C-Programming LanguagePhilipp von Neumann's Digital Garden

Overview

C is one of the widest spread Programming Languagesand was established and developed by Dennis Ritichie in the early 70s at Bell Laboratories.

C is a compiling language and offers a low-level acces to memory.

In the following you will find some notes on the C-Programming Language and how i use it in Emacs on MacOS.

Content

C Compiler

GCC
Clang

C Standards

Program Structure and Syntax

Every C-Program has exactly one main function and is defined like every other function in C:

it consists of the function name, e.g., main
the function name is prefixed by its return type, e.g., int main
after this, it is followed by a pair of parentheses that defines the input parameter, e.g., int main()
- here main doesn’t expect any input arguments
then, the function body follows, enclosed in {}
- every statement in the body needs to be closed with a ;
- the execution of the function is terminated by the return statement
  - return 0; for succesful execution
  - return 1; for execution with error

a example:

int main()
{
  printf("Hello World!\n");
 
  return 0; // the return value is 0, which is of the data type integer
}

For this function you need the stdio.h librarie. Libraries and other header files are included, via #include, at the beginnging of the c-file.

#include <stdio.h>
 
int main()
{
  printf("Hello World!\n");
 
  return 0; // the return value is 0, which is of the data type integer
}

Variables and Datatypes

A variable consists of a data type (dtype) and a name. The dtype describes the type of information the variable holds. It can be a integer, a floating-point number, or a subeset of these. There are also other variable types. The name should be descriptive. It must not start with a number or contain whitespace. The declaration for a variable only happens once in your code. A second delcaration for the same variable is a redefinition and this will produce a error.

Declaring a variable:

int my_age;

Assigning a value to a variable:

// if this is the first assignement, it is called 'initialization'
my_age = 23;

Declaring and initializing a variable:

int my_age = 23;

dtypes

Standard dtype

The standard dtypes are depending on the machine you’re working on. They can differ in bit size. That’s why this table gives the bit sizes ind minimum bit sizes. For printing values of a specific dtype you also need the correct format specifier. These are also included in the table. (Don’t forget, that you can also print a string with "%s".)

TIP

Remember these bit sizes and numbers:

8 Bit → 256

16Bit → 65.536

24 Bit → 16 Million

32 Bit → 4 Billion

64 Bit → so huge!

dtype	Remark	min. bit size	value-range	format specifier	suffix
char	smallest integer dtype	8	0-255	%c
signed char	like char, always signed	8	-128-127	%c,%hhi
unsigned char	like char, always unsigned	8	0-255	%c,%hhu
(signed) short (int)	always signed	16	-32.767-32.767	%hi,%hd
unsigned short (int)	always unsigned	16	0,65.535	%u
(signed) int	basic signed integer type	16	-32.767,32.767	%i,%d
unsigned int	basic unsigned integer	16	0,65.535	%u	u,U
(signed) long (int)	always signed	32	−2bil,2bil	%li,%ld	l,L
unsigned long (int)	always unsigned	32	4billion	%lu	ul,UL
(signed) long long (int)	always signed	64	-9quint.,9quint	%lli,%lld	ll,LL
unsigned long long (int)	always unsigned	64	0,18quintillion	%lu	ull,ULL
float	single precision floating point	32		%f	f,F
double	double precision	64		%lf

In the C standard C99 a header stdint.h file exists for predefined data types, that will always have the same size on every machine.

