Jan Kończak

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===== Blocking vs non-blocking operations =====

Most POSIX functions complete in a limited number of steps.

But when the user invokes certain functions, then in well-defined circumstances 
it makes sense to wait until a particular thing happens.

For instance, when a ''php -S 0:8080 2>&1 | tee -a log'' command is executed in
a shell, then it is desired that when the ''tee'' program attempts to read from
the output of ''php'' program, then the ''read'' function waits until the ''php''
wrote something.

When a function stops not because it is not given the CPU time but because it
waits for something to happen, then it **blocks**.
\\
Functions that may block are called **blocking**. (Cf. definition of
[[https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap03.html#tag_03_77|blocking]]
in POSIX standard.)

**Blocking may take indefinite time. When a blocking function is used, the
programmer must always account that a call to the function may stop the thread
that invoked it for an arbitrary time.**

There is usually a way to invoke blocking functions in a non-blocking mode.
\\
**When a blocking functions is used in non-blocking mode, then it either does
what it's supposed to do without waiting, or it returns ''-1'' and sets
''errno'' to EWOULDBLOCK or EAGAIN**.
\\
When one uses non-blocking mode, one must handle the case when the function
failed to do (a part of) what it was supposed to do.

For functions related to file descriptors, the blocking / non-blocking mode
is selected by a O_NONBLOCK flag for a file descriptor.
\\
To set/clear the O_NONBLOCK flag, one shall first read the flags with 
''int flags = [[https://pubs.opengroup.org/onlinepubs/9699919799/functions/fcntl.html|fcntl]](fd, F_GETFL);'',
then set/clear the flag (e.g., ''flags |= O_NONBLOCK;'') and finally set the
new flags with ''fcntl(fd, F_SETFL, flags);''.

===== Pipes & FIFOs =====

A **pipe** is an unidirectional communication channel – a pair of file descriptors
such that any data written to the second descriptor can be read from the first
descriptor. A pipe is created with the following function:
\\
<html><span style="float:right"><small>Needs header: <code>unistd.h</code></small></span>
<a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/pipe.html"></html>
''int **pipe**(int //fildes//[2])''
<html></a></html>
\\
The ''//fildes//[0]'' is opened for reading, and the ''//fildes//[1]'' is opened
for writing.
\\
Pipes can be used to send data from one process to another process, or from 
one thread to another thread of the same process((While using a pipe to communicate
threads is inefficient, it has its use cases. For instance, it enables waking a
thread that is waiting for I/O from multiple sources with ''select'' or ''poll''.)).

By default, pipes are **blocking**, that is reading data from a pipe will
stall the thread that invoked ''read'' until some data is written to a pipe.
Also, writing data to a pipe will block when sufficiently many bytes were 
already written and are not yet read from the pipe.
\\
<small>
In non-blocking mode the ''write'' function may write only a part of the data.
</small>

When all file descriptors that allowed writing to a pipe are closed, a read from
the pipe will return ''0''.
\\
When all file descriptors that allowed reading from a pipe are closed, a write
to the pipe will first raise SIGPIPE, then return ''-1'' and set ''errno'' to
''EPIPE'' (provided the process did not terminate upon SIGPIPE).

To share a pipe between two processes, one must create a pipe in one process and
''fork'' – file descriptors are copied upon forking.
\\
A **FIFO** file (or a named pipe) is a special file that allows opening either
end of a pipe by providing a path to the file. A FIFO file can be created with
''mkfifo'' shell utility or by the
''[[https://pubs.opengroup.org/onlinepubs/9699919799/functions/mkfifo.html|mkfifo]]''
function.
\\
A call to ''open'' on a path to a FIFO file is blocking. ''open'' returns only
once at least one process invoked ''open'' with O_RDONLY and at least one
process invoked ''open'' with O_WRONLY((POSIX defines only what happens with FIFO
files opened with O_RDONLY and O_WRONLY. Result of using O_RDWR with a FIFO file
is explicitly undefined in POSIX.)). From that point on the file descriptors
act as those of an (anonymous) pipe.

