C

Appeared in:
1969-1973
Influenced by:
Influenced:
Paradigm:
Typing discipline:
File extensions:
.c, .h
Dialects:
Versions and implementations (Collapse all | Expand all):
Programming language

C is a standardized imperative (procedural) general-purpose computer programming language.

It was developed in early 1970s by Dennis Ritchie at the Bell Telephone Laboratories as a successor of B.

In 1978 Kernighan and Ritchie published “The C Programming Language” book which served as an informal language standard known as K&R C.

The first official standard ANSI X3.159-1989 was ratified in 1989 and was adopted as ISO/IEC 9899:1990 the year after that; it is known as ANSI C. Next standards were ISO 9899:1999 (C99) and ISO/IEC 9899:2011 (C11).

The language was designed to be compiled using a relatively straightforward compiler, to provide low-level access to memory, to provide language constructs that map efficiently to machine instructions, and to require minimal run-time support. C was therefore useful for many applications that had formerly been coded in assembly language. Eventually it became the most popular language for system programming.

Despite its low-level capabilities, the language was designed to encourage machine-independent programming. A standards-compliant and portably written C program can be compiled for a very wide variety of computer platforms and operating systems with little or no change to its source code. The language has become available on a very wide range of platforms, from embedded microcontrollers to supercomputers.

Language features:

  • small number of keywords.
  • most language functionality is separated from core language and moved out to libraries.
  • a preprocessor is used for macro definition, source code files inclusion and conditional compilation.
  • static weak typing: all data has fixed types but implicit casts are allowed.
  • user-defined and compound data types are possible.
  • low-level memory access is possible by converting machine addresses to pointers.
  • a procedure is a kind of function which returns a special type void.
  • source code files can be compiled independently and linked together.

Elements of syntax:

Inline comments // (C99 only)
Nestable comments #if 0 ... #endif
Non-nestable comments /* ... */
Case-sensitivity yes
Variable identifier regexp [_a-zA-Z][_a-zA-Z0-9]*
Variable assignment =
Variable declaration type variable
Variable declaration with assignment type variable = value
Block { ... }
Physical (shallow) equality ==
Physical (shallow) inequality !=
Comparison < > <= >=, strcmp for strings
Function definition returntype f(type1 p1, type2 p2, ...)
Function call f(a, b, ...)
Function call with no parameters f()
If - then if (condition) ...
If - then - else if (condition) ... else ... OR ( condition ? ... : ... )
Loop forever while (1) ...
While condition do while (condition) ...
Do until condition do ...while (!condition)
For each value in a numeric range, 1 increment for (int i = 1; i <= 10; i++) ...
For each value in a numeric range, 1 decrement for (int i = 10; i >= 1; i--) ...

Links:

Examples:

Factorial:

Example for versions gcc 3.4.5, gcc 3.4.5 (Objective-C), gcc 4.2.4, tcc 0.9.25

This example uses recursive factorial definition. Note that 13! and larger causes an overflow, so the last lines of the output look like this:

13! = 1932053504
14! = 1278945280
15! = 2004310016
16! = 2004189184

#include <stdio.h>

unsigned long long factorial(unsigned long long n)
{
    if (n == 0) {
        return 1;
    } else {
        return n * factorial (n - 1);
    }
}

int main(void)
{
    int n;
    for (n = 0; n <= 16; n++) {
        printf("%i! = %lld\n", n, factorial(n));
    }
    return 0;
}

CamelCase:

Example for versions gcc 3.4.5, gcc 4.2.4, tcc 0.9.25

This example is based on character-wise string processing. fgets here reads at most 99 characters into the string, and stops when it finds end-of-string character, so a long line might be split. C doesn’t provide boolean data type, so it has to be simulated using integer variable. 

#include <stdio.h>

void main() {
    char text[100],cc[100];
    fgets(text, sizeof text, stdin);
    int i,j=0,lastSpace=1;
    for (i=0; text[i]!='\0'; i++) 
        if (text[i]>='A' && text[i]<='Z' || text[i]>='a' && text[i]<='z')
        {   if (lastSpace>0)
                cc[j] = toupper(text[i]);
            else
                cc[j] = tolower(text[i]);
            j++;
            lastSpace = 0;
        }
        else
            lastSpace = 1;
    cc[j]='\0';
    printf("%s\n",cc);
}

Quadratic equation:

Example for versions gcc 4.2.4

This example requires C99, since it uses complex type introduced in it. 

#include <stdio.h>
#include <complex.h>
#include <math.h>

void print(int ind, double complex x) {
    printf("x%d = ", ind);
    if (fabs(cimag(x)) < 1e-6)
        printf("%f\n", creal(x));
    else printf("(%f, %f)\n", creal(x), cimag(x));
}

int main() {
    double A, B, C;
    double D;
    printf("A = ");
    scanf("%lf", &A);
    if (fabs(A)<1E-3) {
        printf("Not a quadratic equation\n");
        return 1;
    }
    printf("B = ");
    scanf("%lf", &B);
    printf("C = ");
    scanf("%lf", &C);

    A *= 2;
    D = B*B-A*C*2.0;
    if (fabs(D)<1E-3)
        printf("x = %f", creal(-B/A));
    else {
        print(1, (-B+csqrt(D))/A);
        print(2, (-B-csqrt(D))/A);
    }
    return 0;
}

Quadratic equation:

Example for versions Borland C++ Builder 6, Microsoft Visual C++ 9 (2008), g++ 3.4.5, gcc 3.4.5, gcc 3.4.5 (Objective-C), gcc 4.2.4, tcc 0.9.25

This example works both for C and C++, as well as for Objective-C which is superset of C. 

#include <math.h> 
#include <stdio.h>

int main()
{
  int A, B, C;
  double D;
  printf("A = ");
  scanf("%d", &A);
  if (A == 0) {
    printf("Not a quadratic equation.\n");
    return 0;
  }
  
  printf("B = ");
  scanf("%d", &B);
  printf("C = ");
  scanf("%d", &C);

  D = B * B - 4 * A * C;
  if (D == 0) {
    printf("x = %f\n", -B / 2.0 / A);
    return 0;
  }
  
  if (D > 0) {
    printf("x1 = %f\nx2 = %f\n",
           (-B + sqrt(D)) / 2.0 / A, (-B - sqrt(D))/ 2.0 / A);
  } else {
    printf("x1 = (%f, %f)\nx2 = (%f, %f)\n",
           -B / 2.0 / A, sqrt(-D) / 2.0 / A, -B / 2.0 / A, -sqrt(-D) / 2.0 /A);
  }
  return 0;
}

Hello, World!:

Example for versions Borland C++ Builder 6, Turbo C++ 1.01, g++ 3.4.5, gcc 3.4.5, gcc 3.4.5 (Objective-C), gcc 4.2.4, tcc 0.9.25 
#include <stdio.h>

int main()
{
    printf("Hello, World!\n");
    return 0;
}

Fibonacci numbers:

Example for versions gcc 3.4.5, gcc 3.4.5 (Objective-C), tcc 0.9.25

This example uses recursive definition of Fibonacci numbers. Note the difference from C++ example: loop counter must be declared outside of the loop, and printf is used for output instead of std::cout. 

#include <stdio.h>

int fibonacci(int n)
{
    return ( n<=2 ? 1 : fibonacci(n-1) + fibonacci(n-2) );
}

int main(void)
{
    int n;
    for (n=1; n<=16; n++)
        printf("%d, ", fibonacci(n));
    printf("...\n");
    return 0;
}