Question

Starting New Processes

We can create a new process from within another program using the system library function:

#include <stdlib.h> int system ( const char *str );

The system function runs the command passed to it as str and waits for it to complete. The command is executed as if the command,

$ sh -c str

Use the "man" command to study both the "sh" and "system" command. i.e. Execute

$ man sh

$ man system

For example,

$ sh -c "echo 'Hello, CSE 460'"

prints out the message "Hello, CSE 460".

Try the following program ( test_system.cpp ) that uses system to run ps.

//test_system.cpp #include <stdlib.h> #include <iostream> using namespace std; int main() { cout << "Running ps with system\n"; system ( "ps -ax" ); cout << "Done \n"; return 0; }

Compile and run the program by:

$ g++ -o test_system test_system.cpp

$ ./test_system

What do you see? Change the system statement in test_system.cpp to system ( "ps -ax &" );. Compile and run it again. What happens? What does '&' here do?

Answer 1

this line execute first in both 
cout << "Running ps with system\n";

then

system ( "ps -ax" );

will execute and then

cout << "Done \n";

will execute

now after modification to system ( "ps -ax &" );.

cout << "Running ps with system\n";
system ( "ps -ax &" );. // this command will execute in background whatever time it may take but next instruction will execute to avoid waiting
cout << "Done \n";

if you have any doubt then please ask me without any hesitation in the comment section below , if you like my answer then please thumbs up for the answer , before giving thumbs down please discuss the question it may possible that we may understand the question different way and we can edit and change the answers if you argue, thanks :)