List of types

dtype	bit size	value range	format specifier	suffix
uint8_t	8	0 to 255	%hhu
int8_t	8	-128 to 127	%hhi
uint16_t	16	0 to 65.535	%hu
int16_t	16	-32.768 to 32.767	%hi,%hd
uint32_t	32	0 to 4 billion	%u	u
int32_t	32	−2 bil. to 2 bil.	%i,%d
uint64_t	64	0 to 18 quintillion	%llu	u,U
int64_t	64	-9 quint. to 9 quint.	%lli,%lld	l,L

Using single characters as variables

You can also use single characters as variables. They are saved as ASCII values. Use for this the smallest, char, dtype for this. You have to put your single character inside single '.

char character = 'A';
 
printf("Print as dezimal number: %d\n Print as character: %c\n", character);

Since characters are saved as numbers one can also do artihmetics with these:

char chracter_a = 'a';
char chracter_b = 'b';
 
char new_char = chracter_a + character_b;

TIP

Important ASCII numbers to remember:

0 → ASCII: 48

a → ASCII: 97

A → ASCII: 65

Type casting

Type casting is the conversion from one data type to another. You can always cast a smaller data type to a bigger one. The internal binary representation is then extended with bits on the left side, which take the value of the most significant bit (MSB) in case of signed types (sign extension) or are set to zero in case of unsigned types (zero extension).

In C you can cast via putting the new dtype in parentheses as prefix to the variable you want to cast:

char my_char = 8;
int my_int = (int) my_char;

Enumerations (Enum)

A enum is a user-defined data type that represents a set of constants. It provides a way to create a group of related named values. Under the hood, the c-compiler assigns integer values to each of the enum categories, starting from 0 by default. These categories are global and constant (immutable). By convention, enum categories are written in ALL_UPPERCASE. Including a default category is a good practice for initialization and error handling.

enum Cheese
{
  MOZZARELLA,     // implicitly assigned 0
  GOUDA,          // implicitly assigned 1
  APPENZELLER,    // implicitly assigned 2
  GRUYER,         // implicitly assigned 3
  PECORINO,       // implicitly assigned 4
  CAMEMBERT,      // implicitly assigned 5
  INVALID_CHEESE, // your default category
};

But you can also assign freely choosen integer values to each constant:

enum Cheese
{
  MOZZARELLA = 0,
  GOUDA = 3,
  APPENZELLER = 1,
  GRUYER = 2,
  PECORINO = 4,
  CAMEMBERT = 6,
  INVALID_CHEESE = 5,
};

If you want to create decalare and initialize a enum variable you have to use the data type enum and the name of the enum you want to use:

#include <stdio.h>
 
enum Cheese
{
  MOZZARELLA,
  GOUDA,
  APPENZELLER,
  GRUYER,
  PECORINO,
  CAMEMBERT,
  INVALID_CHEESE,
};
 
int main()
{
  enum Cheese my_favorite_cheese = INVALID_CHEESE;
 
  printf("What is your favorite cheese?\nMozzarella (0), Gouda (1), Appenzeller (2), Gruyer (3), Pecorino (4) or Camembert (5)?");
  scanf("%d", &my_favorite_cheese);
 
  printf("MMMhhhh! You selected %d,\nGreat taste!", my_favorite_cheese);
 
  return 0;
}

How to do a Switch Case with enum:

#include <stdio.h>
 
enum Cheese
{
  MOZZARELLA,
  GOUDA,
  APPENZELLER,
  GRUYER,
  PECORINO,
  CAMEMBERT,
  INVALID_CHEESE,
};
 
int main()
{
  enum Cheese my_favorite_cheese = INVALID_CHEESE;
 
  printf("What is your favorite cheese?\nMozzarella (0), Gouda (1), Appenzeller (2), Gruyer (3), Pecorino (4) or Camembert (5)?\n");
  scanf("%d", &my_favorite_cheese);
 
  switch (my_favorite_cheese)
	{
	  case MOZZARELLA:
		{
		  printf("Mozzarella! So delicious!\n");
		  break;
		}
	  case GOUDA:
		{
		  printf("Gouda! A real classic!\n");
		  break;
		}
	  case APPENZELLER:
		{
		  printf("Appenzeller! Do you want to be my friend?\n");
		  break;
		}
	  case GRUYER:
		{
		  printf("Gruyer! I like!\n");
		  break;
		}
	  case PECORINO:
		{
		  printf("Pecorino! Super salty!\n");
		  break;
		}
	  case CAMEMBERT:
		{
		  printf("Camembert! Welcome to France!\n");
		  break;
		}
	  default:
	    {
		printf("Are you not a real fan of Cheese? :(\nSomething went wrong.");
			break;
	    }
	}
 
  return 0;
}

Input and Ouput

Via the stdio.h librarie you can print some value to the console or get values from the user, who is using the console.