<small>Pipe is unidirectional. The unix //socket// is its bidirectional
equivalent. See ''[[https://man7.org/linux/man-pages/man7/unix.7.html|man 7 unix]]''
for details.</small>

~~Exercise.#~~ Write a program that:
\\     • creates a pipe
\\     • forks
\\     • in the child process:
\\           · calculates a computationally expensive mathematical equation (say, "2+2")
\\           · writes the result to the pipe
\\           · terminates
\\     • in the parent process:
\\           · reads from pipe
\\           · writes result to the standard output

~~Exercise.#~~ Write a program that:
\\     • creates a pipe
\\     • creates three child processes 
\\     • in each child process:
\\           · calculates a computationally expensive mathematical equation (say, "2+2")
\\           · writes the result to the pipe as a four-byte integer
\\           · terminates
\\     • in the parent process:
\\           · calculates a computationally expensive mathematical equation (say, "2+2")
\\           · reads from the pipe all three results <small>(and calls ''wait()'' to reap defunct children)</small>
\\           · writes the sum of all four results to the standard output

~~Exercise.#~~ Write a program that prints the result of ''ls -l'' in uppercase.
\\
<small> You may do this e.g., by: ''pipe'', ''fork'', in one process: ''dup2''
and ''exec'', in the other process: reading from pipe and changing case.
\\
The ''[[https://en.cppreference.com/w/c/string/byte/toupper|toupper]]'' function (available from ''ctype.h'') converts a single character to upper case.

~~Exercise.#~~ Write a program that does ''ps -eF | sort -nk6''.</small>

===== Signals =====

Disclaimer: these materials contain the very basics of signals. 
For comprehensive informations, see the
[[https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_04|POSIX standard]].

==== Sending signals ====

To send a signal, one can use the function
<html><span style="float:right"><small>Needs header: <code>signal.h</code></small></span>
<a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/kill.html"></html>
''int **kill**(pid_t //pid//, int //sig//)''
<html></a></html>
\\
This function works just like the shell ''kill'' utility.

==== Setting up signal handlers ====

To handle a signal delivered to the process, one can replace the default signal
handler with a custom function.
\\
The function must return ''void'' and take an ''int'' argument, that is its
prototype has to be as follows:
\\
''void //your_function_name//(int //the_number_of_the_signal_delivered//);''

<small>
**The following paragraph explains C syntax for function pointers.**
\\
Notice that in C, to represent the type of (pointer to) such function, one has
to write ''void (*)(int)''.
\\
To take (a pointer to) such function as a parameter (or declare a variable of
this type), one has to write:
\\
''void (*//some_name//)(int)''
\\
So a function //some_function_1// that takes a ''void (*)(int)'' function as an
argument and returns, say, ''float'', looks like this: \\
''float //some_function_1//(void (*//some_name//)(int))''
\\
And a function ''//some_function_2(…)//'' that returns a function of
''void (*)(int)'' type is written as: \\
''void (*//some_function_2//(…))(int)'' \\
++++ An example code that uses function pointers |
<code c>
int foo(char a) {return a;}
int baz(char a) {return a;}

int (* getFoo     (void)  )(char) {return foo;}
int (* getFooOrBaz(long w))(char) {if(w) return foo; return baz;}

int (*savedFunc)(char); // a variable that stores (a pointer to) a function
int (*getFunc())(char){ // a function returning (a pointer to) a function
    return savedFunc;
}

void setFunc(int (*arg)(char)) {savedFunc = arg;}
int (*getOldSetNewFunc(int (*newFunc)(char)))(char) {
    int (*oldFunc)(char) = savedFunc;
    savedFunc = newFunc;
    return oldFunc;
}

void doSomething(){
    setFunc(&foo);  // both lines are correct, a function name is an implicit
    setFunc(baz);   // pointer to the function
    
    int result1 = savedFunc('a'); // calls the function
    
    int (*x)(char) = getFooOrBaz(0); // saves (a pointer to) a function returned by getFooOrBaz
    
    int result2 = x('a'); // calls the x (that points to baz) with the argument 'a'
    
    // getFooOrBaz(1) returns a function foo, then foo('a') is called and returns the result
    int result3 = getFooOrBaz(1)('a');
    
    // passes 'foo' (= result of getFooOrBaz) to getOldSetNewFunc
    // and calls the resut of getOldSetNewFunc ('baz') with argument 'a'
    int result4 = getOldSetNewFunc(getFooOrBaz(1))('a');
}
</code>
++++
C+%%%%+ has a MUCH better support for functional programming.
</small>

To replace a signal handler, one can use the following function:
\\
<html><span style="float:right"><small>Needs header: <code>signal.h</code></small></span>
<a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/signal.html"></html>
''void (*<nowiki></nowiki>**signal**(int //sig//, void (*//func//)(int)))(int)''
<html></a></html>
\\
That definition looks ugly, but hey, it's even a part of [[https://en.cppreference.com/w/c/program/signal|the C standard]].
\\
There's a way to make it look better: let's define a type that refers to 
functions taking an int and returning nothing:\\
''typedef void (*sighandler_t)(int);''\\
Now, __the same__ ''signal'' function looks like this: \\
''sighandler_t **signal**(int //signum//, sighandler_t //func//);'' \\
(This way of writing ''signal'' function is preferred in the [[https://man7.org/linux/man-pages/man2/signal.2.html|Linux manual]].)

