- The start of the execution
of a C program:
- The execution of a C program begins with the main() function
- The Hello World program in C:
#include <stdio.h> int main( int argc, char* argv[] ) { printf( "Hello World !\n" ); }
- Example Program:
(Demo above code)
- Prog file: click here
How to run the program:
- Right click on link and
save in a scratch directory
- To compile: gcc hello.c (output file is named a.out)
- To run: ./a.out
Or:
- Right click on link and
save in a scratch directory
- To compile: gcc -o hello hello.c (option -o renames output file to hello)
- To run: ./hello
- Explantion:
- #include <stdio.h>
- The # symbol
starts a command for the
C pre-processor
- The #include
command instructs
the C pre-processor to
read in the file
stdio.h
from the System include directory
(this is traditionally the directory:
/usr/include)
- The file
stdio.h
(/usr/include/stdio.h) is
C's
standard IO include file
This file contains constant and variable definitions to allow C programs to perform commonly used input/output operations.
Take a look at the file /usr/include/stdio.h....
- The # symbol
starts a command for the
C pre-processor
- int main( int argc, char* argv[] )
- This line is the header of the
definition of the main()
function
- The function main()
will return an integer error code
(may be used by a shell script to check for the outcome)
- The parameters of main()
are:
int argc = number of parameter strings char* argv[] = array of String argv[0] = first argument argv[1] = second argument ... argv[argc-1] = last argument
- This line is the header of the
definition of the main()
function
- printf("Hello World !\n");
- This is the statement inside the
body of the
main()
function
- printf is the C library function to print outputs to the terminal
- This is the statement inside the
body of the
main()
function
- #include <stdio.h>
- Some terminilogy used in C:
- stdio.h = header file
containing definitions for the
standard Input/Output operations
- stdin = the
name of the
standard input device
(which is the keyboard)
- stdout = the name of the standard output device (which is the screen or terminal)
- stdio.h = header file
containing definitions for the
standard Input/Output operations
- Fact:
- Unlike Java, the C compiler translates the C program source code into machine executable instructions (code)
- This is the process used to
compile and
then run a
Java program:
- Compile the Java source code
using a Java compiler
(javac
(output is a file containing
Java byte code -
instruction code for a virtual machine)
- Execute the Java source code
using a Java byte code
interpreter (java)
(execution may generate output to the terminal or file(s)).
- Compile the Java source code
using a Java compiler
(javac
(output is a file containing
Java byte code -
instruction code for a virtual machine)
- This is the process used to
compile and
then run a
C program:
- Compile the C source code
using a C compiler
(gcc
(output is a file containing
machine executable code -
instruction code for a actual computer !!!)
- Execute the machine code
directly by the
computer:
(execution may generate output to the terminal or file(s)).
- Compile the C source code
using a C compiler
(gcc
(output is a file containing
machine executable code -
instruction code for a actual computer !!!)
- Facts:
- Interpretation (executing code
using an interpreter) is
very inefficient.
- Due to the fact that C program source is translated machine code, C programs run multiple times (at least 10) faster than Java programs.
- Interpretation (executing code
using an interpreter) is
very inefficient.