Lecture 04: Files, Memory, and Processes

Principles of Computer Systems

Winter 2021

Stanford University

Computer Science Department

Lecturer: Chris Gregg and

Nick Troccoli

PDF of this presentation

You've seen how a file system (e.g., the System 6 file system, s6fs) layers on top of a block device to present an abstraction of files and directories
Layering: decomposing systems into components with well-defined responsibilities, specifying repcise APIs between them (above and below)
- s6fs works on top of anything that provides a block interface: hard disk, solid state disk, RAM disk, loopback disk, etc.
- Many different file systems can sit on top of a block device: s6fs, ext2fs, ext4fs, btfs, ntfs, etc., etc.
Abstraction: defining an API of an underlying resource that is simultaneously simple to use, allows great flexibility in implementation, and can perform well
- Userspace programs operate on files and directories
- File system has great flexibility in how it represents files and directories on a disk
- Not always perfect: block interface for flash and FTLs
Names and name resolution: files are resources, directory entries (file names) are the way we name and refer to those resources

Recap of Lectures 1-3

This diagram shows a vertical list of connections from userspace processes down to harder. Specifically: userspace processes<--system calls-->operating system<--vnode interface-->file system (ext4fs, s6fs, btfs)<--block interface-->device driver (SATA, iSCSI)<--hardare interface-->hardware (PCI-E, DMA, RDMA, etc.)

How files on disk are presented to a program as file descriptors
How programs open, control and manipulate files
- How does the command below work?
- $cat people.txt | sort | uniq > list.txt
File descriptors vs. open files (many-to-one mapping)
vnode abstraction of a file within the kernel
Memory mapped files and the buffer cache
The concept of a process and what it represents
- Address space: virtualization of memory
- Seamless thread(s) of execution: virtualization of CPU
Creating and managing processes
Concurrency: challenges when you have multiple processes running and how you manage them

Today's Lecture

Linux maintains a data structure for each active process. These data structures are called process control blocks, and they are stored in the process table
- We'll explain exactly what a process is later in lecture
Process control blocks store many things (the user who launched it, what time it was launched, CPU state, etc.). Among the many items it stores is the file descriptor table
A file descriptor (used by your program) is a small integer that's an index into this table
- Descriptors 0, 1, and 2 are standard input, standard output, and standard error, but there are
  no predefined meanings for descriptors 3 and up. When you run a program from the terminal, descriptors 0, 1, and 2 are most often bound to the terminal

File Descriptor Table and File Descriptors

This diagram shows the "Process Control Blocks", handled by the OS. Specifically, it shows the "descriptor table for process ID 1000", "descriptor table for process ID 1001", and "descriptor table for process ID 1002". Each sub-diagram has an array from 0 to 9.

A file descriptor is the identifier needed to interact with a resource (most often a file) via system calls (e.g., read, write, and close)
A name has semantic meaning, an address denotes a location; an identifier has no meaning
- /etc/passwd vs.34.196.104.129 vs. file descriptor 5
Many system calls allocate file descriptors
- read: open a file
- pipe: create two unidirectional byte streams (one read, one write) between processes
- accept: accept a TCP connection request, returns descriptor to new socket
When allocating a new file descriptor, kernel chooses the smallest available number
- These semantics are important! If you close stdout (1) then open a file, it will be assigned to file descriptor 1 so act as stdout (this is how $ cat in.txt > out.txt works)

Creating and Using File Descriptors

E.g., a file table entry (for a regular file) keeps track of a current position in the file
- If you read 1000 bytes, the next read will be from 1000 bytes after the preceding one
- If you write 380 bytes, the next write will start 380 bytes after the preceding one
If you want multiple processes to write to the same log file and have the results be intelligible, then you have all of them share a single file table entry: their calls to write will be serialized and occur in some linear order

File Descriptor vs. File Table Entries

A entry in the file descriptor table is just a pointer to a file table entry
Multiple entries in a table can point to the same file table entry
Entries in different file descriptor tables (different processes!) can point to the same file table entry

Almost the same as previous, with the addition of an expansion of one of the elements in process 1001. The expansion has an entry that reads: "mode: r, cursor: 0, refcount: 1, vnode: (blank)"

