summaryrefslogtreecommitdiffhomepage
path: root/c.html.markdown
blob: 3da269f9b1cfa1719d61e37ed5d3b5e4711104c9 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
---
language: c
filename: learnc.c
contributors:
    - ["Adam Bard", "http://adambard.com/"]
    - ["Árpád Goretity", "http://twitter.com/H2CO3_iOS"]
    - ["Jakub Trzebiatowski", "http://cbs.stgn.pl"]
    - ["Marco Scannadinari", "https://marcoms.github.io"]
    - ["Zachary Ferguson", "https://github.io/zfergus2"]
    - ["himanshu", "https://github.com/himanshu81494"]
---

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.
*/

/*
Multi-line comments don't nest /* Be careful */  // comment ends on this line...
*/ // ...not this one!

// Constants: #define <keyword>
// Constants are written in all-caps out of convention, not requirement
#define DAYS_IN_YEAR 365

// Enumeration constants are also ways to declare constants.
// All statements must end with a semicolon
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();
int function_2(void);

// Must declare a 'function prototype' before main() when functions occur after
// your main() function.
int add_two_ints(int x1, int x2); // function prototype
// although `int add_two_ints(int, int);` is also valid (no need to name the args),
// it is recommended to name arguments in the prototype as well for easier inspection

// Your program's entry point is a function called
// main with an integer return type.
int main(void) {
  // your program
}

// The command line arguments used to run your program are also passed to main
// argc being the number of arguments - your program's name counts as 1
// argv is an array of character arrays - containing the arguments themselves
// argv[0] = name of your program, argv[1] = first argument, etc.
int main (int argc, char** argv)
{
  // print output using printf, for "print formatted"
  // %d is an integer, \n is a newline
  printf("%d\n", 0); // => Prints 0

  ///////////////////////////////////////
  // Types
  ///////////////////////////////////////

  // All variables MUST be declared at the top of the current block scope
  // we declare them dynamically along the code for the sake of the tutorial

  // 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 = 0.0f; // 'f' suffix here denotes floating point literal

  // doubles are usually 64-bit floating-point numbers
  double x_double = 0.0; // real numbers without any suffix are doubles

  // integer types may be unsigned (greater than or equal to zero)
  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 in the ASCII character set.
  'A'; // => 65 in 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 is 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 is an unsigned integer type of at least 2 bytes used to represent
  // the size of an object.
  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
  int array_size;
  fscanf(stdin, "%d", &array_size);
  int var_length_array[array_size]; // declare the VLA
  printf("sizeof array = %zu\n", sizeof var_length_array);

  // Example:
  // > Enter the array size: 10
  // > sizeof array = 40

  // Strings are just arrays of chars terminated by a NULL (0x00) byte,
  // represented in strings as the special character '\0'.
  // (We don't have to include the NULL 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)

  // Multi-dimensional arrays:
  int multi_array[2][5] = {
    {1, 2, 3, 4, 5},
    {6, 7, 8, 9, 0}
  };
  // access elements:
  int array_int = multi_array[0][2]; // => 3

  ///////////////////////////////////////
  // Operators
  ///////////////////////////////////////

  // Shorthands for multiple declarations:
  int i1 = 1, i2 = 2;
  float f1 = 1.0, f2 = 2.0;

  int b, c;
  b = c = 0;

  // Arithmetic is straightforward
  i1 + i2; // => 3
  i2 - i1; // => 1
  i2 * i1; // => 2
  i1 / i2; // => 0 (0.5, but truncated towards 0)

  // You need to cast at least one integer to float to get a floating-point result
  (float)i1 / i2; // => 0.5f
  i1 / (double)i2; // => 0.5 // Same with double
  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.
  // Warning: The line below will compile, but it means `(0 < a) < 2`.
  // This expression is always true, because (0 < a) could be either 1 or 0.
  // In this case it's 1, because (0 < 1).
  int between_0_and_2 = 0 < a < 2;
  // Instead use:
  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 ternary expression ( ? : )
  int e = 5;
  int f = 10;
  int z;
  z = (e > f) ? e : f; // => 10 "if e > f return e, else return f."

  // Increment and decrement operators:
  int j = 0;
  int s = j++; // Return j THEN increase j. (s = 0, j = 1)
  s = ++j; // Increase j THEN return j. (s = 2, j = 2)
  // same with j-- and --j

  // Bitwise operators!
  ~0x0F; // => 0xFFFFFFF0 (bitwise negation, "1's complement", example result for 32-bit int)
  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 << 31)
  // - 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 less than ten is true.
    printf("%d, ", ii++); // ii++ increments ii AFTER using its 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 its 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 and Functions MUST have a body. If no body is needed:
  int i;
  for (i = 0; i <= 5; i++) {
    ; // use semicolon to act as the body (null statement)
  }
  // Or
  for (i = 0; i <= 5; i++);

  // branching with multiple choices: switch()
  switch (a) {
  case 0: // labels need to be integral *constant* expressions
    printf("Hey, 'a' equals 0!\n");
    break; // if you don't break, control flow falls over labels
  case 1:
    printf("Huh, 'a' equals 1!\n");
    break;
    // Be careful - without a "break", execution continues until the
    // next "break" is reached.
  case 3:
  case 4:
    printf("Look at that.. 'a' is either 3, or 4\n");
    break;
  default:
    // if `some_integral_expression` didn't match any of the labels
    fputs("Error!\n", stderr);
    exit(-1);
    break;
  }
  /*
  using "goto" in C
  */
  typedef enum { false, true } bool;
  // for C don't have bool as data type :(
  bool disaster = false;
  int i, j;
  for(i=0;i<100;++i)
  for(j=0;j<100;++j)
  {
    if((i + j) >= 150)
        disaster = true;
    if(disaster)
        goto error;
  }
  error :
  printf("Error occured at i = %d & j = %d.\n", i, j);
  /*
  https://ideone.com/GuPhd6
  this will print out "Error occured at i = 52 & j = 99."
  */