float my_float;
 
printf("Please enter a float value: ") // print this line in the console
scanf("%f", &my_float); // get user input and write it to the adress of =my_float=
printf("float value: %f\n", my_float) // print the value of =my_float=

Control Structures and Logic

Boolean

With logical operators, you can check whether a statement is either true or false. Since C99, the datatype bool has been available for this, which you can find in stdbool.h. The values true and false correspond to 1 and 0, respectively.

#include <stdbool.h>
#include <stdio.h>
 
int main()
{
  int a = 22;
  int b = 42;
 
  // boolean: true or false
  bool comparison = false;
 
  // a greater then b
  comparison = a > b;
  printf("a > b = %d\n", comparison);
 
  // a less then b
  comparison = a < b;
  printf("a < b = %d\n", comparison);
 
  // a and b are equal
  comparison = a == b;
  printf("a == b = %d\n", comparison);
 
  // a and b are not equal
  comparison = a != b;
  printf("a != b = %d\n", comparison);
 
  // a greater then or equal to b
  comparison = a >= b;
  printf("a >= b = %d\n", comparison);
 
  // a less than or equal to b
  comparison = a <= b;
  printf("a <= b = %d\n", comparison);
 
  return 0;
}

You can also concetenate logical statements with && and ||. With && both statements must evaluate to true, for the whole statement to be true; otherwise, it is false. With ||, only one statement needs to be true, for the whole statement to evaluate to true. If no statement is true or both are false the whole statement is false.

#include <stdio.h>
#include <stdbool.h>
 
int ()
{
  bool A = true;
  bool B = false;
  bool C = true;
 
  // and: && (ampersand)
  // or: || (pipes)
 
  bool C1 = A && B;
  bool C2 = A || C;
 
  return 0;
}

If-Else Statement

If-Statements are used to decide whether something is executed or not. When the logical statement evaluates to true the if-branch is executed. When the logical comparison evaluates to false, the the else-branch is executed instead. The else-branch does not need to exist. If it does not exist and the if-statement is false, the code following the if-branch is excuted as normal.

Between the if- and else-statement, there can be an arbitrary number of else-if statements, each of which checks it’s own condition.

#include <stdio.h>
 
int main()
{
  int x = 10;
  int y = 12;
 
  if (x < y) // the same as if ((x < y) == true)
	{
	  printf("x is smaller then y.\n");
	}
  else if (x == y)
	{
	  printf("x and y are equal.\n");
	}
  else
	{
	  printf("x is bigger then y.\n");
	}
 
  return 0;
}

Ternary Operator

The Ternary Operator (conditional operator) is used to set or initialize a value based on a condition. It works like a simple if-else statement, but is shorter and compiler optimized. A ternary operator consits of a ?, a : and a boolean expression:

int value_1 = 23;
int value_2 = 123;
 
//            boolean expr       ? if-val  : else-val
int value_3 = value_1 <= value_2 ? value_1 : 345;

In this example:

checks if value_1 < value_2
if true, value_3 = value_1
if false, value_3 = 345

Switch Case

A Switch Case is a control structure which can be used with enums, integer values or other data types. This control structure is useful for menu selection, controlling programm structures or branching based on variable values. In C a switch consists of a switch(VALUE){} statement, a arbitrary number of case VALUE: checks and a default: execution branch. Every case VALUE and the default: execution needs the break; signal, for leaving the switch case after executing it’s body. Without break; there would be a fall-through behaviour. This means after the succesfull case check every other case would also be executed.