File Descriptors vs. File Table Entries Example

$ ./main 1> log.txt 2> log.txt

$ ./main 1> log.txt 2>&1

Opens log.txt twice (two file table entries)

Opens log.txt once, two descriptors for same file table entry

// file: testfd.c
#include <stdio.h>
#include <unistd.h>
#include <string.h>

int main(int argc, char **argv)
{
    const char* error = "One plus one is\ntwo.\n";
    const char* msg   = "One plus two is\n";

    write(2, error, strlen(error));
    write(1, msg, strlen(msg));
    return 0;
}

File Descriptors vs. File Table Entries Example

pos: 0

log.txt

fd table

file table

vnode

pos: 0

log.txt

fd table

file table

vnode

cgregg@myth60:$ ./testfd 1> log.txt 2> log.txt
cgregg@myth60:$ cat log.txt
One plus two is
two.
cgregg@myth60:$

cgregg@myth60:$ ./testfd 1> log.txt 2>&1
cgregg@myth60:$ cat log.txt
One plus one is
two.
One plus two is
cgregg@myth60:$

Each process maintains its own descriptor table, but there is one, system-wide open ﬁle table. This allows for ﬁle resources to be shared between processes, as we've seen
As drawn above, descriptors 0, 1, and 2 in each of the three PCBs alias the same three open files. That's why each of the referred table entries have refcounts of 3 instead of 1.
This shouldn't surprise you. If your bash shell calls make, which itself calls g++, each of them inserts text into the same terminal window: those three files could be stdin, stdout, and stderr for a terminal

File Table Details

Almost the same as the previous diagram, except that there are many descriptor table entries. The idea is that file descriptior 0 from each descriptor table points to the same open file table entry, corresponding to STDIN. Likewise for descriptor 1 (all point to the same open file table entry, for STDOUT, which has a reference count of 3, because there are three files that have STDOUT open), and for descriptor 2 (STDERR). Each descriptor table also has some entries (above 2) that point to open files that are not shared between other processes.

vnodes

The vnode is the kernel's abstraction of an actual file: it includes information on what kind of file it is, how many file table entries reference it, and function pointers for performing operations.
A vnode's interface is file-system independent, but its implementation is file-system specific; any file system (or file abstraction) can put state it needs to in the vnode (e.g., inode number)
The term vnode comes from BSD UNIX; in Linux source it's called a generic inode (CONFUSING!)

Each open ﬁle entry has a pointer to a vnode, which is a structure housing static information about a ﬁle or ﬁle-like resource.

Almost the same as the last image, except that one of the vnode entries is further expanded to show the details in the "vnode table". In particular: "type: regfile, refcount: 1, fnptrs ***, inode: 0644; 8:23pm; cgregg"

File Decriptors -> File Table -> vnode Table

There is one system-wide vnode table for the same reason there is one system-wide open ﬁle table. Independent ﬁle sessions reading from the same ﬁle don't need independent copies of the vnode. They can all alias the same one.

Almost the same as the last diagram, except now many of the open file table vnode entries are expanded for the vnode table.

UNIX File Abstractions Summary

Userspace programs interact through files through file descriptors, small integers which are indexes into a per-process file descriptor table
Many file descriptors can point to the same file table entry
- They share seek pointers (writes and reads are serialized)
- Multiple programs share stdout/stderr in a terminal
Many file table entries can point to the same file (vnode)
- They concurrently access the file with different seek pointers
- You run two instances of a python script in parallel: each invocation of Python opens the file separately, with a different file table entry
Exactly how vnodes are implemented is filesystem/resource dependent
- A terminal (tty) vnode is different than an ext4fs one
Reference counting throughout
- Free a file table entry when the last file descriptor closes it
- Free a vnode when the last file table entry is freed
- Free a file when its reference count is 0 and there is no vnode
Key principles: abstraction, layers, naming

Same diagram as earlier: This diagram shows a vertical list of connections from userspace processes down to harder. Specifically: userspace processes<--system calls-->operating system<--vnode interface-->file system (ext4fs, s6fs, btfs)<--block interface-->device driver (SATA, iSCSI)<--hardare interface-->hardware (PCI-E, DMA, RDMA, etc.)

