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530 lines
20 KiB
Markdown
530 lines
20 KiB
Markdown
---
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- name: c
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- category: language
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- language: c
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- filename: learnc.c
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- contributors:
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- [Adam Bard](http://adambard.com/)
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- [Árpád Goretity](http://twitter.com/H2CO3_iOS)
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---
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Ah, C. Still **the** language of modern high-performance computing.
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C is the lowest-level language most programmers will ever use, but
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it more than makes up for it with raw speed. Just be aware of its manual
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memory management and C will take you as far as you need to go.
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```c
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// Single-line comments start with // - only available in C99 and later.
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/*
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Multi-line comments look like this. They work in C89 as well.
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*/
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// Import headers with #include
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#include <stdlib.h>
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#include <stdio.h>
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#include <string.h>
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// file names between <angle brackets> are headers from the C standard library.
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// They are searched for by the preprocessor in the system include paths
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// (usually /usr/lib on Unices, can be controlled with the -I<dir> option if you are using GCC or clang.)
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// For your own headers, use double quotes instead of angle brackets:
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#include "my_header.h"
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// The C preprocessor introduces an almost fully-featured macro language. It's useful, but
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// it can be confusing (and what's even worse, it can be misused). Read the
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// Wikipedia article on the C preprocessor for further information:
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// http://en.wikipedia.org/wiki/C_preprocessor
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// Declare function signatures in advance in a .h file, or at the top of
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// your .c file.
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void function_1();
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void function_2();
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// Your program's entry point is a function called
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// main with an integer return type.
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int main() {
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// print output using printf, for "print formatted"
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// %d is an integer, \n is a newline
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printf("%d\n", 0); // => Prints 0
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// All statements must end with a semicolon
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///////////////////////////////////////
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// Types
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///////////////////////////////////////
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// You have to declare variables before using them. A variable declaration
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// requires you to specify its type; a variable's type determines its size
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// in bytes.
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// ints are usually 4 bytes
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int x_int = 0;
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// shorts are usually 2 bytes
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short x_short = 0;
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// chars are guaranteed to be 1 byte
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char x_char = 0;
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char y_char = 'y'; // Char literals are quoted with ''
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// longs are often 4 to 8 bytes; long longs are guaranteed to be at least
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// 64 bits
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long x_long = 0;
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long long x_long_long = 0;
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// floats are usually 32-bit floating point numbers
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float x_float = 0.0;
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// doubles are usually 64-bit floating-point numbers
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double x_double = 0.0;
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// Integral types may be unsigned. This means they can't be negative, but
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// the maximum value of an unsigned variable is greater than the maximum
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// signed value of the same size.
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unsigned char ux_char;
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unsigned short ux_short;
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unsigned int ux_int;
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unsigned long long ux_long_long;
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// Other than char, which is always 1 byte (but not necessarily 8 bits!),
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// these types vary in size depending on your machine and compiler.
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// sizeof(T) gives you the size of a variable with type T in
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// bytes so you can express the size of these types in a portable way.
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// sizeof(obj) yields the size of an actual expression (variable, literal, etc.).
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// For example,
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printf("%zu\n", sizeof(int)); // => 4 (on most machines with 4-byte words)
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// It's worth noting that if the argument of the `sizeof` operator is not a type but an expression,
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// then its argument is not evaluated except VLAs (see below). Also, `sizeof()` is an operator, not a function,
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// furthermore, the value it yields is a compile-time constant (except when used on VLAs, again.)
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int a = 1;
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size_t size = sizeof(a++); // a++ is not evaluated
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printf("sizeof(a++) = %zu where a = %d\n", size, a);
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// the above code prints "sizeof(a++) = 4 where a = 1" (on a usual 32-bit architecture)
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// Arrays must be initialized with a concrete size.
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char my_char_array[20]; // This array occupies 1 * 20 = 20 bytes
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int my_int_array[20]; // This array occupies 4 * 20 = 80 bytes
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// (assuming 4-byte words)
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// You can initialize an array to 0 thusly:
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char my_array[20] = {0};
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// Indexing an array is like other languages -- or,
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// rather, other languages are like C
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my_array[0]; // => 0
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// Arrays are mutable; it's just memory!
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my_array[1] = 2;
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printf("%d\n", my_array[1]); // => 2
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// In C99 (and as an optional feature in C11), variable-length arrays (VLAs) can be declared as well.