Here is a example with a enum:

#include <stdio.h>
 
enum Cheese
{
  MOZZARELLA,
  GOUDA,
  APPENZELLER,
  GRUYER,
  PECORINO,
  CAMEMBERT,
  INVALID_CHEESE,
};
 
int main()
{
  enum Cheese my_favorite_cheese = INVALID_CHEESE;
 
  printf("What is your favorite cheese?\nMozzarella (0), Gouda (1), Appenzeller (2), Gruyer (3), Pecorino (4) or Camembert (5)?\n");
  scanf("%d", &my_favorite_cheese);
 
  switch (my_favorite_cheese)
	{
	  case MOZZARELLA:
		{
		  printf("Mozzarella! So delicious!\n");
		  break;
		}
	  case GOUDA:
		{
		  printf("Gouda! A real classic!\n");
		  break;
		}
	  case APPENZELLER:
		{
		  printf("Appenzeller! Do you want to be my friend?\n");
		  break;
		}
	  case GRUYER:
		{
		  printf("Gruyer! I like!\n");
		  break;
		}
	  case PECORINO:
		{
		  printf("Pecorino! Super salty!\n");
		  break;
		}
	  case CAMEMBERT:
		{
		  printf("Camembert! Welcome to France!\n");
		  break;
		}
	  default:
	    {
		printf("Are you not a real fan of Cheese? :(\nSomething went wrong.");
			break;
	    }
	}
 
  return 0;
}

Loops

Increment / Decrement

There are few ways to increment or decrement a value that ar interchangeable:

int i = 0;   // declare and initialize a variable
 
i = i + 1;   // increment to 1
i += 1;      // increment to 2
i++;         // increment to 3
 
i = i - 1;    // decrement to 2
i -= 1;      // decrement to 1
i--;         // decrement to 0

While-Loops

A while-loop performs its body while a condition evaluates to true. The condition is checked at the start of every iteration. The while-loop is useful, when we don’t know in advance, how many iteration we need.

#include <stdio.h>
 
int main()
{
  int i = 0;
 
  while (i < 3)
  {
	printf("Hello World!\n");
	i++;
  }
 
  return 0;
}

We can also implement a endless while-loop and work with the break keyword, to end the loop. The break keyword is useful for special conditions for ending a loop.

#include <stdbool.h>
#include <stdio.h>
 
int main()
{
  float user_input;
 
  while (true)
  {
	scanf("%f", &user_input);
 
	if (user_input < 0.0) // if 'true' stop the loop and continue with the main body
	  {
		break;
	  }
  }
 
  return 0;
}

With the continue keyword we can interrupt the current iteration without ending the loop. After continue the loop starts at it’s beginning.

int main()
{
  float user_input;
  float sum = 0.0f;
 
  while (true)
  {
	scanf("%f", &user_input);
 
	if (user_input > 10.0) // if 'true' interrupt the current iteration and go to loop head
	  {
		continue;
	  }
 
	if (user_input < 0.0)
	  {
		break;
	  }
 
	sum += user_input;
  }
 
  printf("sum: %f\n", sum);
 
  return 0;
}

Do-While-Loops

A do-while-loop performs it’s body at least one time. This happens, because the condition is checked after the iteration and not at the beginning of the iteration. While the condition is evaluated to true the loop continues.
```
#include <stdio.h>
 
int main()
{
  int i = 0;
 
  do
  {
	printf("Hello World!\n");
	i++;
  } while (i < 3);
 
  return 0;
}
```

For-Loops

A For-Loop consists of a count-variable, a condition and a operation on the count-variable. The count-variable is delcared and initalized only once, befor the first iteration starts. The condition is checked at the beginning of every iteration. If the condition evaluates to true the loop body is executed. After the execution of the iteration the operation on the coun-variable is ecxecuted and the next check on the condition starts. For-Loops are used when we know in advance, how many iteration we need.
```
#include <stdio.h>
 
int main()
{
  int num_iterations = 3;
 
  // for (COUNT-VARIABLE; CONDITION; OPERATION)
  for (int i = 0; i < num_iterations; i++)
	{
	  printf("%d\n", i);
	}
 
  return 0;
}
 
```

Nested-Loops

Nested-Loops are necessary when we need combinations of two count-variables, e.g. when we are indexing a 2D Array.