The ''signal'' function replaces the handler for signal //signum// with the
function //func//. A single function can be used for multiple signals, hence
when the function is called, the signal number is passed as an argument.

See the following example:
<code c>
#include <signal.h>                           
#include <stdio.h>                            
#include <unistd.h>                           
                                                                 // A fprintf in a signal handler
void handleSignals(int num) {                                    // is an undefined behaviour,
  if (num == SIGINT)                                             // hence this program is not
    fprintf(stderr, "You probably pressed Ctrl+c\n");            // correct. It's still probably
  else if (num == SIGTERM)                                       // going to work as expected in
    fprintf(stderr, "You probably run 'kill %d'\n", getpid());   // in this simple case.
}

int main() {
  signal(SIGINT,  handleSignals);
  signal(SIGTERM, handleSignals);
  while (1)
    pause();
}
</code>

There are two special values for signal handlers: ''SIG_DFL'' and **''SIG_IGN''**.
The former resets the signal handler to the default handler, and the latter
ignores the signal.
For instance, when one wants the program to ignore the HUP signal, one can simply write:
\\
''signal(SIGHUP, SIG_IGNORE);''

==== Signal handlers ====

=== How they work ===

When a signal is delivered to a process by the operating system, the operating
system stops one of the threads of the process, modifies the stack so that it
looks as if the previous instruction called the signal handler, and sets the
program counter (the CPU register that tells which instruction should be
executed next) to the first instruction of the signal handler.
\\
**This means that a signal handler can be executed in any, possibly completely
inconvenient, moment.**
\\
When the signal handler returns, the control flow returns to the place where
it's been interrupted by the signal.
\\
By default, interrupted system calls restart – for instance, when a signal is
received while waiting in ''read'' for input, then after returning from the
signal handler the ''read'' is continued.

=== Restrictions ===

Since a signal handler can be called anytime, possibly in the middle of executing
a complex action, only a subset of functions can be safely called from within
the signal handler. Such functions are marked as **async-signal-safe** functions.
\\
[[https://man7.org/linux/man-pages/man7/signal-safety.7.html|Linux manual]] and
[[https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_04_03|POSIX standard]]
provide a list of async-signal-safe functions.

Even worse, "anytime" means even in the middle of an assignment – did you know
that an innocently looking ''i = 0;'' is split into two machine instructions if
''i'' is a ''long long int'' and you run this code on a 32-bit machine?
\\
Thus, naturally, there are restrictions on accessing variables declared outside
the signal handler from the handler code.
\\
It is only guaranteed that variables of the ''volatile sig_atomic_t'' type
(and [[https://en.cppreference.com/w/c/atomic/ATOMIC_LOCK_FREE_consts|lock-free atomic variables]])
can be safely accessed.
\\
One may also use ''errno'' provided one saves it beforehand and restores its 
previous value after use.

**These restrictions are not enforced anyhow.** If you don't follow these rules,
you end up in the broad field of undefined behaviour.

<small>

==== [Extra] ====

To control actions done by the operating system on signal delivery more accurately,
one can use the ''sigaction'' function.
\\
The ''sigaction'' can also be used to set up a signal handler that gets more
information on the incoming signal.
\\
Delivery of signals can be blocked (and later unblocked) e.g., by calling
''sigprocmask''.
\\
Signal handlers can have their own list of blocked signals (to prevent nesting
signals).
\\
There is a number of functions that wait until a signal occurs, e.g., ''sigwait'',
''sigsuspend'', ''pause''.
\\
''kill'' delivers a signal to an arbitrary thread of a given process.
\\
''raise'' delivers a signal to the current thread. ''pthread_kill''
delivers a signal to a specified thread of the current process.
\\
While there is no POSIX function that delivers signal to a specified thread of
another process, there is a Linux-specific ''tgkill'' function that does that.
</small>

==== Exercises ====

~~Exercise.#~~ Write a program that prints ''Shutting down...'' and exits when
it receives SIGINT (Ctrl+c).
\\
<small>The program should either sleep (''sleep''), or wait for an input
(''read'' / ''getchar''), or explicitly wait for a signal (''pause'').</small>

