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03 Exec

The document explains the exec() system call used in systems programming to load and execute new programs after forking a process. It details the execve function, its parameters, and how it replaces the current process's address space without creating a new process. Additionally, it discusses the concepts of parent and child processes, waiting for child processes, and the implications of orphaned and zombie processes.

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46 views11 pages

03 Exec

The document explains the exec() system call used in systems programming to load and execute new programs after forking a process. It details the execve function, its parameters, and how it replaces the current process's address space without creating a new process. Additionally, it discusses the concepts of parent and child processes, waiting for child processes, and the implications of orphaned and zombie processes.

Uploaded by

viraguber3
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PPT, PDF, TXT or read online on Scribd
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IT628: Systems Programming

Load and execute programs, exec

1
exec(): Loading and Running
Programs
 After fork, the child process is an identical
duplicate of the parent process
 How do we start a new program, instead of

copying the parent?


 Use the exec() system call

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int execve( char *filename, char *argv[ ],
char *envp );
filename: name of the executable file to run

argv: command line arguments

envp: environment variable settings (e.g. $PATH,

$HOME, etc.)
returns -1 if error, otherwise doesn’t return

int main() { /* exec1.c */


char *args[2];
args[0] = “/bin/echo”;
args[1] = NULL;
execv(“/bin/echo”, args);
return 0;
}

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int main() { /* exec2.c */
char *args[2];
args[0] = “/bin/echo”;
args[1] = NULL;
printf(“About to exec from process %d\n”, getpid());
sleep(1);
execv(“./exec2”, args);
printf(“Done exec-ing ...\n”);
return 0;
}

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 exec() does not create a new process!
 Replaces the address space and CPU state of the current
process
 Loads the address space from the executable file and starts it
from main()
 On success, exec does not return!

 UNIX shells use fork-then-exec to run


programs

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execv(new_program, argv[ ])

Initial process

fork
fork() returns pid=0 and runs as a
Returns a cloned parent until execv is called
new PID

Original New
process new_Program
Copy of
Continues (replacement)
Parent

execv(new_program)

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Exercise
Write a program that creates a child process,
the child executes /bin/ls, and then the parent
prints “done”. Make sure the word “done” is
printed after the output of ls.

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int main() { /* exec3.c */
if (fork() == 0) { /* Child process */
char *args[2];
args[0] = “/bin/ls”; /* Not required!! */
args[1] = NULL; /* Indicate end of args array */
execv(“/bin/ls”, args);
exit(0); /* in case exec fails! */
}
wait(NULL);
printf(“Done\n”);
return 0;
}

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Waiting for a Child Process
 If a process (the parent) calls fork() to create a
process (the child), the parent doesn't automatically
wait for the child to finish. The parent must call
wait.
 So if wait is not called, which process finishes first?

 Either one could finish first.

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Zombies
 If the parent finishes first, the child becomes an
orphan and is adopted by a system process called
init whose pid is 1.
 If the child finishes first, it becomes a zombie.

 The child is mostly dead, but the parent might call

waitpid. So it's termination information must be


retained until the parent either terminates or calls
waitpid.

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Summary
 Basic functions
 fork spawns new processes
 exit terminates own process
 wait and waitpid wait for and reap terminated children
 execve runs new program in existing process

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