1. fork() and exec() David Ferry CSCI 3500 – Operating Systems
Saint Louis University St. Louis, MO 63103

2. CSCI 3500 - Operating Systems
Process Creation
Two functions to create and initialize a new process:
fork() – creates an identical clone of the calling process
Creates a parent-child relationship
Child is identical to the parent except for a new PID and the return value from fork(). Some process-specific info is modified as well (e.g. accounting). See docs for details.
Implemented efficiently with lazy copy-on-write
exec() – replaces existing process with another program
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3. CSCI 3500 - Operating Systems
fork()
I.e. a fork in the road
int x = 10;
int y = 20;
fork();
if( child )
x = 30;
fork()
parent
parent x = 10
child x = 30
time
x = 10
x = 10
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4. CSCI 3500 - Operating Systems
fork()
fork() is a unique function
Called once, returns twice
Return value in parent is the PID of the child
Return value in the child is zero
Can be useful for concurrent programming – two loosely related processes executing alongside one another
parent
ret = fork()
time
parent
ret = child PID
child
ret = 0
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5. CSCI 3500 - Operating Systems
Lazy Copy-On-Write
The “cloning” behavior of fork() makes it very efficient to implement.
Very little needs to be changed immediately.
Some things might never need to be changed.
Use existing data until a write operation modifies it, then make a copy and modify the copy
If you want to execute a whole new program, exec() will overwrite everything anyway, and we don’t want to do that twice
.stack
.stack
.heap
.heap
.data
.data
.text
.text
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6. CSCI 3500 - Operating Systems
exec()
Replaces process space with that of a new program
Replaces contents of program with a new binary image
Despite name, only makes a program eligible for execution
Typical Usage: int ret = fork(); if ( ret == 0 ){ //child
newCmd = … newArgv = … exec( newCmd, newArgv); }
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7. CSCI 3500 - Operating Systems
wait()
waitpid() and related functions wait for a child to finish executing:
int ret = fork();
if ( ret == 0 ){ //child stuff } waitpid( ret, NULL, 0 );
Prevents parent from progressing until the child terminates.
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8. CSCI 3500 - Operating Systems
kill()
General purpose function for sending signals
Good for event notification, but carry no information.
Originally used to forcibly stop processes, but now an established method for inter-process communication.
Many different signals with different meanings, see man 7 signal for details.
kill( child_pid, SIGINT ) is same as CTRL-C
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