~~Exercise.#~~ Write a program that on getting a USR1 signal:
\\     • opens a file called ''signal_log'',
\\     • appends current [[https://en.wikipedia.org/wiki/Unix_time|timestamp]] to it,
\\     • closes the file.
\\
Use to following code to write the current timestamp:
<small><code c>
#include <time.h>
#include <unistd.h>

void writeDateTo(int fd) {
  struct timespec now;
  char buf[22];
  clock_gettime(CLOCK_REALTIME, &now);
  buf[20] = '\n';
  for (int i = 0; i < 10; ++i, now.tv_nsec /= 10)
    buf[19 - i] = '0' + now.tv_nsec % 10;
  buf[10] = '.';
  for (int i = 0; i < 10; ++i, now.tv_sec /= 10)
    buf[9 - i] = '0' + now.tv_sec % 10;
  write(fd, buf, 21);
}
</code></small>

===== Shared memory =====

It is possible for two processes to use the same physical address range of the
main memory.
\\
Obviously any modification to such memory done by one process is visible to the
other processes that access the memory.
\\
Such memory is called [[https://en.wikipedia.org/wiki/Shared_memory#In_software|shared memory]].

Keep in mind that although two processes use the same physical addresses, the
virtual addresses are usually different.
\\
So do not store any pointers in the shared memory - they will not be valid for
other processes sharing the memory. Storing offsets (differences between
addresses) within a continuous shared memory address range is fine.
\\
There is an exception to this – when two processes share memory as a result of
a ''fork'', then the addresses naturally match.

To use shared memory, a program must explicitly request the kernel to set up a
(possibly shared) memory address range, and tell the kernel what memory should
be associated with the address range.
\\
This is done using the ''**[[https://pubs.opengroup.org/onlinepubs/9699919799/functions/mmap.html|mmap(…)]]**''
<html><span style="float:right"><small>Needs header: <code>sys/mman.h</code></small></span></html>
function with ''**MAP_SHARED**'' flag.
\\
On success, ''mmap'' returns an address to the start of the newly created
address range.

To tell what memory should back the address rage, one must either pass a file
descriptor to ''mmap'' or ask it to allocate some memory.
\\
The latter can be shared only by this process and its newly created child
processes, and is done by adding the ''**MAP_ANONYMOUS**'' flag, passing the
file descriptor of ''-1'' and passing the offset of ''0''.
\\
<small>
''MAP_ANONYMOUS'' is not part of the POSIX standard. However, virtually any 
UNIX-like system supports it.
</small>

When the file descriptor passed to ''mmap'' refers to an ordinary file, the
file is automatically copied by the operating system from disk to memory
<small>(a memory page is fetched upon first access within the page)</small>.
Writing the changes back to the file must be done manually by calling the
''**[[https://pubs.opengroup.org/onlinepubs/9699919799/functions/msync.html|msync]]**''
function.

The file descriptor passed to ''mmap'' can also refer to a shared memory object.
Such descriptors are returned by the ''**[[https://pubs.opengroup.org/onlinepubs/9699919799/functions/shm_open.html|shm_open]]**''
function, which has identical arguments as the ''open'' function, but the
returned descriptor refers to a region of main memory associated with a given
name (rather then with a disk file associated with a path). \\
<small>(In portable code, the name shall be one word starting with a slash.)</small>

In ''mmap'' one has to specify the size of the memory range. If this size is
larger than the backing file, then ''mmap'' succeeds, but any accesses to
the mapped addresses beyond the file result in a ''SIGBUS'' signal.
\\
To ensure that the file is large enough, one can use the following functions:
  * ''[[https://pubs.opengroup.org/onlinepubs/9699919799/functions/ftruncate.html|ftruncate]]'' <html><span style="float:right"><small>Needs header: <code>unistd.h</code></small></span></html> resizes file to a given size. \\ Whenever the file is larger it is truncated.
  * ''[[https://pubs.opengroup.org/onlinepubs/9699919799/functions/posix_fallocate.html|posix_fallocate]]'' <html><span style="float:right"><small>Needs header: <code>fcntl.h</code></small></span></html> ensures that the file at least of a given size. \\ Whenever the file is larger it is left unchanged. \\ <small>Warning: ''posix_fallocate'' is guaranteed to work for ordinary files. The result of using ''posix_fallocate'' on a shared memory object is undefined.</small>

To clean up the memory mapping, one has to use the
''[[https://pubs.opengroup.org/onlinepubs/9699919799/functions/munmap.html|munmap]]''
function. (Provided one wishes to flush data to a backing ordinary file, one
must call ''msync'' before ''munmap''.)

