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Process Processes
Lecture Management January 17, 2005
A process is an instance of program during execution, e.g., this Powerpoint program. I can run Powerpoint several times. Other processes are running at the same time (calendar, email, browser, ). Only a process can create another process. Linux: init process.
Boot Procedure
Whats a Process?
BIOS: lowest-level interface between computer & peripherals. BIOS: integrity check in memory and seeks instructions in MBR (master boot record) on disk. Linux: MBR points to bootloader (LILO, GRUB). Bootloader loads kernel image and starts the init process (/sbin/init). Init is the root/parent of all processes in Linux. Init starts processes defined in /etc/inittab to make system functional. Linux selects run-level (0=shutdown, 3+=fully operational)
PID (identifier) executing program (code) memory (code, data) program counter signal handlers set of registers -> state
How to Determine my ID?
#include <sys/types.h> #include <unistd.h> pid_t getpid(void); //my pid pid_t getppid(void); //my parents pid
Creating New Processes
#include <sys/types.h> #include <unistd.h> pid_t fork(void);
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fork()
Example
pid_t x; printf(pid=%d\n, getpid()); printf(ppid=%d\n, getppid()); x = fork(); printf(pid=%d\n, getpid()); printf(ppid=%d\n, getppid()); printf(x=%d\n, x);
Child is EXACT copy of parent. Same program, same PC position, memory is exact copy, signal handlers, file descriptors are copied. Whats different?
child: fork() returns 0 parent: fork() return childs PID or -1 (error)
fork()
exec()
Switch to another program. Code/data is destroyed. Environment variables and file descriptors kept are identical. New code loaded, then started. No way back. int x=3; int err=exec(hello); printf(x=%d\n,x);
Use PID to diverge (if, else)
Question: what happens here? while (1) { fork(); }
More About exec()
Stopping a Process
Fork/exec often used together. 4 versions: absolute path n parameters execl() 1 array execv()
search in $PATH execlp() execvp()
char *params[4]={ls, -l, /home/cpoellab, NULL}; execv(/bin/ls, params);
Parameter list must start from argv[0] and end with NULL pointer.
main() returns. Explicit exit(). A process can send a signal to another process. Signal handler determines action, may stop. SIGINT: stops process (^C), handler can be overwritten. SIGKILL: same effect, cant be overwritten.
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Sending Signals
#include <sys/types.h> #include <signal.h> int kill (pid_t pid, int sig);
Catching Signals
void myhandler(int sig) { printf(I received signal %d\n, sig); signal(SIGINT, myhandler); } int main () { signal(SIGINT, myhandler); }
Zombie Processes
Waiting for Child Process
#include <sys/types.h> #include <sys/wait.h> pid_t wait(int *status);
Finished execution, stays until parent received termination status. If parent is gone, its adopted by process 1 (init). Must be dealt with (steal resources). Parent must wait for it, block until children are dead or set up signal handler.
wait() waits for completion of child (any), including zombies; status is process return status. Also: waitpid(), offers more control.
waitpid()
void sig_chld(int sig) { pid_t pid; int stat; while ((pid=waitpid(-1, &status, WNOHANG))>0) { printf(Child %d terminated with %d\n, pid, status); } signal(SIGCHLD, sig_chld); }
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