Process Management I CS 351: Systems Programming Michael Saelee - - PowerPoint PPT Presentation

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process management i

Process Management I CS 351: Systems Programming Michael Saelee - - PowerPoint PPT Presentation

May 31, 2023 269 likes •754 views

Process Management I CS 351: Systems Programming Michael Saelee <lee@iit.edu> 1 Computer Science Science Creating Processes 2 Computer Science Science #include <unistd.h> pid_t fork (); 3 Computer Science Science fork

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Process Management I

CS 351: Systems Programming Michael Saelee <lee@iit.edu>

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Computer Science Science

§Creating Processes

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Computer Science Science

#include <unistd.h> pid_t fork();

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fork traps to OS to create a new process … which is (mostly) a duplicate of the calling process!

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e.g., the new (child) process runs the same program as the creating (parent) process

and starts with the same PC,
the same SP

, FP , regs,

the same open files, etc., etc.

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Computer Science Science

Pparent

int main () { fork(); foo(); }

OS

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Computer Science Science

Pparent

int main () { fork(); foo(); }

OS

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Computer Science Science

Pparent

int main () { fork(); foo(); }

OS Pchild

int main () { fork(); foo(); }

creates

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Computer Science Science

Pparent

int main () { fork(); foo(); }

OS Pchild

int main () { fork(); foo(); }

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fork, when called, returns twice (to each process @ the next instruction)

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int main () { fork(); printf("Hello world!\n"); } Hello world! Hello world!

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Computer Science Science

int main () { fork(); fork(); printf("Hello world!\n"); } Hello world! Hello world! Hello world! Hello world!

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Computer Science Science

int main () { fork(); fork(); fork(); printf("Hello world!\n"); } Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world!

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Computer Science Science

the “fork bomb” (I didn’t show you this)

int main() { while(1) fork(); }

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# processes waiting to be scheduled

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system-wide unique process identifier
child’s pid (> 0) is returned in the parent
sentinel value 0 is returned in the child

pid_t fork(); typedef int pid_t;

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void fork0() { int pid = fork(); if (pid == 0) printf("Hello from Child!\n"); else printf("Hello from Parent!\n"); } main() { fork0(); } Hello from Child! Hello from Parent! Hello from Parent! Hello from Child!

(or)

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Computer Science Science

i.e., order of execution is nondeterministic

parent & child run concurrently!

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void fork1 () { int x = 1; if (fork() == 0) { printf("Child has x = %d\n", ++x); } else { printf("Parent has x = %d\n", --x); } } Parent has x = 0 Child has x = 2

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Computer Science Science

important: post-fork, parent & child are identical, but separate!

OS allocates and maintains

separate data/state

control flow can diverge

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Computer Science Science

void fork2() { printf("L0\n"); fork(); printf("L1\n"); fork(); printf("Bye\n"); } L0 L1 L1 Bye Bye Bye Bye

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Computer Science Science

void fork2() { printf("L0\n"); fork(); printf("L1\n"); fork(); printf("Bye\n"); }

process tree:

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void fork2() { printf("L0\n"); fork(); printf("L1\n"); fork(); printf("Bye\n"); } L1 L0 L1 Bye Bye Bye Bye

Which are possible?

L0 L1 Bye Bye L1 Bye Bye L0 L1 Bye Bye Bye L1 Bye L1 Bye Bye L0 L1 Bye Bye L0 Bye Bye L1 L1 Bye Bye

A. B. C. D. E.

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main() { fork(); fork(); while(1) ; } $ ./a.out & [1] 8198 $ pstree -p 8198 a.out(8198)a.out(8199)a.out(8201) a.out(8200)

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void fork3() { printf("L0\n"); fork(); printf("L1\n"); fork(); printf("L2\n"); fork(); printf("Bye\n"); }

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void fork4() { printf("L0\n"); if (fork() != 0) { printf("L1\n"); if (fork() != 0) { printf("L2\n"); fork(); } } printf("Bye\n"); } L0 L1 L2 Bye Bye Bye Bye L0 L1 Bye Bye L2 Bye Bye Bye L0 Bye L1 Bye L2 Bye L0 Bye L1 Bye L2 Bye Bye L0 L1 Bye Bye Bye L2 Bye

A. B. C. D. E.

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void fork4() { printf("L0\n"); if (fork() != 0) { printf("L1\n"); if (fork() != 0) { printf("L2\n"); fork(); } } printf("Bye\n"); }

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void fork5() { printf("L0\n"); if (fork() == 0) { printf("L1\n"); if (fork() == 0) { printf("L2\n"); fork(); } } printf("Bye\n"); }

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a good question: what if fork fails?

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most syscalls return -1 on failure global var errno populated with “cause”

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#include <errno.h> extern int errno; /* get error string */ char *strerror(int errnum); /* print error string w/ message */ void perror(const char *s);

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int fd = open("/etc/shadow", O_RDONLY); if (fd == -1) { perror("Uh-oh"); exit(1); } $ ./errtest Uh-oh: Permission denied

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$ man errno NAME errno - number of last error SYNOPSIS #include <errno.h> DESCRIPTION ... E2BIG Argument list too long (POSIX.1) EACCES Permission denied (POSIX.1) EADDRINUSE Address already in use (POSIX.1) EADDRNOTAVAIL Address not available (POSIX.1) EAFNOSUPPORT Address family not supported (POSIX.1) EAGAIN Resource temporarily unavailable (may be the same value as EWOULDBLOCK) (POSIX.1) EALREADY Connection already in progress (POSIX.1) EBADE Invalid exchange ...

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§Terminating Processes

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int main () { return 0; }

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void exit(int status);

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void foo() { exit(1); /* no return */ } int main () { foo(); /* no return */ return 0; }

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Unix convention:

normal termination → exit status 0
other exit status values = error

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void foo() { exit(1); } int main () { ... foo(); release(resource); /* cleanup */ return 0; } /* $^@#%!! */

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int atexit(void (*fn)());

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int atexit(void (*fn)());

registers function to call before exiting
can call multiple times; functions are

invoked in reverse order

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void cleanup() { printf("Cleaning up\n"); } void foo() { printf("Self-destructing\n"); exit(1); } int main() { atexit(cleanup); foo(); /* no return */ return 0; } Self-destructing Cleaning up

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void cleanup() { printf("Cleaning up\n"); } void foo() { fork(); exit(1); } int main() { atexit(cleanup); foo(); /* no return */ return 0; } Cleaning up Cleaning up

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i.e., atexit handlers are “inherited” by child processes on fork

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void fork7() { if (fork() == 0) { printf("Terminating Child, PID = %d\n", getpid()); exit(0); } else { printf("Running Parent, PID = %d\n", getpid()); while (1) ; /* Infinite loop */ } }

(demo)

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All terminating processes turn into zombies

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“dead” but still tracked by OS

pid remains in use
exit status can be queried

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§Reaping Processes (& Synchronization)

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