The ''shm_open'', ''mmap'', ''msync'' and ''munmap'' functions need **''#include <sys/mman.h>''**
(__m__emory __man__agement).
\\
The ''shm_open'' function needs also ''#include <fcntl.h>'' for the file open mode flags (such as ''O_RDWR'').

<html><p style="text-align: center"><object id="svg-object" data="/jkonczak/_media/os_cp:mmap.svg" type="image/svg+xml"></object></p></html>

**To compile programs that use ''shm_open'' with older glibc versions, one must add ''-lrt'' to compile options.**

~~Exercise.#~~
Test the following program that uses shared memory. Run it concurrently in multiple terminals.
\\
Answer the following questions:
\\       • what is the size of the shared memory object?
\\       • how does a ''struct'' help in laying out memory of the shared object?
\\ <small>Try to modify the following:
</small>\\       • <small>print the address of the shared memory
</small>\\       • <small>add a new field called ''counter'' in the ''struct myData'' and implement ''i num'' and ''d num'' commands to increment/decrement the counter
</small>\\       • <small>change the ''prot'' argument of mmap to PROT_READ (without PROT_WRITE) and check how the program works
</small>\\       • <small>change the ''size'' argument of mmap to ''1024*1024'' and try to access an address:
</small>\\               · <small>within mapping, but outside shared memory object (e.g., ''%%putchar(*((char*)data+1025*1024));%%'')
</small>\\               · <small>outside mapping (e.g., ''%%putchar(*((char*)data-1));%%'').
</small>\\         <small>what is the difference in handling those two accesses by the OS?
</small>

<code c simple_cli.c>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <unistd.h>

#define CHECK(result, textOnFail)                                              \
  if (((long int)result) == -1) {                                              \
    perror(textOnFail);                                                        \
    exit(1);                                                                   \
  }

struct myData {
  int version;
  char text[1020];
};

int main() {
  int fd = shm_open("/os_cp", O_RDWR | O_CREAT, 0600);
  CHECK(fd, "shm_open failed");
  int r = ftruncate(fd, sizeof(struct myData));
  CHECK(r, "ftruncate failed");
  struct myData *data = mmap(NULL, sizeof(struct myData),
                             PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
  CHECK(data, "mmap failed");
  close(fd);

  printf("commands:\n"
         "  r           - reads the text\n"
         "  w <text>    - writes new text\n"
         "  q           - quits\n");

  while (1) {
    printf("> ");
    fflush(stdout);

    char c, text[1022] = {0};
    scanf("%1021[^\n]", text);
    do { // this reads all remaining characters in this line including '\n'
      c = getchar();
      CHECK(c, "getchar EOF'ed");
    } while (c != '\n');

    if (!strlen(text)) // empty line
      continue;

    switch (text[0]) {
    case 'r':
      printf("version: %d\n   text: %s\n", data->version, data->text);
      break;
    case 'w':
      data->version++;
      strcpy(data->text, text + 2);
      break;
    case 'q':
      munmap(data, sizeof(struct myData));
      exit(0);
      break;
    }
  }
}
</code>

~~Exercise.#~~
The following program uses the same shared memory object named ''/os_cp''.
Run it in one terminal, and run in parallel the program from the previous exercise.
Try to read the text.

<code c writer.c>
#include <fcntl.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <unistd.h>

#define CHECK(result, textOnFail)                                              \
  if (((long int)result) == -1) {                                              \
    perror(textOnFail);                                                        \
    exit(1);                                                                   \
  }

struct myData {
  int version;
  char text[1020];
};

volatile sig_atomic_t stopFlag = 0;
void ctrlC(int num) { stopFlag = 1; }

int main() {
  int fd = shm_open("/os_cp", O_RDWR | O_CREAT, 0600);
  CHECK(fd, "shm_open failed");
  int r = ftruncate(fd, sizeof(struct myData));
  CHECK(r, "ftruncate failed");
  struct myData *data = mmap(NULL, sizeof(struct myData),
                             PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
  CHECK(data, "mmap failed");
  close(fd);

  signal(SIGINT, ctrlC);
  while (!stopFlag) {
    for (char letter = 'a'; letter <= 'z'; ++letter) {
      data->version++;
      for (int i = 0; i < 1020 - 1; ++i) {
        data->text[i] = letter;
      }
      if (stopFlag)
        break;
    }
  }
  munmap(data, sizeof(struct myData));
  return 0;
}
</code>

~~META:
language = en
~~
Jan Kończak

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