Memory mapped files

read(2) is not the only way to for a program to access a file
Read requires making a copy: program provides a buffer to read into
- What if many programs want read-only access to the file at the same time?
Example: libc.so
- Almost program wants to read libc.so for some of the functions it provides
- Imagine if every program had to read() all of its libraries into local memory
- Let's use pmap to look at how much memory libraries take up
Solution: memory mapped files
- Ask the operating system: "please map this file into memory for me"

Process Address Spaces

Recall that each process operates as if it owns all of main memory.
The diagram on the right presents a 64-bit process's general memory playground that stretches from address 0 up through and including 2⁶⁴ - 1.
CS107 and CS107-like intro-to-architecture courses present the diagram on the right, and discuss how various portions of the address space are cordoned oﬀ to manage traditional function call and return, dynamically allocated memory, access global data, and machine code storage and execution.
No process actually uses all 2⁶⁴ bytes of its address space. In fact, the vast majority of processes use a miniscule fraction of what they otherwise think they own.
The OS virtualizes memory: each process thinks it as the complete system memory (but obviously it doesn't)

This diagram shows main memory layout as a large, vertical rectangle, going from address 0x0 to address 0xFFFFFFFFFFFFFFF

Memory Regions in a Process

Most of a process's memory isn't used: valid regions are defined by segments, blocks of memory for a particular use
- Quick quiz: what's a SEGV (segmentation violation)?
Some segments you know quite well are the stack, heap, BSS, data, rodata, and code (where your executable is)
- Quick quiz: differences between bss, data, and rodata?
There are also segments for shared libraries
- We just pmapped a process on myth
Code is usually not read in through read: instead, it's memory mapped
A memory mapped file acts like the whole file is read into a segment of memory, but it a single copy can be shared across many processes

This image is another vertical memory diagram, with the segment details, as follows, from top to bottom (starting in the middle): "user stack, shared libraries, heap, bss, data, rodata, code"

Memory Mapped Files

void *mmap(void *addr,
size_t len,
int prot,
int flags,
int fd,
off_t offset);
"The mmap() system call causes the pages starting at addr and continuing for at most len bytes to be mapped from the object described by fd, starting at byte offset offset. If offset or len is not a multiple of the pagesize, the mapped region may extend past the specified range. Any extension beyond the end of the mapped object will be zero-filled."
- A page (typically 4kB) is an operating system's unit of memory management, defined by hardware
You can also mmap() anonymous memory, memory that has no backing file: pages in an anonymous region are zero (until written)
- This is how the heap, stack, data, and bss are set up

0xFFFFFFFFFFFFFFFF

0x0

libc.so

bash

heap

stack

libdl.so

data

Memory Mapped Files and the Buffer Cache

The operating system maintains a buffer cache: a pool of (page-sized) pieces of files that are in memory
- Caching: keeping pieces of used data in faster storage to improve performance
- Buffer cache: keeping parts of files in use in memory so you don't have to hit disk
Calls to read() and write() operate on the buffer cache
Blocks are read from disk and put into the buffer cache as needed
Dirty pages in the buffer cache are written to disk when needed
- sync() system call flushes buffers associated with file
Two memory maps of the same file can point to the same buffer cache entry
There's a bit more to this, but you'll have to wait for CS140 (we could spend 4 lectures on just the basics)

libc.so

Process A

Process B

Buffer Cache

Memory Mapped Files Summary

A program can map a file into its memory with mmap()
- Virtualization: every process thinks it has its own copy, but in reality there's a single one in memory (exception: MAP_PRIVATE)
Memory mapped files unify the idea of the process address space with its file descriptors
Used parts of files are kept in memory in the buffer cache
- Caching: don't force every process to read every file in entirety when it loads

libc.so

Process A

Process B

Buffer Cache

Review

The UNIX file system provides a naming and name resolution system for user data, through files and directories
The UNIX file system is designed to use abstraction and layering so that it's easy to use new file systems and use existing file systems on new devices
- Processes use the abstraction of a file descriptor, which refers to an open file in the file entry table
- A file entry refers to a vnode, which describes the actual file itself
- There can be many file descriptors for the same single file table entry, and many file table entries for the same vnode
- This layering has important semantics which give you a lot of power in how you manipulate files
Programs can directly map files into their memory with mmap(), which allows the OS to use caching
- In-use parts of files are kept in memory by a system called the buffer cache
- The buffer cache allows many programs to share a single copy of data (e.g., library code)

Topic #2: Multiprocessing

Key Question: How can my program create and interact with other programs?

