Rendezvous: Difference between revisions

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m (→‎{{header|Raku}}: fix up Perl 6 -> Raku in comments, minor update)
(→‎{{header|C}}: Add C implementation using pthreads)
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=={{header|C}}==
=={{header|C}}==
== Pthreads implementation ==
{{works with|gcc|10.1.0}}
{{libheader|pthread}}
This uses POSIX threads to implement a subset of the Ada functionality and primarily focuses on the synchronization aspect. C does not have exceptions, so return values are used to signal errors. Multiple threads can enter a rendezvous at once, and a single thread can accept them. No attempt is made to implement selective accept statements or timeouts (though pthreads does have ''pthread_cond_timedwait()'').
<lang C>
#include <stdlib.h>
#include <stdio.h>
#include <pthread.h>

/* The language task, implemented with pthreads for POSIX systems. */

/* Each rendezvous_t will be accepted by a single thread, and entered
* by one or more threads. accept_func() only returns an integer and
* is always run within the entering thread's context to simplify
* handling the arguments and return value. This somewhat unlike an
* Ada rendezvous and is a subset of the Ada rendezvous functionality.
* Ada's in and out parameters can be simulated via the void pointer
* passed to accept_func() to update variables owned by both the
* entering and accepting threads, if a suitable struct with pointers
* to those variables is used. */
typedef struct rendezvous {
pthread_mutex_t lock; /* A mutex/lock to use with the CVs. */
pthread_cond_t cv_entering; /* Signaled when a thread enters. */
pthread_cond_t cv_accepting; /* Signaled when accepting thread is ready. */
pthread_cond_t cv_done; /* Signaled when accept_func() finishes. */
int (*accept_func)(void*); /* The function to run when accepted. */
int entering; /* Number of threads trying to enter. */
int accepting; /* True if the accepting thread is ready. */
int done; /* True if accept_func() is done. */
} rendezvous_t;

/* Static initialization for rendezvous_t. */
#define RENDEZVOUS_INITILIZER(accept_function) { \
.lock = PTHREAD_MUTEX_INITIALIZER, \
.cv_entering = PTHREAD_COND_INITIALIZER, \
.cv_accepting = PTHREAD_COND_INITIALIZER, \
.cv_done = PTHREAD_COND_INITIALIZER, \
.accept_func = accept_function, \
.entering = 0, \
.accepting = 0, \
.done = 0, \
}

int enter_rendezvous(rendezvous_t *rv, void* data)
{
/* Arguments are passed in and out of the rendezvous via
* (void*)data, and the accept_func() return value is copied and
* returned to the caller (entering thread). A data struct with
* pointers to variables in both the entering and accepting
* threads can be used to simulate Ada's in and out parameters, if
* needed. */
pthread_mutex_lock(&rv->lock);

rv->entering++;
pthread_cond_signal(&rv->cv_entering);

while (!rv->accepting) {
/* Nothing is accepting yet, keep waiting. pthreads will
* queue all waiting entries. The loop is needed to handle
* both race conditions and spurious wakeups. */
pthread_cond_wait(&rv->cv_accepting, &rv->lock);
}

/* Call accept_func() and copy the return value before leaving
* the mutex. */
int ret = rv->accept_func(data);

/* This signal is needed so that the accepting thread will wait
* for the rendezvous to finish before trying to accept again. */
rv->done = 1;
pthread_cond_signal(&rv->cv_done);

rv->entering--;
rv->accepting = 0;
pthread_mutex_unlock(&rv->lock);

return ret;
}

void accept_rendezvous(rendezvous_t *rv)
{
/* This accept function does not take in or return parameters.
* That is handled on the entry side. This is only for
* synchronization. */
pthread_mutex_lock(&rv->lock);
rv->accepting = 1;

while (!rv->entering) {
/* Nothing to accept yet, keep waiting. */
pthread_cond_wait(&rv->cv_entering, &rv->lock);
}

pthread_cond_signal(&rv->cv_accepting);