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// The size of such an array need not be a compile time constant:
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printf("Enter the array size: "); // ask the user for an array size
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char buf[0x100];
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fgets(buf, sizeof buf, stdin);
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size_t size = strtoul(buf, NULL, 10); // strtoul parses a string to an unsigned integer
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int var_length_array[size]; // declare the VLA
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printf("sizeof array = %zu\n", sizeof var_length_array);
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// A possible outcome of this program may be:
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Enter the array size: 10
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sizeof array = 40
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// Strings are just arrays of chars terminated by a NUL (0x00) byte,
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// represented in strings as the special character '\0'.
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// (We don't have to include the NUL byte in string literals; the compiler
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// inserts it at the end of the array for us.)
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char a_string[20] = "This is a string";
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printf("%s\n", a_string); // %s formats a string
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/*
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You may have noticed that a_string is only 16 chars long.
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Char #17 is the NUL byte.
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Chars #18, 19 and 20 are 0 as well - if an initializer list (in this case, the string literal)
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has less elements than the array it is initializing, then excess array elements are implicitly
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initialized to zero. This is why int ar[10] = { 0 } works as expected intuitively.
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*/
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printf("%d\n", a_string[16]); // => 0
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// So string literals are strings enclosed within double quotes, but if we have characters
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// between single quotes, that's a character literal.
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// It's of type `int`, and *not* `char` (for historical reasons).
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int cha = 'a'; // fine
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char chb = 'a'; // fine too (implicit conversion from int to char - truncation)
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///////////////////////////////////////
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// Operators
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///////////////////////////////////////
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int i1 = 1, i2 = 2; // Shorthand for multiple declaration
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float f1 = 1.0, f2 = 2.0;
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// Arithmetic is straightforward
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i1 + i2; // => 3
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i2 - i1; // => 1
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i2 * i1; // => 2
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i1 / i2; // => 0 (0.5, but truncated towards 0)
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f1 / f2; // => 0.5, plus or minus epsilon - floating-point numbers and calculations are not exact
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// Modulo is there as well
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11 % 3; // => 2
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// Comparison operators are probably familiar, but
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// there is no boolean type in c. We use ints instead.
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// (Or _Bool or bool in C99.)
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// 0 is false, anything else is true. (The comparison
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// operators always yield 0 or 1.)
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3 == 2; // => 0 (false)
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3 != 2; // => 1 (true)
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3 > 2; // => 1
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3 < 2; // => 0
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2 <= 2; // => 1
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2 >= 2; // => 1
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// C is not Python - comparisons don't chain.
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int a = 1;
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// WRONG:
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int between_0_and_2 = 0 < a < 2;
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// Correct:
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int between_0_and_2 = 0 < a && a < 2;
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// Logic works on ints
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!3; // => 0 (Logical not)
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!0; // => 1
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1 && 1; // => 1 (Logical and)
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0 && 1; // => 0
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0 || 1; // => 1 (Logical or)
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0 || 0; // => 0
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// Bitwise operators!
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~0x0F; // => 0xF0 (bitwise negation, "1's complement")
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0x0F & 0xF0; // => 0x00 (bitwise AND)
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0x0F | 0xF0; // => 0xFF (bitwise OR)
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0x04 ^ 0x0F; // => 0x0B (bitwise XOR)
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0x01 << 1; // => 0x02 (bitwise left shift (by 1))
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0x02 >> 1; // => 0x01 (bitwise right shift (by 1))
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// Be careful when shifting signed integers - the following are all undefined behavior:
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// - shifting into the sign bit of a signed integer (int a = 1 << 32)
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// - left-shifting a negative number (int a = -1 << 2)
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// - shifting by an offset which is more than or equal to the width of the type of the LHS:
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// int a = 1 << 32; // UB if int is 32 bits wide
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///////////////////////////////////////
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// Control Structures
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///////////////////////////////////////
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if (0) {
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printf("I am never run\n");
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} else if (0) {
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printf("I am also never run\n");
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} else {
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printf("I print\n");
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}
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// While loops exist
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int ii = 0;
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while (ii < 10) {
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printf("%d, ", ii++); // ii++ increments ii in-place, after yielding its value ("postincrement").
