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Árpád Goretity
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c.html.markdown
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c.html.markdown
@ -5,19 +5,20 @@ 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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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 //
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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.
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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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@ -25,6 +26,11 @@ Multi-line comments look like this.
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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
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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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@ -33,264 +39,317 @@ 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 hystorical 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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// 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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// 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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///////////////////////////////////////
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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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///////////////////////////////////////
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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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// 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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|
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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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|
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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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|
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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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|
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// Other than char, which is always 1 byte, these types vary in size depending
|
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// on your machine. 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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// For example,
|
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printf("%lu\n", sizeof(int)); // => 4 (on machines with 4-byte words)
|
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|
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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)
|
||||
|
||||
|
||||
// You can initialize an array to 0 thusly:
|
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char my_array[20] = {0};
|
||||
|
||||
// Indexing an array is like other languages -- or,
|
||||
// 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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|
||||
// 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.)
|
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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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/*
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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 have undefined values.
|
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*/
|
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|
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printf("%d\n", a_string[16]); // => 0
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|
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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
|
||||
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");
|
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} else if (0) {
|
||||
printf("I am also never run\n");
|
||||
} else {
|
||||
printf("I print\n");
|
||||
}
|
||||
|
||||
// While loops exist
|
||||
int ii = 0;
|
||||
while (ii < 10) {
|
||||
printf("%d, ", ii++); // ii++ increments ii in-place, after using its value.
|
||||
} // => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "
|
||||
|
||||
printf("\n");
|
||||
|
||||
int kk = 0;
|
||||
do {
|
||||
printf("%d, ", kk);
|
||||
} while (++kk < 10); // ++kk increments kk in-place, before using its value
|
||||
// => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "
|
||||
|
||||
printf("\n");
|
||||
|
||||
// For loops too
|
||||
int jj;
|
||||
for (jj=0; jj < 10; jj++) {
|
||||
printf("%d, ", jj);
|
||||
} // => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "
|
||||
|
||||
printf("\n");
|
||||
|
||||
///////////////////////////////////////
|
||||
// Typecasting
|
||||
///////////////////////////////////////
|
||||
|
||||
// Every value in C has a type, but you can cast one value into another type
|
||||
// if you want.
|
||||
|
||||
int x_hex = 0x01; // You can assign vars with hex literals
|
||||
|
||||
// Casting between types will attempt to preserve their numeric values
|
||||
printf("%d\n", x_hex); // => Prints 1
|
||||
printf("%d\n", (short) x_hex); // => Prints 1
|
||||
printf("%d\n", (char) x_hex); // => Prints 1
|
||||
|
||||
// Types will overflow without warning
|
||||
printf("%d\n", (char) 257); // => 1 (Max char = 255)
|
||||
|
||||
// Integral types can be cast to floating-point types, and vice-versa.
|
||||
printf("%f\n", (float)100); // %f formats a float
|
||||
printf("%lf\n", (double)100); // %lf formats a double
|
||||
printf("%d\n", (char)100.0);
|
||||
|
||||
///////////////////////////////////////
|
||||
// Pointers
|
||||
///////////////////////////////////////
|
||||
|
||||
// A pointer is a variable declared to store a memory address. Its declaration will
|
||||
// also tell you the type of data it points to. You can retrieve the memory address
|
||||
// of your variables, then mess with them.
|
||||
|
||||
int x = 0;
|
||||
printf("%p\n", &x); // Use & to retrieve the address of a variable
|
||||
// (%p formats a pointer)
|
||||
// => Prints some address in memory;
|
||||
|
||||
|
||||
// Pointers start with * in their declaration
|
||||
int *px, not_a_pointer; // px is a pointer to an int
|
||||
px = &x; // Stores the address of x in px
|
||||
printf("%p\n", px); // => Prints some address in memory
|
||||
printf("%d, %d\n", (int)sizeof(px), (int)sizeof(not_a_pointer));
|
||||
// => Prints "8, 4" on 64-bit system
|
||||
|
||||
// To retreive the value at the address a pointer is pointing to,
|
||||
// put * in front to de-reference it.
|
||||
printf("%d\n", *px); // => Prints 0, the value of x, which is what px is pointing to the address of
|
||||
|
||||
// You can also change the value the pointer is pointing to.
|
||||
// We'll have to wrap the de-reference in parenthesis because
|
||||
// ++ has a higher precedence than *.
|
||||
(*px)++; // Increment the value px is pointing to by 1
|
||||
printf("%d\n", *px); // => Prints 1
|
||||
printf("%d\n", x); // => Prints 1
|
||||
|
||||
int x_array[20]; // Arrays are a good way to allocate a contiguous block of memory
|
||||
int xx;
|
||||
for (xx=0; xx<20; xx++) {
|
||||
x_array[xx] = 20 - xx;
|
||||
} // Initialize x_array to 20, 19, 18,... 2, 1
|
||||
|
||||
// Declare a pointer of type int and initialize it to point to x_array
|
||||
int* x_ptr = x_array;
|
||||
// x_ptr now points to the first element in the array (the integer 20).
