Process Groups, Sessions and Terminals

Processes

Review of Concepts

• http://www.win.tue.nl/~aeb/linux/lk/lk-10.html
• A process is an entity that is started by the fork system call.

Creation

pid_t p; p = fork(); if (p == (pid_t) -1) /* ERROR */ else if (p == 0) /* CHILD */ else /* PARENT */

Termination

• A process is terminated normally by the exit “C” library call, or an _exit system call
• It also returns by executing return(n) from the main function
• The status of the system call may be collected by the parent using the wait system call

Collecting Status

pid_t p; int status; p = wait(&status);

• A process that has terminated, but not yet been waited-for is called a “zombie”.
• A zombie process stores a 2-byte status.
• On the other hand, if the parent dies first, theinit process inherits the child, and becomes its parent.

Signal

• Signals may force abnormal termination of a process.
• The default action on receipt of a signal by a process is as follows
  • Terminate the process
  • Terminate the process with a core-dump
  • Ignore the signal

SIGSTOP and SIGCONT

• The default action on all signals except SIGKILL and SIGSTOP can be overridden
• The SIGSTOP signal pauses the given process
• The SIGCONT signal continues the process from where it left off.
• SIGTTIN (input requested by background process) and SIGTTOUT (output requested by background process), also cause a process to pause.
• Several related processes may be configured as part of the same process group.
• An example of this is when we pipe several processes together

Process Group

Process group

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• A process group is created with the setpgid(pid, pgid) call
• Only the process or its parent can set the process group-id.
  • The parent must set the process group-id before the child does the exec, and may move it within the session only
• A process may set itself as a process-group leader by giving itself as the process-group-id
• A session leader cannot set the process group-id
• An example to set the process group

Example of setting a process group

p = fork();if (p == (pid_t) -1) {/* ERROR */ } else if (p == 0) {    /* CHILD */ setpgid(0, pgid); … } else {                /* PARENT */ setpgid(p, pgid); … }

Signaling and Waiting

• One can signal to all members of a process group as part of the kill system call

kill( -getpgid(), signal );

• One can also wait for all children of a specified process group

wait( 0, &status ); // By default, wait waits for all processes of the// process group of the current processwait( -pid, &status ); // Waits for pid of the current process

Foreground Process Groups

• Among the several process-groups associated with a session, at most one is called a “foreground process group”.
• The terminals associated with a “session” send their input and signals to the processes associated with the “foreground process group”.
• A process can get / set the “foreground process group” associated with its session using
  • fd denotes the controlling terminal of the process

pid = tcgetpgrp(fd);tcsetpgrp(pgid,fd);

• The file descriptor is obtained by opening the device /dev/tty. This works irrespective of the redirects of standard inputs and standard outputs.
• All process groups, except “foreground process group”, are called “background process groups”.
• Since the foreground process group is interacting with the terminal, the background processes stay out of this
  • When they try to read the Terminal input, they get a SIGTTIN signal. If this signal is blocked or ignored, then the function returns an error
• Similarly, when they try to read the Terminal output, they get a SIGTTOU signal.
  • If background processes are allowed to write to the terminal, verified by an stty setting
  • then, they may be allowed to write to the terminal.

Background Process Group

stty -tostop

Orphaned Process Group

• Whenever a Process Group Leader dies, it leaves the entire process group orphaned.
• When the process leader dies, all the members of the process group are sent SIGHUP or SIGCONT.
• Each process group belongs to a unique session.
• A session is often created by a user-login
• The terminal on which one is logged in becomes the controlling tty of the session.
• By convention, the session-id is the process-id of the first member of the session, also known as “Session Leader”.
• One can obtain the session-id with the getsessid() call.
• The id can be changed with a setsessid() calls
• The setsid creates a new session with the specified process-id, and makes it the leader of a group.

Sessions

What are sessions

Creation and Propagation of Sessions

p = fork(); if (p) exit(0); pid = setsid();

Controlling Terminal

Getting a Controlling terminal

• System V specifies that the first tty that is opened by the process becomes its controlling tty.
• As per BSD, a control tty is explicitly set using the process,
• Linux uses a combination of these techniques.

ioctl(fd, TIOCSCTTY, …);

Getting rid of the controlling terminal

• When the controlling terminal is not needed, as for a daemon,
  • we need to detach the process from the controlling tty.
• This is done using the function setsid(),
• Or by the function ioctl with flag TIOCNOTTY

if ((fork()) != 0)   exit(0); setsid(); if ((fork()) != 0)    exit(0);

The daemon function

• The daemon function enables the process to close the controlling terminals, and to start as a system daemon

int daemonize( int nochdir, int noclose )// Specifies whether the current directory should be changed// and whether or not the stdin, stdout and stderr file descriptors should be closed

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Coroutines in C

This is a very readable and understandable article, that I have just rephrased a little. Enjoy reading it …

References

• http://www.chiark.greenend.org.uk/~sgtatham/coroutines.html
• http://www.linuxhowtos.org/C_C++/coroutines.htm
• Consider two routines
• The first decompresses the output
  • Loop
    • Read a character
    • If character is FF, then read the length and the character
    • Emit the character

Motivating example

Decompressor

/* Decompression code */while (1) { c = getchar(); if (c == EOF) break; if (c == 0xFF) { len = getchar(); c = getchar(); while (len–) emit(c); } else emit(c); } emit(EOF);

Parser

• The second routine is a parser
  • This function scans continuous tokens of alphabets as a word
  • The rest of the characters are output individually as tokens.