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} // => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "
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printf("\n");
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int kk = 0;
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do {
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printf("%d, ", kk);
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} while (++kk < 10); // ++kk increments kk in-place, and yields the already incremented value ("preincrement")
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// => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "
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printf("\n");
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// For loops too
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int jj;
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for (jj=0; jj < 10; jj++) {
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printf("%d, ", jj);
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} // => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "
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printf("\n");
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// branching with multiple choices: switch()
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switch (some_integral_expression) {
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case 0: // labels need to be integral *constant* epxressions
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do_stuff();
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break; // if you don't break, control flow falls over labels - you usually don't want that.
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case 1:
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do_something_else();
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break;
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default:
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// if `some_integral_expression` didn't match any of the labels
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fputs("error!\n", stderr);
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exit(-1);
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break;
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}
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///////////////////////////////////////
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// Typecasting
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///////////////////////////////////////
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// Every value in C has a type, but you can cast one value into another type
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// if you want (with some constraints).
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int x_hex = 0x01; // You can assign vars with hex literals
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// Casting between types will attempt to preserve their numeric values
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printf("%d\n", x_hex); // => Prints 1
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printf("%d\n", (short) x_hex); // => Prints 1
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printf("%d\n", (char) x_hex); // => Prints 1
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// Types will overflow without warning
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printf("%d\n", (unsigned char) 257); // => 1 (Max char = 255 if char is 8 bits long)
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// printf("%d\n", (unsigned char) 257); would be undefined behavior - `char' is usually signed
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// on most modern systems, and signed integer overflow invokes UB.
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// Also, for determining the maximal value of a `char`, a `signed char` and an `unisigned char`,
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// respectively, use the CHAR_MAX, SCHAR_MAX and UCHAR_MAX macros from <limits.h>
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// Integral types can be cast to floating-point types, and vice-versa.
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printf("%f\n", (float)100); // %f formats a float
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printf("%lf\n", (double)100); // %lf formats a double
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printf("%d\n", (char)100.0);
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///////////////////////////////////////
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// Pointers
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///////////////////////////////////////
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// A pointer is a variable declared to store a memory address. Its declaration will
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// also tell you the type of data it points to. You can retrieve the memory address
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// of your variables, then mess with them.
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int x = 0;
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printf("%p\n", (void *)&x); // Use & to retrieve the address of a variable
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// (%p formats an object pointer of type void *)
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// => Prints some address in memory;
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// Pointers start with * in their declaration
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int *px, not_a_pointer; // px is a pointer to an int
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px = &x; // Stores the address of x in px
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printf("%p\n", (void *)px); // => Prints some address in memory
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printf("%zu, %zu\n", sizeof(px), sizeof(not_a_pointer));
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// => Prints "8, 4" on a typical 64-bit system
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// To retreive the value at the address a pointer is pointing to,
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// put * in front to de-reference it.
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// Note: yes, it may be confusing that '*' is used for _both_ declaring a pointer and dereferencing it.
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printf("%d\n", *px); // => Prints 0, the value of x, which is what px is pointing to the address of
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// You can also change the value the pointer is pointing to.
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// We'll have to wrap the de-reference in parenthesis because
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// ++ has a higher precedence than *.
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(*px)++; // Increment the value px is pointing to by 1
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printf("%d\n", *px); // => Prints 1
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printf("%d\n", x); // => Prints 1
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int x_array[20]; // Arrays are a good way to allocate a contiguous block of memory
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int xx;
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for (xx = 0; xx < 20; xx++) {
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x_array[xx] = 20 - xx;
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} // Initialize x_array to 20, 19, 18,... 2, 1
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// Declare a pointer of type int and initialize it to point to x_array
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int* x_ptr = x_array;
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// x_ptr now points to the first element in the array (the integer 20).
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// This works because arrays often decay into pointers to their first element.
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// For example, when an array is passed to a function or is assigned to a pointer,
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// it decays into (implicitly converted to) a pointer.
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// Exceptions: when the array is the argument of the `&` (address-od) operator:
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int arr[10];
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int (*ptr_to_arr)[10] = &arr; // &arr is NOT of type `int *`! It's of type "pointer to array" (of ten `int`s).