  ///////////////////////////////////////
  // 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 `unsigned 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 retrieve the value at the address a pointer is pointing to,
  // put * in front to dereference 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 dereference 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-of) 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 otherarr[] = "foobarbazquirk";
  // or when it's the argument of the `sizeof` or `alignof` operator:
  int arraythethird[10];
  int *ptr = arraythethird; // equivalent with int *ptr = &arr[0];
  printf("%zu, %zu\n", sizeof arraythethird, 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)

  // Note that there is no standard way to get the length of a
  // dynamically allocated array in C. Because of this, if your arrays are
  // going to be passed around your program a lot, you need another variable
  // to keep track of the number of elements (size) of an array. See the
  // functions section for more info.
  int size = 10;
  int *my_arr = malloc(sizeof(int) * size);
  // Add an element to the array
  size++;
  my_arr = realloc(my_arr, sizeof(int) * size);
  my_arr[10] = 5;

  // 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
}

/*
Functions are call by value. When a function is called, the arguments passed to
the function are copies of the original arguments (except arrays). Anything you
do to the arguments in the function do not change the value of the original
argument where the function was called.

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;
  }
}
//NOTE: string.h header file needs to be included to use strlen()

/*
char c[] = "This is a test.";
str_reverse(c);
printf("%s\n", c); // => ".tset a si sihT"
*/
/*
as we can return only one variable
to change values of more than one variables we use call by reference
*/
void swapTwoNumbers(int *a, int *b)
{
    int temp = *a;
    *a = *b;
    *b = temp;
}
/*
int first = 10;
int second = 20;
printf("first: %d\nsecond: %d\n", first, second);
swapTwoNumbers(&first, &second);
printf("first: %d\nsecond: %d\n", first, second);
// values will be swapped
*/

/*
With regards to arrays, they will always be passed to functions
as pointers. Even if you statically allocate an array like `arr[10]`,
it still gets passed as a pointer to the first element in any function calls.
Again, there is no standard way to get the size of a dynamically allocated
array in C.
*/
// Size must be passed!
// Otherwise, this function has no way of knowing how big the array is.
void printIntArray(int *arr, int size) {
    int i;
    for (i = 0; i < size; i++) {
        printf("arr[%d] is: %d\n", i, arr[i]);
    }
}
/*
int my_arr[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
int size = 10;
printIntArray(my_arr, size);
// will print "arr[0] is: 1" etc
*/

// 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
}

// make external variables private to source file with static:
static int j = 0; //other files using testFunc2() cannot access variable j
void testFunc2() {
  extern int j;
}
//**You may also declare functions as static to make them private**

///////////////////////////////////////
// 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 areaptr(const rect *r)
{
  return r->width * r->height;
}

///////////////////////////////////////
// Function pointers
///////////////////////////////////////
/*
At run time, 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 run time)
  // 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;

//Special characters:
/*
'\a'; // alert (bell) character
'\n'; // newline character
'\t'; // tab character (left justifies text)
'\v'; // vertical tab
'\f'; // new page (form feed)
'\r'; // carriage return
'\b'; // backspace character
'\0'; // NULL character. Usually put at end of strings in C.
//   hello\n\0. \0 used by convention to mark end of string.
'\\'; // backslash
'\?'; // question mark
'\''; // single quote
'\"'; // double quote
'\xhh'; // hexadecimal number. Example: '\xb' = vertical tab character
'\0oo'; // 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";    // hexadecimal
"%o";    // octal
"%%";    // prints %
*/

///////////////////////////////////////
// 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 //
//---------------------------------------------------//

/******************************* Header Files **********************************

Header files are an important part of C as they allow for the connection of C
source files and can simplify code and definitions by separating them into
separate files.

Header files are syntactically similar to C source files but reside in ".h"
files. They can be included in your C source file by using the precompiler
command #include "example.h", given that example.h exists in the same directory
as the C file.
*/

/* A safe guard to prevent the header from being defined too many times. This */
/* happens in the case of circle dependency, the contents of the header is    */
/* already defined.                                                           */
#ifndef EXAMPLE_H /* if EXAMPLE_H is not yet defined. */
#define EXAMPLE_H /* Define the macro EXAMPLE_H. */

/* Other headers can be included in headers and therefore transitively */
/* included into files that include this header.                       */
#include <string.h>

/* Like c source files macros can be defined in headers and used in files */
/* that include this header file.                                         */
#define EXAMPLE_NAME "Dennis Ritchie"
/* Function macros can also be defined. */
#define ADD(a, b) (a + b)

/* Structs and typedefs can be used for consistency between files. */
typedef struct node
{
    int val;
    struct node *next;
} Node;

/* So can enumerations. */
enum traffic_light_state {GREEN, YELLOW, RED};

/* Function prototypes can also be defined here for use in multiple files,  */
/* but it is bad practice to define the function in the header. Definitions */
/* should instead be put in a C file.                                       */
Node createLinkedList(int *vals, int len);

/* Beyond the above elements, other definitions should be left to a C source */
/* file. Excessive includes or definitions should, also not be contained in */
/* a header file but instead put into separate headers or a C file.          */

#endif /* End of the if precompiler directive. */

```
## 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 Dennis Ritchie, the creator of C, and Brian Kernighan. 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 style](https://www.kernel.org/doc/Documentation/process/coding-style.rst).

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