Program: code you write to execute tasks

Process: an instance of your program running; consists of program and execution state.

Key idea: multiple processes can run the same program

Multiprocessing Terminology

int main(int argc, char *argv[]) {
    printf("Hello, world!\n");
    printf("Goodbye!\n");
    return 0;
}

Process 5621

Your computer runs many processes simultaneously - even with just 1 processor core (how?)

"simultaneously" = switch between them so fast humans don't notice
Your program thinks it's the only thing running
OS schedules processes - who gets to run when
Each process gets a little time, then has to wait
Many times, waiting is good! E.g. waiting for key press, waiting for disk
Caveat: multicore computers can truly multitask

Multiprocessing

Playing With Processes

When you run a program from the terminal, it runs in a new process.

The OS gives each process a unique "process ID" number (PID)
PIDs are useful once we start managing multiple processes
getpid() returns the PID of the current process

// getpid.c
#include <stdio.h>
#include <unistd.h>

int main(int argc, char *argv[]) {
    pid_t myPid = getpid();
    printf("My process ID is %d\n", myPid);
    return 0;
}

$ ./getpid 
My process ID is 18814

$ ./getpid 
My process ID is 18831

$ ./myprogram

fork()

fork() creates a second process that is a clone of the first:

pid_t fork();

int main(int argc, char *argv[]) {
    printf("Hello, world!\n");
    fork();
    printf("Goodbye!\n");
    return 0;
}

Process A

$ ./myprogram 
Hello, world!

fork()

fork() creates a second process that is a clone of the first:

pid_t fork();

int main(int argc, char *argv[]) {
    printf("Hello, world!\n");
    fork();
    printf("Goodbye!\n");
    return 0;
}

Process A

int main(int argc, char *argv[]) {
    printf("Hello, world!\n");
    fork();
    printf("Goodbye!\n");
    return 0;
}

Process A

$ ./myprogram 
Hello, world!

fork()

fork() creates a second process that is a clone of the first:

pid_t fork();

int main(int argc, char *argv[]) {
    printf("Hello, world!\n");
    fork();
    printf("Goodbye!\n");
    return 0;
}

Process A

int main(int argc, char *argv[]) {
    printf("Hello, world!\n");
    fork();
    printf("Goodbye!\n");
    return 0;
}

Process B

$ ./myprogram 
Hello, world!
Goodbye!
Goodbye!

fork()

fork() creates a second process that is a clone of the first:

pid_t fork();

int main(int argc, char *argv[]) {
    printf("Hello, world!\n");
    fork();
    printf("Goodbye!\n");
    return 0;
}

Process A

int main(int argc, char *argv[]) {
    printf("Hello, world!\n");
    fork();
    printf("Goodbye!\n");
    return 0;
}

Process A

int main(int argc, char *argv[]) {
    printf("Hello, world!\n");
    fork();
    printf("Goodbye!\n");
    return 0;
}

Process B

$ ./myprogram2

fork()

fork() creates a second process that is a clone of the first:

Process A

pid_t fork();

int main(int argc, char *argv[]) {
    int x = 2;
    printf("Hello, world!\n");
    fork();
    printf("Goodbye, %d!\n", x);
    return 0;
}

$ ./myprogram2 
Hello, world!

fork()

fork() creates a second process that is a clone of the first:

Process A

pid_t fork();

int main(int argc, char *argv[]) {
    int x = 2;
    printf("Hello, world!\n");
    fork();
    printf("Goodbye, %d!\n", x);
    return 0;
}

