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52 lines
2.1 KiB
C
52 lines
2.1 KiB
C
#pragma once
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#include "arch.h"
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// Lock-free, single-producer / single-consumer (SPSC) message queue.
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// Safe to write from an ISR and read from the main loop without locking,
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// as long as only one context writes and one context reads at a time.
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// Each message is stored with an internal 32-bit length prefix; the buf
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// pointers returned by mg_queue_book() and mg_queue_next() already skip it.
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//
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// Producer pattern:
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// char buf[1024];
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// if (mg_queue_book(&q, &buf, len) >= len) {
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// memcpy(buf, data, len);
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// mg_queue_add(&q, len);
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// }
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//
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// Consumer pattern:
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// char *buf;
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// size_t len = mg_queue_next(&q, &buf);
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// if (len > 0) { process(buf, len); mg_queue_del(&q, len); }
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struct mg_queue {
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char *buf; // Caller-supplied backing buffer
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size_t size; // Size of buf in bytes
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volatile size_t tail; // Read offset; updated by the consumer
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volatile size_t head; // Write offset; updated by the producer
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};
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// Initialises q to use the caller-supplied buf/size. buf must remain valid
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// for the lifetime of the queue.
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void mg_queue_init(struct mg_queue *q, char *buf, size_t size);
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// Reserves space for a message of len bytes. Sets *buf to the write location.
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// Returns the number of bytes available (>= len means the reservation
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// succeeded). Does NOT commit the message; call mg_queue_add() after writing.
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size_t mg_queue_book(struct mg_queue *q, char **buf, size_t len);
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// Commits the most recently reserved message of len bytes. Must be called
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// with the same len passed to the preceding mg_queue_book() call, after
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// the data has been written into the buffer it returned.
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void mg_queue_add(struct mg_queue *q, size_t len);
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// Returns the length of the oldest unconsumed message and sets *buf to its
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// start. Returns 0 if the queue is empty. Does NOT remove the message;
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// call mg_queue_del() after processing to advance the read position.
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size_t mg_queue_next(struct mg_queue *q, char **buf);
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// Removes the oldest message of len bytes. Must be called with the same len
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// returned by the preceding mg_queue_next() call.
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void mg_queue_del(struct mg_queue *q, size_t len);
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