Processes!

Address Space

What is Copied Over?

Forkbomb

Process Flowchart

Utilities Unleashed

What are you going to learn?

• What the fork-exec-wait pattern is.
• Why we use it.
• String Manipulation.

All Fork-Exec-Wait Code

pid_t pid = fork();
if(pid == -1){
	//fork failed
}else if(pid == 0){
	//I am the child
	exec(...)
}else{
	//I Am the parent
	wait(pid);
}

Exec Family

• There are three modifiers/mnemonic
  • Either past a list or argv (null terminated array) of arguments
  • Search for the executable either in the current directory or in the path
  • Use the parent’s environment variables or use an environment setting

• Here are the list exec
  • execl( path, arg, … ),execute the file in current directory
  • execlp( file, arg, … ), executes a file only searching in the path
  • execle( path, arg, …, envp[]), execute the file in path + environment settings

• Pass an array of string as arguments
  • execv( path, argv[]), execute the file in current directory
  • execvp( file, argv[]), execute the file in the path only
  • execvpe( file, argv[]), envp[]) // environment setting

time

• ./time <command> <args> ...
• Try measuring the time of running sleep 2
• Should use fork-exec scheme.
• Should take care of programs do not terminate successfully.
• Command line arguments are not limited two one.
• Use the format.h functions to print out time/etc.

time Workflow

Helper Functions

• struct timespec
  • time_t tv_sec;
  • long tv_nsec;
  • tv_sec = 10, tv_nsec = 992300000 -> 10.9923 sec
• int clock_gettime(clockid_t, timespec *);
  • clockid_t: should use CLOCK_MONOTONIC in this lab
• return 0 when success, -1 otherwise

env

• ./env [key=val1] [key2=val1] ... -- cmd [args] ..
• ./env TZ=EST5EDT -- date - execute date under enviornment TZ=EST5EDT
• ./env TEMP=EST5EDT TZ=%TEMP -- date - why is this the same as above?

env Workflow

Helper Functions

• int setenv(const char* name, const char* value,
int overwrite)
• char *getenv(const char *name)
• Write a function that can find all %notation in a string
• Extend that function so that you can replace variables
  with environment variables
• Use getenv to get environment variables
• Be familiar with: return array of strings, clear an array
  of strings-> camelCasers