while (!rv->done) {
/* Wait for accept_func() to finish. */
pthread_cond_wait(&rv->cv_done, &rv->lock);
}
rv->done = 0;

rv->accepting = 0;
pthread_mutex_unlock(&rv->lock);
}

/* The printer use case task implemented using the above rendezvous
* implementation. Since C doesn't have exceptions, return values are
* used to signal out of ink errors. */

typedef struct printer {
rendezvous_t rv;
struct printer *backup;
int id;
int remaining_lines;
} printer_t;

typedef struct print_args {
struct printer *printer;
const char* line;
} print_args_t;

int print_line(printer_t *printer, const char* line) {
print_args_t args;
args.printer = printer;
args.line = line;
return enter_rendezvous(&printer->rv, &args);
}

int accept_print(void* data) {
/* This is called within the rendezvous, so everything is locked
* and okay to modify. */
print_args_t *args = (print_args_t*)data;
printer_t *printer = args->printer;
const char* line = args->line;

if (printer->remaining_lines) {
/* Print the line, character by character. */
printf("%d: ", printer->id);
while (*line != '\0') {
putchar(*line++);
}
putchar('\n');
printer->remaining_lines--;
return 1;
}
else if (printer->backup) {
/* "Requeue" this rendezvous with the backup printer. */
return print_line(printer->backup, line);
}
else {
/* Out of ink, and no backup available. */
return -1;
}
}

printer_t backup_printer = {
.rv = RENDEZVOUS_INITILIZER(accept_print),
.backup = NULL,
.id = 2,
.remaining_lines = 5,
};

printer_t main_printer = {
.rv = RENDEZVOUS_INITILIZER(accept_print),
.backup = &backup_printer,
.id = 1,
.remaining_lines = 5,
};

void* printer_thread(void* thread_data) {
printer_t *printer = (printer_t*) thread_data;
while (1) {
accept_rendezvous(&printer->rv);
}
}

typedef struct poem {
char* name;
char* lines[];
} poem_t;

poem_t humpty_dumpty = {
.name = "Humpty Dumpty",
.lines = {
"Humpty Dumpty sat on a wall.",
"Humpty Dumpty had a great fall.",
"All the king's horses and all the king's men",
"Couldn't put Humpty together again.",
""
},
};

poem_t mother_goose = {
.name = "Mother Goose",
.lines = {
"Old Mother Goose",
"When she wanted to wander,",
"Would ride through the air",
"On a very fine gander.",
"Jack's mother came in,",
"And caught the goose soon,",
"And mounting its back,",
"Flew up to the moon.",
""
},
};

void* poem_thread(void* thread_data) {
poem_t *poem = (poem_t*)thread_data;

for (unsigned i = 0; poem->lines[i] != ""; i++) {
int ret = print_line(&main_printer, poem->lines[i]);
if (ret < 0) {
printf(" %s out of ink!\n", poem->name);
exit(1);
}
}
return NULL;
}

int main(void)
{
pthread_t threads[4];

pthread_create(&threads[0], NULL, poem_thread, &humpty_dumpty);
pthread_create(&threads[1], NULL, poem_thread, &mother_goose);
pthread_create(&threads[2], NULL, printer_thread, &main_printer);
pthread_create(&threads[3], NULL, printer_thread, &backup_printer);

pthread_join(threads[0], NULL);
pthread_join(threads[1], NULL);
pthread_cancel(threads[2]);
pthread_cancel(threads[3]);

return 0;
}
</lang>

== OpenMP implementation ==
Basically just synched threads doing printing: since task didn't ask for service type or resource enumeration, and "message passing is stupid" (c.f. talk), the guarding thread is no more than a glorified mutex, hence completely cut out, leaving the threads directly check ink and do print.
Basically just synched threads doing printing: since task didn't ask for service type or resource enumeration, and "message passing is stupid" (c.f. talk), the guarding thread is no more than a glorified mutex, hence completely cut out, leaving the threads directly check ink and do print.
<lang C>#include <stdio.h>
<lang C>#include <stdio.h>
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