diff options
| -rw-r--r-- | c.html.markdown.orig | 641 |
1 files changed, 0 insertions, 641 deletions
diff --git a/c.html.markdown.orig b/c.html.markdown.orig deleted file mode 100644 index 47996cb2..00000000 --- a/c.html.markdown.orig +++ /dev/null @@ -1,641 +0,0 @@ ---- -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 |