/* Parser code */ while (1) {     c = getchar();     if (c == EOF)        break;     if (isalpha(c)) {       do {         add_to_token(c);         c = getchar();       } while (isalpha(c));       got_token(WORD);     }     add_to_token(c);     got_token(PUNCT); }

Putting them together

• These functions need to be put together. There are two approaches to doing this
• In the first approach, we create two programs around each of these processes, and send the output of one to the input of the other.
•  This involves two programs, and is typically a performance overhead.
• Or one could create two threads, one thread running the decompressor, and the other thread running the parser
• The decompressor produces a character, and places it into a buffer. The parser reads data from the buffer.
• The threads co-ordinate by means of a lock or a semaphore.
• This is still a performance overhead, especially if
• An efficient way of putting the programs together within a single program is to rewrite one of the routines to output a character at a time, so that it can be used within the loop of another program.
• The following is the routine to output the decompressed character
  • There are two static variables – replen and repchar, that represent repeated characters.

 UNIX Pipes

POSIX Threads

Rewriting the routines

Per-character Decompressor

int decompressor(void) {   static int repchar;   static int replen;     // If repeated characters have not ended, send the character back   if (replen > 0) {     replen--;     return repchar;   }     c = getchar();   if (c == EOF)     return EOF;   if (c == 0xFF) {     replen = getchar();     repchar = getchar();     replen--;     return repchar;   } else     return c; }

• This function can be used to replace the getchar() method in the second function

/* Parser code */ while (1) {   c = decompressor();   if (c == EOF)     break;   if (isalpha(c))     do {       add_to_token(c);       c = decompressor();     } while (isalpha(c));     got_token(WORD);   }   add_to_token(c);   got_token(PUNCT); }

Per-character Tokenizer

• The following variant of the parser, returns token, if the next character forms a token
• The actual implementation is more complex
  • The routine returns no token if the current character is alphabetic, and instead adds the token to its internal word.
  • If the next character is non-alphabetic, the internal word is output, as well as the next token.

void parser(int c) {     // The state of whether there is a word being built or not   static enum {     START, IN_WORD   } state;     switch (state) {       // If we are already in the word, and we get a token, we fall through     case IN_WORD:       if (isalpha(c)) {         add_to_token(c);         return;       }       got_token(WORD);       state = START;       /* fall through */       // If the state is already Start, if we found a non-alphanumeric token,     //   we return that token as well     //   else, we add to the internal array     case START:       add_to_token(c);       if (isalpha(c))          state = IN_WORD;       else          got_token(PUNCT);       break;     } }

Knuth’s Co-routines

Knuth’s concept is to avoid the caller-callee relationship, and treat the processes as co-operating equals.

1. Each co-routine gets its own co-location frame, where it gets to store its own local variables, and the return address
2. When calling a co-routine, the function saves its location on the stack, and jumps to the address specified in the co-routine frame.
3. Inside the co-routine, local variables are accessed from the co-routine frame, rather than from the stack.
4. When the co-routine returns, the control returns to its old location, based on the return address saved on the stack as part of the co-routine call.

Co-routines in a stack-based Environment

Co-routines can also be implemented with the current chip architectures with the current compilers

1. Every local variable needs to be static. This will ensure that all variables get allocated on a separate co-routine frame.
2. One such variable – state denotes the location from where the co-routine previously returned.
3. Based on this state, we go-to the previous point, where the computation left off.

A Toy Example

• A routine to iterate through a list is specified as follows
• The following is the original “C” function

int function(void) {   int i;   for (i = 0; i < 10; i++)     return i;   /* won't work, but wouldn't it be nice */ }

• When expressed as a co-routine, the following is the co-routine

int function(void) {static int state = 0; static int i; switch(state) { case 0: goto BEGIN;  break; case 1: goto LABEL1; break; } for( i = 0; i < 10; i++ ) { state = 1; return i; LABEL1:; } }

Framework in C, and solution to the problem

Macros to achieve the same effect

• The following are a good set of macros, that will achieve the same effect

#define crBegin static int state=0; switch(state) { case 0: #define crReturn(i,x) do { state=i; return x; case i:; } while (0) #define crFinish }

• The co-routine may therefore be coded as

int function(void) {     static int i;     crBegin;     for (i = 0; i < 10; i++)         crReturn(1, i);     crFinish; }

• The macro crReturn could be simplified even further, as follows

#define crReturn(x) do { state=__LINE__; return x; \                          case __LINE__:; } while (0)

The Decompressor code

• The decompressor function may be modified to a co-routine as follows

int decompressor(void) {   static int c, len;   crBegin;   while (1) {     c = getchar();     if (c == EOF)       break;     if (c == 0xFF) {       len = getchar();       c = getchar();       while (len--)         crReturn(c);     } else       crReturn(c);   }   crReturn(EOF);   crFinish; }

• This could be used directly with the parser code.
• The parser code could continue to be a direct subroutine call.
• Alternatively, we could convert the Parser code into subroutine code as follows

The Parser Code

void parser(int c) {   crBegin;   while (1) {     /* first char already in c */     if (c == EOF)       break;     if (isalpha(c)) {       do {         add_to_token(c);         crReturn( );       } while (isalpha(c));       got_token(WORD);     }     add_to_token(c);     got_token(PUNCT);     crReturn( );   }   crFinish; }

Scalability of the Model

Multi-threading and Redundancy

In a multi-threaded environment, the above semantics are unlikely to be useful, because of the use of static variables.

The Use of Contexts

The way to solve this is to

1. Have multiple contexts.
2. Have an additional argument to each call – the Context itself. One variable in the Context State can store the last exit-point, or the expected entry-point.
3.  Pass the Context into every call of the subroutine.
4. All static variables need to dereference the context, instead of being static variables.

The use of Macros

The macros can make it easier to achieve this

• Macros in http://www.chiark.greenend.org.uk/~sgtatham/coroutine.h achieve the desired effect.