$ ./myprogram2
Hello, world!

fork()

fork() creates a second process that is a clone of the first:

pid_t fork();

Process B

int main(int argc, char *argv[]) {
    int x = 2;
    printf("Hello, world!\n");
    fork();
    printf("Goodbye, %d!\n", x);
    return 0;
}

Process A

int main(int argc, char *argv[]) {
    int x = 2;
    printf("Hello, world!\n");
    fork();
    printf("Goodbye, %d!\n", x);
    return 0;
}

$ ./myprogram2
Hello, world!
Goodbye, 2!
Goodbye, 2!

fork()

fork() creates a second process that is a clone of the first:

pid_t fork();

Process B

int main(int argc, char *argv[]) {
    int x = 2;
    printf("Hello, world!\n");
    fork();
    printf("Goodbye, %d!\n", x);
    return 0;
}

Process A

int main(int argc, char *argv[]) {
    int x = 2;
    printf("Hello, world!\n");
    fork();
    printf("Goodbye, %d!\n", x);
    return 0;
}

fork()

fork() creates a second process that is a clone of the first:

pid_t fork();

parent (original) process forks off a child (new) process
The child starts execution on the next program instruction. The parent continues execution with the next program instruction.
fork() is called once, but returns twice (why?)
Everything is duplicated in the child process
- File descriptors (increasing reference counts on file table entries)
- Mapped memory regions (the address space)
- Regions like stack, heap, etc. are copied

fork()

Process B

int main(int argc, char *argv[]) {
    int x = 2;
    printf("Hello, world!\n");
    fork();
    printf("Goodbye, %d!\n", x);
    return 0;
}

Process A

int main(int argc, char *argv[]) {
    int x = 2;
    printf("Hello, world!\n");
    fork();
    printf("Goodbye, %d!\n", x);
    return 0;
}

(Am I the parent or the child?)

Is there a way for the processes to tell which is the parent and which is the child?

Key Idea: the return value of fork() is different in the parent and the child.

fork()

fork() creates a second process that is a clone of the first:

pid_t fork();

parent (original) process forks off a child (new) process
In the parent, fork() will return the PID of the child (only way for parent to get child's PID)
In the child, fork() will return 0 (this is not the child's PID, it's just 0)

$ ./myprogram

fork()

int main(int argc, char *argv[]) {
    printf("Hello, world!\n");
    pid_t pidOrZero = fork();
    printf("fork returned %d\n", pidOrZero);
    return 0;
}

Process 110

In the parent, fork() will return the PID of the child (only way for parent to get child's PID)
In the child, fork() will return 0 (this is not the child's PID, it's just 0)

$ ./myprogram2
Hello, world!

fork()

int main(int argc, char *argv[]) {
    printf("Hello, world!\n");
    pid_t pidOrZero = fork();
    printf("fork returned %d\n", pidOrZero);
    return 0;
}

Process 110

In the parent, fork() will return the PID of the child (only way for parent to get child's PID)
In the child, fork() will return 0 (this is not the child's PID, it's just 0)

$ ./myprogram2
Hello, world!

fork()

int main(int argc, char *argv[]) {
    printf("Hello, world!\n");
    pid_t pidOrZero = fork(); // 111
    printf("fork returned %d\n", pidOrZero);
    return 0;
}

Process 110

In the parent, fork() will return the PID of the child (only way for parent to get child's PID)
In the child, fork() will return 0 (this is not the child's PID, it's just 0)

int main(int argc, char *argv[]) {
    printf("Hello, world!\n");
    pid_t pidOrZero = fork(); // 0
    printf("fork returned %d\n", pidOrZero);
    return 0;
}

Process 111

$ ./myprogram
Hello, world!
fork returned 111
fork returned 0

fork()

int main(int argc, char *argv[]) {
    printf("Hello, world!\n");
    pid_t pidOrZero = fork(); // 111
    printf("fork returned %d\n", pidOrZero);
    return 0;
}