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// or when the array is a string literal used for initializing a char array:
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char arr[] = "foobarbazquirk";
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// or when it's the argument of the `sizeof` or `alignof` operator:
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int arr[10];
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int *ptr = arr; // equivalent with int *ptr = &arr[0];
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printf("%zu %zu\n", sizeof arr, sizeof ptr); // probably prints "40, 4" or "40, 8"
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// Pointers are incremented and decremented based on their type
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// (this is called pointer arithmetic)
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printf("%d\n", *(x_ptr + 1)); // => Prints 19
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printf("%d\n", x_array[1]); // => Prints 19
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// You can also dynamically allocate contiguous blocks of memory with the
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// standard library function malloc, which takes one argument of type size_t
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// representing the number of bytes to allocate (usually from the heap, although this
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// may not be true on e. g. embedded systems - the C standard says nothing about it).
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int *my_ptr = malloc(sizeof(*my_ptr) * 20);
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for (xx = 0; xx < 20; xx++) {
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*(my_ptr + xx) = 20 - xx; // my_ptr[xx] = 20-xx would also work here, and it's also more readable
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} // Initialize memory to 20, 19, 18, 17... 2, 1 (as ints)
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// Dereferencing memory that you haven't allocated gives
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// "unpredictable results" - the program is said to invoke "undefined behavior"
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printf("%d\n", *(my_ptr + 21)); // => Prints who-knows-what? It may even crash.
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// When you're done with a malloc'd block of memory, you need to free it,
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// or else no one else can use it until your program terminates
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// (this is called a "memory leak"):
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free(my_ptr);
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// Strings are arrays of char, but they are usually represented as a
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// pointer-to-char (which is a pointer to the first element of the array).
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// It's good practice to use `const char *' when referring to a string literal,
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// since string literals shall not be modified (i. e. "foo"[0] = 'a' is ILLEGAL.)
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const char *my_str = "This is my very own string literal";
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printf("%c\n", *my_str); // => 'T'
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// This is not the case if the string is an array (potentially initialized with a string literal)
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// that resides in writable memory, as in:
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char foo[] = "foo";
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foo[0] = 'a'; // this is legal, foo now contains "aoo"
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function_1();
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} // end main function
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///////////////////////////////////////
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// Functions
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///////////////////////////////////////
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// Function declaration syntax:
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// <return type> <function name>(<args>)
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int add_two_ints(int x1, int x2)
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{
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return x1 + x2; // Use return to return a value
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}
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/*
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Functions are pass-by-value, but you can make your own references
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with pointers so functions can mutate their values.
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Example: in-place string reversal
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*/
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// A void function returns no value
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void str_reverse(char *str_in)
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{
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char tmp;
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int ii = 0;
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size_t len = strlen(str_in); // `strlen()` is part of the c standard library
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for (ii = 0; ii < len / 2; ii++) {
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tmp = str_in[ii];
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str_in[ii] = str_in[len - ii - 1]; // ii-th char from end
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str_in[len - ii - 1] = tmp;
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}
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}
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/*
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char c[] = "This is a test.";
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str_reverse(c);
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printf("%s\n", c); // => ".tset a si sihT"
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*/
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///////////////////////////////////////
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// User-defined types and structs
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///////////////////////////////////////
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// Typedefs can be used to create type aliases
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typedef int my_type;
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my_type my_type_var = 0;
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// Structs are just collections of data, the members are allocated sequentially, in the order they are written:
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struct rectangle {
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int width;
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int height;
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};
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// it's generally not true that sizeof(struct rectangle) == sizeof(int) + sizeof(int) due to
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// potential padding between the structure members (this is for alignment reasons. Probably won't
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// happen if all members are of the same type, but watch out!
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// See http://stackoverflow.com/questions/119123/why-isnt-sizeof-for-a-struct-equal-to-the-sum-of-sizeof-of-each-member
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// for further information.
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void function_1()
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{
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struct rectangle my_rec;
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// Access struct members with .
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my_rec.width = 10;
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my_rec.height = 20;
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// You can declare pointers to structs
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struct rectangle *my_rec_ptr = &my_rec;
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// Use dereferencing to set struct pointer members...
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(*my_rec_ptr).width = 30;
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// ... or even better: prefer the -> shorthand for the sake of readability
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my_rec_ptr->height = 10; // Same as (*my_rec_ptr).height = 10;
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}
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// You can apply a typedef to a struct for convenience
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typedef struct rectangle rect;
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int area(rect r)
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{
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return r.width * r.height;
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}
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// 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;
|
|
|
|
```
|
|
|
|
## 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.
|