C Programming

main()

• All C programs should contain a main() function that follows the format

<optional return value type> main(<optional argument>) {
  <optional procedure statements or functional calls>;
}

// argc integer holds the number of arguments 
// argv holds the input arguments (strings)
// Name of the program is always stored at offset argv[0]
<optional return value type> main(int argc, char * argv[]){
}

Functions

• Functions are self-contained bundles of code that can be called for execution by main()

// C Function format
<optional return value type> function name (<optional function argument>) {
}

// Simple example
#include <stdio.h>
#include <stdlib.h>

int foo() {
    return 8;
}
int main(void){
    int val_x;
    val_x = foo();
    printf("The value returned is: %d\n", val_x);
    exit(0);
}

Variables

• Used in programs to store pieces of information that may change and may be used to dynamically influence the program.

Variable Type	Use	Typical Size
int	Stores a signed integer value such as 314 or -314	8 bytes for 64-bit machines 4 bytes for 32-bit machines 2 bytes for 16-bit machines
float	Stores a signed floating-point number such as 3.234	4 bytes
double	Stores a large floating-point number	8 bytes
char	Stores a single character	1 byte

printf

• Prints out to the screen

// Trwo forms of the printf command:
printf(<string>);
printf(<format string>), <list of variables/values>);

Format Type	Meaning	Example
%n	Print nothing	printf("test %n");
%d	Decimal value	printf("test %d, 123);
%s	String Value	printf("test %s", "123");
%x	Hex value	printf("test %x", 0x123);
%f	Float	printf("test %f", 1.308);

// Format string example code
#include <stdio.h>

int main(void){
  double x = 23.5644;
  //Total width of 5 with 2 values after the floating point
  printf("The value of x is %5.2f\n", x);
  // Total width of 4 with 1 value after the floating point 
  printf("The value of x is %4.1f\n", x);
  
  return 0;
}

scanf

• Generally used to get input from the user

// scanf format
scanf(<format string>, <list of variables/values>);

// Example - reads an integer from the user and stores it in a variable called number
scanf("%d", &number);

// You must use & before any variable with scanf

strcpy/strncpy

• One of the most dangerous functions used in C.

• Purpose is to copy each character in the source string into the destination string.

• Dangerous because there is no checking of the source's size before it is copied over to the destination.

  • If the source is is larger than space allocated for the destination, overflow conditions are likely.

// Format
strcpy(<destination>, <source>);

• strncpy is a safer alternative

// strncpy format
strncpy(<destination>, <source>, <width>);