Process 110

In the parent, fork() will return the PID of the child (only way for parent to get child's PID)
In the child, fork() will return 0 (this is not the child's PID, it's just 0)

int main(int argc, char *argv[]) {
    printf("Hello, world!\n");
    pid_t pidOrZero = fork(); // 0
    printf("fork returned %d\n", pidOrZero);
    return 0;
}

Process 111

$ ./myprogram
Hello, world!
fork returned 111
fork returned 0

fork()

int main(int argc, char *argv[]) {
    printf("Hello, world!\n");
    pid_t pidOrZero = fork(); // 111
    printf("fork returned %d\n", pidOrZero);
    return 0;
}

Process 110

In the parent, fork() will return the PID of the child (only way for parent to get child's PID)
In the child, fork() will return 0 (this is not the child's PID, it's just 0)

int main(int argc, char *argv[]) {
    printf("Hello, world!\n");
    pid_t pidOrZero = fork(); // 0
    printf("fork returned %d\n", pidOrZero);
    return 0;
}

Process 111

$ ./myprogram
Hello, world!
fork returned 0
fork returned 111

$ ./myprogram
Hello, world!
fork returned 111
fork returned 0

fork()

int main(int argc, char *argv[]) {
    printf("Hello, world!\n");
    pid_t pidOrZero = fork(); // 111
    printf("fork returned %d\n", pidOrZero);
    return 0;
}

Process 110

In the parent, fork() will return the PID of the child (only way for parent to get child's PID)
In the child, fork() will return 0 (this is not the child's PID, it's just 0)

int main(int argc, char *argv[]) {
    printf("Hello, world!\n");
    pid_t pidOrZero = fork(); // 0
    printf("fork returned %d\n", pidOrZero);
    return 0;
}

Process 111

$ ./myprogram
Hello, world!
fork returned 0
fork returned 111

We can no longer assume the order in which our program will execute! The OS decides the order.

fork()

In the parent, fork() will return the PID of the child (only way for parent to get child's PID)
In the child, fork() will return 0 (this is not the child's PID, it's just 0)
A process can use getppid() to get the PID of its parent
if fork() returns < 0, that means an error occurred

// basic-fork.c
int main(int argc, char *argv[]) {
    printf("Greetings from process %d! (parent %d)\n", getpid(), getppid());
    pid_t pidOrZero = fork();
    assert(pidOrZero >= 0);
    printf("Bye-bye from process %d! (parent %d)\n", getpid(), getppid());
    return 0;
}

$ ./basic-fork 
Greetings from process 29686! (parent 29351)
Bye-bye from process 29686! (parent 29351)
Bye-bye from process 29687! (parent 29686)

$ ./basic-fork 
Greetings from process 29688! (parent 29351)
Bye-bye from process 29689! (parent 29688
Bye-bye from process 29688! (parent 29351)

The parent of the original process is the shell - the program that you run in the terminal.
The ordering of the parent and child output is nondeterministic. Sometimes the parent prints first, and sometimes the child prints first!

Debugging Multiprocess Programs

How do I debug two processes at once? gdb has built-in support for debugging multiple processes

set detach-on-fork off
- This tells gdb to capture any fork'd processes, though it pauses them upon the fork.
info inferiors
- This lists the processes that gdb has captured.
inferior X
- Switch to a different process to debug it.
detach inferior X
- Tell gdb to stop watching the process, and continue it
You can see an entire debugging session on the basic-fork program right here.

Here's a useful (but mind-melting) example of a program where child processes themselves call fork():

// fork-puzzle.c
static const char *kTrail = "abc";

int main(int argc, char *argv[]) {
    for (int i = 0; i < strlen(kTrail); i++) {
        printf("%c\n", kTrail[i]);
        pid_t pidOrZero = fork();
        assert(pidOrZero >= 0);
    }
    return 0;
}

Fork Tree

Parent

// fork-puzzle.c
static const char *kTrail = "abc";

int main(int argc, char *argv[]) {
    for (int i = 0; i < strlen(kTrail); i++) {
        printf("%c\n", kTrail[i]);
        pid_t pidOrZero = fork();
        assert(pidOrZero >= 0);
    }
    return 0;
}