|
||||
// This works because arrays are actually just pointers to their first element.
|
||||
|
||||
// Arrays are pointers to their first element
|
||||
printf("%d\n", *(x_ptr)); // => Prints 20
|
||||
printf("%d\n", x_array[0]); // => Prints 20
|
||||
|
||||
// Pointers are incremented and decremented based on their type
|
||||
printf("%d\n", *(x_ptr + 1)); // => Prints 19
|
||||
printf("%d\n", x_array[1]); // => Prints 19
|
||||
|
||||
// You can also dynamically allocate contiguous blocks of memory with the
|
||||
// standard library function malloc, which takes one integer argument
|
||||
// representing the number of bytes to allocate from the heap.
|
||||
int* my_ptr = (int*) malloc(sizeof(int) * 20);
|
||||
for (xx=0; xx<20; xx++) {
|
||||
*(my_ptr + xx) = 20 - xx; // my_ptr[xx] = 20-xx would also work here
|
||||
} // Initialize memory to 20, 19, 18, 17... 2, 1 (as ints)
|
||||
|
||||
// Dereferencing memory that you haven't allocated gives
|
||||
// unpredictable results
|
||||
printf("%d\n", *(my_ptr + 21)); // => Prints who-knows-what?
|
||||
|
||||
// When you're done with a malloc'd block of memory, you need to free it,
|
||||
// or else no one else can use it until your program terminates
|
||||
free(my_ptr);
|
||||
|
||||
// Strings can be char arrays, but are usually represented as char
|
||||
// pointers:
|
||||
char* my_str = "This is my very own string";
|
||||
|
||||
printf("%c\n", *my_str); // => 'T'
|
||||
|
||||
function_1();
|
||||
// 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 would also work here, and it's also more readable
|
||||
} // 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
|
||||
|
||||
///////////////////////////////////////
|
||||
@ -300,7 +359,8 @@ function_1();
|
||||
// Function declaration syntax:
|
||||
// <return type> <function name>(<args>)
|
||||
|
||||
int add_two_ints(int x1, int x2){
|
||||
int add_two_ints(int x1, int x2)
|
||||
{
|
||||
return x1 + x2; // Use return to return a value
|
||||
}
|
||||
|
||||
@ -312,10 +372,12 @@ Example: in-place string reversal
|
||||
*/
|
||||
|
||||
// A void function returns no value
|
||||
void str_reverse(char* str_in){
|
||||
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++){
|
||||
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;
|
||||
@ -336,15 +398,20 @@ printf("%s\n", c); // => ".tset a si sihT"
|
||||
typedef int my_type;
|
||||
my_type my_type_var = 0;
|
||||
|
||||
// Structs are just collections of data
|
||||
// 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 generally not true that sizeof(struct rectangle) == sizeof(int) + sizeof(int) due to
|
||||
// potential padding between the structure members (this is for alignment reasons. Probably won't
|
||||
// happen if all members are of the same type, but watch out!
|
||||
// See http://stackoverflow.com/questions/119123/why-isnt-sizeof-for-a-struct-equal-to-the-sum-of-sizeof-of-each-member
|
||||
// for further information.
|
||||
|
||||
void function_1(){
|
||||
|
||||
void function_1()
|
||||
{
|
||||
struct rectangle my_rec;
|
||||
|
||||
// Access struct members with .
|
||||
@ -352,22 +419,29 @@ void function_1(){
|
||||
my_rec.height = 20;
|
||||
|
||||
// You can declare pointers to structs
|
||||
struct rectangle* my_rec_ptr = &my_rec;
|
||||
struct rectangle *my_rec_ptr = &my_rec;
|
||||
|
||||
// Use dereferencing to set struct pointer members...
|
||||
(*my_rec_ptr).width = 30;
|
||||
|
||||
// ... or use the -> shorthand
|
||||
// ... 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){
|
||||
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
|
||||
///////////////////////////////////////
|
||||
@ -379,10 +453,11 @@ However, definition syntax may be initially confusing.
|
||||
|
||||
Example: use str_reverse from a pointer
|
||||
*/
|
||||
void str_reverse_through_pointer(char * str_in) {
|
||||
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.
|
||||
}
|
||||
@ -403,7 +478,15 @@ typedef void (*my_fnp_type)(char *);
|
||||
## 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/)
|
||||
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.
|
||||
|
||||
Loading…
Reference in New Issue
Block a user