- Syntax of the
fork( ) system call:
int fork( )
Effect:
- The running program
that executes the
fork( ) function will
be
cloned:
- The running program
that executes the
fork( ) function will
be
cloned:
- Example:
int main(int argc, char *argv[] ) { printf("Hello World !\n"); fork( ); printf("Good-bye World !\n"); }Output:
Hello World ! Good-bye World ! // Printed twice !!! Good-bye World !
- Example Program:
(Demo above code)
- Prog file: click here
How to run the program:
- Right click on link(s) and
save in a scratch directory
- To compile: gcc fork1.c
- To run: a.out
- Note:
- It may sound stupid
at first to
clone a
running program because:
- why on earth would you want to run the same program twice ???
- It may sound stupid
at first to
clone a
running program because:
- Using the
fork( ) system call:
- The is a way
(discuss soon) to
make the 2 identical programs
continue
2 different ways:
- Use an if-statement
- Then:
- One copy of the
program will execute
the if-part
- The other copy of the program will execute the else-part
- One copy of the
program will execute
the if-part
- The is a way
(discuss soon) to
make the 2 identical programs
continue
2 different ways:
- Parallel server:
- Parallel server = a server than can process multiple connection requests from clients at the same time
- Structure of a
parallel server:
- Make a clone of
yourself (= the server program)
- Have the clone
provide service to
the client
- In the mean time, you (= the original server program) can accept other connection requests made by other clients !!!
- Make a clone of
yourself (= the server program)
- Parent process and
child process:
- Parent process =
the original running program
that called the
fork( ) function
- Child process = the clone of the original running program
- Parent process =
the original running program
that called the
fork( ) function
- Very important fact:
- The return value of
the fork( ) function is
different:
- For the parent process, and
- for the child process
- The return value of
fork( ) for
the parent process is:
- The process id ( ≠ 0) of the child process
- The return value of
fork( ) for
the child process is:
- 0 (always)
- The return value of
the fork( ) function is
different:
- Example:
int main(int argc, char *argv[] ) { printf("Hello World !\n"); int i = fork( ); printf("i = %d\n", i); // Show the return value of fork() }Sample output:
Hello World ! i = 10814 <--- This line was printed by the parent process i = 0 <--- This line was printed by the child process
- Example Program:
(Demo above code)
- Prog file: click here
How to run the program:
- Right click on link(s) and
save in a scratch directory
- To compile: gcc fork2.c
- To run: a.out
- fork( ) is
always used inside
an if-statement:
if ( fork( ) != 0 ) { code executed by the parent process } else { code executed by the child process }
- Example:
int main(int argc, char *argv[] ) { printf("Hello World !\n"); if ( fork( ) != 0 ) { printf("Hello, I am the parent process\n"); } else { printf("Hello, I am the child process\n"); } }
- Example Program:
(Demo above code)
- Prog file: click here
How to run the program:
- Right click on link(s) and
save in a scratch directory
- To compile: gcc fork3.c
- To run: a.out
- A more advanced example:
- The parent process
will keep created child processes
when you enter a line:
int main(int argc, char *argv[] ) { char line[1000]; while ( 1 ) { printf("Enter a line:\n"); scanf( "%[^\n]", line ); getchar(); if ( fork( ) != 0 ) { printf("Parent process: I will go read a line again\n"); } else { /* ------------------------------------------- Code that is executed by the child process --------------------------------------------- */ printf("Child process... your line is: %s\n", line); printf("Child process... here is it again: %s\n", line); printf("Child process: Done !!\n"); exit(0); // Child process exits !! } } }
- The parent process
will keep created child processes
when you enter a line:
- Example Program:
(Demo above code)
- Prog file: click here
How to run the program:
- Right click on link(s) and
save in a scratch directory
- To compile: gcc fork4.c
- To run: a.out