Fork Tree

Child 1

// fork-puzzle.c
static const char *kTrail = "abc";

int main(int argc, char *argv[]) {
    for (int i = 0; i < strlen(kTrail); i++) {
        printf("%c\n", kTrail[i]);
        pid_t pidOrZero = fork();
        assert(pidOrZero >= 0);
    }
    return 0;
}

Parent

Here's a useful (but mind-melting) example of a program where child processes themselves call fork():

// fork-puzzle.c
static const char *kTrail = "abc";

int main(int argc, char *argv[]) {
    for (int i = 0; i < strlen(kTrail); i++) {
        printf("%c\n", kTrail[i]);
        pid_t pidOrZero = fork();
        assert(pidOrZero >= 0);
    }
    return 0;
}

Fork Tree

// fork-puzzle.c
static const char *kTrail = "abc";

int main(int argc, char *argv[]) {
    for (int i = 0; i < strlen(kTrail); i++) {
        printf("%c\n", kTrail[i]);
        pid_t pidOrZero = fork();
        assert(pidOrZero >= 0);
    }
    return 0;
}

Child 2

GChild 1

// fork-puzzle.c
static const char *kTrail = "abc";

int main(int argc, char *argv[]) {
    for (int i = 0; i < strlen(kTrail); i++) {
        printf("%c\n", kTrail[i]);
        pid_t pidOrZero = fork();
        assert(pidOrZero >= 0);
    }
    return 0;
}

// fork-puzzle.c
static const char *kTrail = "abc";

int main(int argc, char *argv[]) {
    for (int i = 0; i < strlen(kTrail); i++) {
        printf("%c\n", kTrail[i]);
        pid_t pidOrZero = fork();
        assert(pidOrZero >= 0);
    }
    return 0;
}

Child 1

Parent

Here's a useful (but mind-melting) example of a program where child processes themselves call fork():

// fork-puzzle.c
static const char *kTrail = "abc";

int main(int argc, char *argv[]) {
    for (int i = 0; i < strlen(kTrail); i++) {
        printf("%c\n", kTrail[i]);
        pid_t pidOrZero = fork();
        assert(pidOrZero >= 0);
    }
    return 0;
}

Fork Tree

// fork-puzzle.c
static const char *kTrail = "abc";

int main(int argc, char *argv[]) {
    for (int i = 0; i < strlen(kTrail); i++) {
        printf("%c\n", kTrail[i]);
        pid_t pidOrZero = fork();
        assert(pidOrZero >= 0);
    }
    return 0;
}

Child 2

GChild 1

// fork-puzzle.c
static const char *kTrail = "abc";

int main(int argc, char *argv[]) {
    for (int i = 0; i < strlen(kTrail); i++) {
        printf("%c\n", kTrail[i]);
        pid_t pidOrZero = fork();
        assert(pidOrZero >= 0);
    }
    return 0;
}

// fork-puzzle.c
static const char *kTrail = "abc";

int main(int argc, char *argv[]) {
    for (int i = 0; i < strlen(kTrail); i++) {
        printf("%c\n", kTrail[i]);
        pid_t pidOrZero = fork();
        assert(pidOrZero >= 0);
    }
    return 0;
}

Child 1

Parent

Child 3

GChild 2

// fork-puzzle.c
static const char *kTrail = "abc";

int main(int argc, char *argv[]) {
    for (int i = 0; i < strlen(kTrail); i++) {
        printf("%c\n", kTrail[i]);
        pid_t pidOrZero = fork();
        assert(pidOrZero >= 0);
    }
    return 0;
}

GChild 3

// fork-puzzle.c
static const char *kTrail = "abc";

int main(int argc, char *argv[]) {
    for (int i = 0; i < strlen(kTrail); i++) {
        printf("%c\n", kTrail[i]);
        pid_t pidOrZero = fork();
        assert(pidOrZero >= 0);
    }
    return 0;
}

// fork-puzzle.c
static const char *kTrail = "abc";

int main(int argc, char *argv[]) {
    for (int i = 0; i < strlen(kTrail); i++) {
        printf("%c\n", kTrail[i]);
        pid_t pidOrZero = fork();
        assert(pidOrZero >= 0);
    }
    return 0;
}