#include <stdio.h>
 
int main()
{
 
  for (int i = 0; i < 4; i++) // this loop iterates 4 times, so the body, which is a loop, is exectued 4 times
	{
	  for (int j = 0; j < 3; j++)   // this loop counts from 0 to 2 for j every time the outer loop is iterated
		{                           // the outer loop will iterate when the inner loop is at its end
		  printf("%d, &d\n", i, j);
		}
	}
 
  return 0;
}

Header Files and Libraries

Header files are included at the beginning of a C-File. They can be libraries, which offer some functionality, or selfwritten header files, with some helper functions to be used in the file which holds the #include.

#include <stdio.h>

When angle brackets < > are used the compiler is searching for the header file in the standard system include dir. These are used for standard system-header files.

You can also user quotation marks "" which allows linking files from somewhere else. With these the compiler is searching first in the current working dir. After this in the standard includ dir. These are used for header files that are specific for the current project.

#include "my_helper_header.h"

Libraries

Name	Description	Functionality	Documentation
stdio.h	standard in- and output	offers some input and output functionality	W3, C++
stdint.h	predefined data types	provide datatypes with fixed sizes	cppreference.com
stdbool.h	boolean data type	provides the `bool` datatype	cppreference.com

Setup for Emacs

Compiling in Emacs

This is a usefull function for compiling the current C-File with minimal compiler settings. Put it in your init.el for convenient compiling and use C-x c to compile on the fly.

(defun pvn/compile-c-file ()
  "compiles the current c file with gcc"
  (interactive)
  (if (buffer-file-name)
      (let* ((file-path (buffer-file-name))
             (file-without-extension (file-name-sans-extension (file-name-nondirectory file-path)))
             (command (format "gcc -Wall -Wextra -o %s %s"
							  file-without-extension file-path)))
        (compile command))
    (message "This buffer has no proper C-file")))
 
(add-hook 'c-mode-hook
          (lambda () (local-set-key (kbd "C-x c") 'pvn/compile-c-file)))

This is a usefull function to run your compiled program inside Emacs. Put this in your init.el file and run your program with C-x r

(defun pvn/run-compiled-buffer-file ()
  "Runs the compiled version of the current C file in a vterm buffer displayed below the current buffer, without replacing it."
  (interactive)
  (let* ((file-without-extension (file-name-sans-extension (file-name-nondirectory (buffer-file-name))))
         (exec-file (concat default-directory file-without-extension))
         (vterm-buffer-name "*vterm*"))
    (if (file-exists-p exec-file)
        (let ((vterm-buffer (or (get-buffer vterm-buffer-name)
                                (vterm))))
          ;; display the vterm buffer below you c buffer
          (display-buffer-in-side-window
           vterm-buffer
           '((side . bottom)
             (slot . 0)
             (window-height . 0.3)
             (preserve-focus . t)))
          (with-current-buffer vterm-buffer
            ;; cd to current working dir
            (vterm-send-string (concat "cd " (shell-quote-argument default-directory)))
            (vterm-send-return)
            ;; run compiled file
            (vterm-send-string (concat "./" file-without-extension))
            (vterm-send-return)))
      (message "Error: File %s does not exist!" exec-file))))
 
(add-hook 'c-mode-hook
          (lambda () (local-set-key (kbd "C-x r") 'pvn/run-compiled-buffer-file)))

von neumann

Mycelium

C-Programming Language

Overview

Content

C Compiler

C Standards

Program Structure and Syntax

Variables and Datatypes

dtypes

Using single characters as variables

Type casting

Enumerations (Enum)

Input and Ouput

Control Structures and Logic

Boolean

If-Else Statement

Ternary Operator

Switch Case

Loops

Header Files and Libraries

Libraries

Setup for Emacs

Compiling in Emacs

Debugging in Emacs

Footer

Links

Graph View

Table of Contents

Backlinks