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  4. Affiliations
    1. Dept. of Programming Languages and Compilers
    2. Ericsson Hungary Ltd

The C programming language – Lecture 2.

The structure of C programs

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/*
 * my first C program      // comment
 *
 */
#include <stdio.h>         // preprocessor directive
int main()
{
  printf( "hello world\n" );  // c tokens...
  return 0;   
}

C programs consist of:

Comments:

single line comment (since C99) from // to end of line

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  /* multi
     line    
     comments
     // hiding single line comments
   */

  /*
      /*  but must not be nested */
   */

not comments inside strings: “/* this is not comment */”

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  /*************************************\
*                                     *
*  exist in various style and format  *
*                                     *
\*************************************/

Preprocessor directives

starting with ‘#’

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#ifdef 
#define MY_HEADER 
#include <header.h>

#if MY_PLATFORM
#define BUFFER 100
#else
#error platform not defined
#endif

#endif

… more on preprocessor in the next lecture

C tokens

keywords

all lowercase

  • C89: 32
  • C99: +5
  • C11: +7

identifyers

starts with letter (incl _underscore) continues with letters or numbers

literals

  • 1 decimal integer literal
  • 0x14 hexadecimal integer literal
  • 3.14 floating point literal
  • ‘x’ character literal
  • “Hello world” string literal

operators

  • unary e.g. -2
  • binary e.g. 2 + 4
  • terciary x<y ? x : y

separators

{ } ,

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/*
 * my first C program    <--- comment 
 *
 */
#include <stdio.h>   <---- preprocessor directvive

int main()               int    <-- type name: keyword
                         main   <-- function name: identifier
                         ()     <-- function call: operator
{                        {      <-- block begin: separator
                         printf <-- function name: identifier
                         (      <-- function call: operator
  printf("Hello world"); "Hello world"  <-- string literal, type char[13]
                         )      <-- function call: operator
                         ; <-- command-end separator
                         return <-- keyword
  return 0;              0      <-- decimal int literal, type int
}                        }      <-- block end: separator

Keywords

we will learn them …

Identifiers

  • starts with letter (underscore ‘_’ is a letter)
  • continues with letters and digits
  • no maximum length
  • but compilers should translate
  • only the first 63 letters (internal linkage)
  • only the first 31 letters (external linkage)
  • must not use keywords as identifiers
  • case sensitive

Different conventions:

  • camelCaseNotation
  • CTypenamesStartsWithUppercase
  • under_score_notation

A research on this: https://whathecode.wordpress.com/2011/02/10/camelcase-vs-underscores-scientific-showdown/

results on February 2015:

  • camelCase: 52.5 %
  • underscore: 47.5 %

MACRO_NAMES_ARE_ALL_UPPERCASE by convention

A paper on this: http://www.cs.loyola.edu/~binkley/papers/icpc09-clouds.pdf

Literals

void

Integrals, type + value

  • decimal integral 12 type = int, value = 12
  • octal integral 014 type = int, value = 12
  • hexadecimal integral 0xC type = int, value = 12

  • long integer 12l type = long int, value = 12
  • long long integer (since C99)

  • unsigned integer 12u type = unsigned int, value = 12
  • signed integers are the same as integers

  • if an integer literal longer than integer 10000000 –> long or long long

  • int size knot known ==> the best for the compiler, machine word
   sizeof(short) <= sizeof(int) <= sizeof(long) <= sizeof(long long)
// at least 16 bit             at least 32 bit    at least 64 bit
  • also exist signed and unsigned versions, e.g. long <==> signed long

Characters

  • character ‘A’ type = char, value = ascii value of ‘A’, likely 65
  • escape sequences:
    • ’'’ single quote
    • ’"’ double quote
    • ’\?’ question mark
    • ’\’ backslash
    • ‘\a’ bell (audio)
    • ‘\b’ backspace
    • ‘\f’ form feed – new page
    • ‘\n’ newline
    • ‘\r’ carriage return
    • ‘\t’ horizontal tab
    • ‘\v’ vertical tab
  • octal value: type = char
    • ‘\377’ -> 11111111
  • hexadecimal value type = char
    • ‘\xff’ -> 11111111
  • unicode value
    • ‘\U1234’ type = char16_t (min 16bit)
    • ‘\U12345678 type = char32_t (min 32bit)
  • wchar_t is the longest character type
  • signed char and unsigned char also exist, but here char not necessary the same as signed char
 1 == sizeof(char) < sizeof(char16_t) <= sizeof(char32_t) <= sizeof(wchar_t)

Boolean

_Bool 1 (true) or 0 (false)

bool, true, false (macro)

true –> 1 false –> 0

In conditional expressions all non-zero values are TRUE values!

Conversion of _Bool differs from int conversions:

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#include <stdio.h>
int main()
{
  printf("_Bool == %d\t int == %d\n", (_Bool) 0.5, (int) 0.5);
  return 0;
}
$ ./a.out
_Bool == 1      int == 0

Floating point numbers

  • float 3.14f
  • double 3.14 (as double precision)
  • long double 3.14l
  sizeof(float) <= sizeof(double) <= sizeof(long double)

Complex floating points

                          #include <complex.h>
  float _Complex      or    float complex
  double _Complex           double complex
  long double _Complex      long double complex

Example;

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#include <complex.h>
#include <stdio.h>
int main(void)
{
    double complex z = 1 + 2*I;
    z = 1/z;
    printf("1/(1.0+2.0i) = %.1f%+.1fi\n", creal(z), cimag(z));
}

Have to link to the math library! Use the “-lm” flag!

$ gcc  -ansi -pedantic -Wall -W complex.c -lm
$ ./a.out
1/(1.0+2.0i) = 0.2-0.4i

Size is not fixed! (C is NOT Java!)

int ==> best for integral computation, “machine word”

double ==> best for floating point computation