Assignment

• What I'm covering:
• General control flow
• fork()
• execlp()
• Parameters
• Makefiles
• Additional Questions

General Control Flow/fork()

• In the Controller:

int main(int argc,char** argv){

.....

for(i = 0;i < 4;i++){

int ret_pid = fork();

if ( ret_pid < 0 ) exit(1); //this means fork() failed

else if ( ret_pid == 0 ) !  //this is the child

else ...  //this is the parent (Controller)

}

return 0;

}

execlp()

• This function call has a lot of gotchas that look very confusing at first glance.
• The first argument to execlp is the string with the file name that will be executed...and so is the second.
• Think about your own program being called from the command-line ( > ./a.out arg1 arg2 ). In this case, the value of argv[0] in the program is "a.out".

Same thing applies when using exec, you just have to be explicit about it.

More execlp() and parameters

• The rest of the parameters to execlp are strings containing the rest of the arguments, in our case, the numbers that the calculator will be operating on.
• These will be passed to Controller as arguments, and from there, they will be passed to Calculator.
• No matter what, the last argument to execlp MUST be a null-terminator (NULL or (char*) 0).

This is because the argument list must be null-terminated, just like any other string.

Makefiles

• This assignment requires that TWO executables be created: the Controller and the Calculator.
• Because of how Eclipse generates its makefiles, if you try to do this in a single project, Eclipse will whine. I recommend doing your coding in a normal text editor (gedit, emacs, vi, bluefish, etc.), and writing your own makefile.
• There are plenty of makefile tutorials online; but this project (and the next one) will not require a particularly complex makefile.

Makefile example

• Here's the general format of a makefile entry:

[target] : [dependency1] ! [dependencyn] [command to execute]

• In this example, when you call make target, the make program checks if any of the dependencies of target are out of date.
• If one or more of them is out of date, the command to execute is run.