10);
- int var_length_array[size]; // 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 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
-
- 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)
-
- ///////////////////////////////////////
- // Operators
- ///////////////////////////////////////
-
- int i1 = 1, i2 = 2; // Shorthand for multiple declaration
- float f1 = 1.0, f2 = 2.0;
-
- //more shorthands:
- int a, b, c;
- a = 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.
- int a = 1;
- // 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 a, b, z;
- z = (a > b) ? a : b; // "if a > b return a, else return b."
-
- //Increment and decrement operators:
- s[j++]; //returns value of j to s THEN increments value of j.
- s[++j]; //increments value of j THEN returns value of j to 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 it's 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 it's 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 MUST always have a body. If no body is needed, do:
- for (i = 0; i <= 5; i++) {
- ; // use semicolon to act as the body (null statement)
- }
-
- // branching with multiple choices: switch()
- switch (some_integral_expression) {
- case 0: // labels need to be integral *constant* epxressions
- 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 `unisigned 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 retreive the value at the address a pointer is pointing to,
- // put * in front to de-reference 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 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
-
- // 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-od) 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 arr[] = "foobarbazquirk";
- // or when it's the argument of the `sizeof` or `alignof` operator:
- int arr[10];
- int *ptr = arr; // equivalent with int *ptr = &arr[0];
- printf("%zu %zu\n", sizeof arr, 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
-}
-
-<<<<<<< HEAD
-// Must declare a 'function prototype' before main() when creating functions
-// in file.
-=======
-// Must declare a 'funtion prototype' when creating functions before main()
->>>>>>> f28d33fb187bc834e6e2956117039f9abe3b6d9b
-void getInt(char c); // function prototype
-int main() { // main function
- return 0;
-}
-void getInt(char w) { //parameter name does not need to match function prototype
- ;
-}
-
-//if function takes no parameters, do:
-int getInt(void); for function prototype
-// and for the function declaration:
-int getInt(void) {}
-// (this is to keep compatibility with older versions of C).
-
-/*
-Functions are call by value. So when a function is called, the arguments passed
-to the function are copies of original arguments (except arrays). Anything you
-do to your arguments do not change the value of the actual argument where the
-function was called.
-
-You can 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;
- }
-}
-
-/////////////////////////////////////
-// Built in functions:
-/////////////////////////////////////
-// from stdio.h:
-// getchar()
-int c = getchar(); //reads character from input.
-// If input = hi, 'h' is returned then next call, 'i' returned.
-while ((c = getchar()) != EOF) { // EOF constant "end of file".
- // Linux: CTRL+D, Windows: CTRL+X
- // must have () around getchar() as != is run before =.
- putchar(c); //prints character (without newline at end)
- char c = getchar();
-}
-
-//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
-}
-
-/*
-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, 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 area(const rect *r)
-{
- return r->width * r->height;
-}
-
-///////////////////////////////////////
-// Function pointers
-///////////////////////////////////////
-/*
-At runtime, 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_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;
-
-
-///////////////////////////////////////
-// 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 stlye](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