GGChild 1

// fork-puzzle.c
static const char *kTrail = "abc";

int main(int argc, char *argv[]) {
    for (int i = 0; i < strlen(kTrail); i++) {
        printf("%c\n", kTrail[i]);
        pid_t pidOrZero = fork();
        assert(pidOrZero >= 0);
    }
    return 0;
}

Here's a useful (but mind-melting) example of a program where child processes themselves call fork():

// fork-puzzle.c
static const char *kTrail = "abc";

int main(int argc, char *argv[]) {
    for (int i = 0; i < strlen(kTrail); i++) {
        printf("%c\n", kTrail[i]);
        pid_t pidOrZero = fork();
        assert(pidOrZero >= 0);
    }
    return 0;
}

Fork Tree

Observations:

One a is printed by the original process.
The original process and its child each print b.
The two bs may not be consecutive - why?

$ ./fork-puzzle
a
b
c
b
c
c
c

$ ./fork-puzzle
a
b
c
b
c
c
$ c

What happened here?

$ ./fork-puzzle
a
b
b
c
c
c
c

Fork Tree

Questions:

1 a is printed.
2 bs are printed.
How many cs get printed?
Who prints nothing?

Child 2

GChild 1

Child 1

Parent

Child 3

GChild 2

GChild 3

GGChild 1

-> 4: parent, child 1, child 2, Gchild 1

-> Child 3, GGchild 1, Gchild 3, Gchild 2

// fork-puzzle.c
static const char *kTrail = "abc";

int main(int argc, char *argv[]) {
    for (int i = 0; i < strlen(kTrail); i++) {
        printf("%c\n", kTrail[i]);
        pid_t pidOrZero = fork();
        assert(pidOrZero >= 0);
    }
    return 0;
}

Fork is used pervasively in applications. A few examples:
- Running a program in a shell: the shell forks a new process to run the program
- Servers: most network servers run many copies of the server in different processes (why?)
Fork is used pervasively in systems. A few examples:
- When your kernel boots, it starts the system.d program, which forks off all of the services and systems for your computer
  - Let's take a look with pstree
- Your window manager spawns processes when you start programs
- Network servers spawn processes when they receive connections
  - E.g., when you ssh into myth, sshd spawns a process to run your shell in (after setting up file descriptors for your terminals over ssh)
Processes are the first step in understanding concurrency, another key principle in computer
systems; we'll look at other forms of concurrency later in the quarter

Why Fork?

Review

A process is an instance of a program
Each process has a unique PID
fork() creates a clone of the current process, and they run concurrently
The parent and child are identical except for fork's return value (child PID for parent, 0 for child)
This concurrency lets your program multitask: much of the quarter will look at the complications
- Nondeterministic ordering of execution across processes
- A parent can wait for its children to terminate

Next time: the power of the fork()

Extra Problems

Fork Tree Round 2

Questions:

How many total processes are there when running this program?
How many times is d printed?
Could a d be printed before an: "a"? "b"? "c"?
How many processes don't print anything?

// fork-puzzle-full.c
static const char *kTrail = "abcd";

int main(int argc, char *argv[]) {
    for (int i = 0; i < strlen(kTrail); i++) {
        printf("%c\n", kTrail[i]);
        pid_t pidOrZero = fork();
        assert(pidOrZero >= 0);
    }
    return 0;
}

Fork Tree Round 2

Questions:

How many total processes are there when running this program?
How many times is d printed?
Could a d be printed before an: "a"? "b"? "c"?
How many processes don't print anything?

// fork-puzzle-full.c
static const char *kTrail = "abcd";

int main(int argc, char *argv[]) {
    for (int i = 0; i < strlen(kTrail); i++) {
        printf("%c\n", kTrail[i]);
        pid_t pidOrZero = fork();
        assert(pidOrZero >= 0);
    }
    return 0;
}

16 total processes
d is printed 8 times
before a "b" or "c"
8 processes print nothing

Child 2

GChild 1

Child 1

Parent

Child 3

GChild 2

GChild 3

GGChild 1

From earlier fork tree: