From 818ce700109c959bb670b48e2b6a7e2adaafeb1c Mon Sep 17 00:00:00 2001
From: Ludovic Pouzenc <ludovic@pouzenc.fr>
Date: Tue, 3 Sep 2019 22:41:26 +0200
Subject: GL and SDL does not mixup. Add mmap-sem.c example to fork this in
 separate processes.

---
 examples/mmap-sem.c | 140 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 140 insertions(+)
 create mode 100644 examples/mmap-sem.c

diff --git a/examples/mmap-sem.c b/examples/mmap-sem.c
new file mode 100644
index 0000000..d5a887d
--- /dev/null
+++ b/examples/mmap-sem.c
@@ -0,0 +1,140 @@
+#include <sys/mman.h>  // mmap()
+#include <unistd.h>    // fork()
+#include <sys/wait.h>  // wait()
+#include <semaphore.h> // sem_init(), Link with -pthread
+#include <errno.h>     // errno
+#include <stdint.h>    // uint32_t
+#include <stdio.h>     // printf()
+
+int do_fork1();
+int do_fork2();
+int parent();
+int worker_gl();
+int worker_sdl();
+
+typedef struct {
+	sem_t worker_gl_can_render, worker_sdl_can_render, parent_can_read_result;
+	int scene;
+	uint32_t framebuf[256*256];
+} shm_t;
+
+shm_t *shm;
+
+int main() {
+	shm = mmap(NULL, sizeof(shm_t), PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, 0, 0);
+	if (sem_init(&shm->worker_gl_can_render, 1, 0) < 0) return 1;
+	if (sem_init(&shm->worker_sdl_can_render, 1, 0) < 0) return 2;
+	if (sem_init(&shm->parent_can_read_result, 1, 0) < 0) return 3;
+	shm->scene = 0;
+	return do_fork1();
+}
+
+int do_fork1() {
+	switch (fork()) {
+		case -1: return 4; break;
+		case 0:  return worker_gl(); break;
+		default: return do_fork2(); break;
+	}
+}
+
+int do_fork2() {
+	switch (fork()) {
+		case -1: return 5; break;
+		case 0:  return worker_sdl(); break;
+		default: return parent(); break;
+	}
+}
+
+int parent() {
+	int i, child_status;
+
+	shm->scene++;
+	printf("parent1 %i\n", shm->scene);
+
+
+	for(i=0;i<4;i++) {
+		if ( sem_post(&shm->worker_sdl_can_render) == -1 ) {
+			return 6;
+		}
+
+		while ( sem_wait(&shm->parent_can_read_result) == -1 ) {
+			switch(errno) {
+				case EINTR:
+				case EAGAIN:
+					break;
+				default:
+					return 7;
+			}
+		}
+		printf("parent2 %i\n", shm->scene);
+
+		if ( sem_post(&shm->worker_gl_can_render) == -1 ) {
+			return 6;
+		}
+
+		while ( sem_wait(&shm->parent_can_read_result) == -1 ) {
+			switch(errno) {
+				case EINTR:
+				case EAGAIN:
+					break;
+				default:
+					return 8;
+			}
+		}
+		printf("parent3 %i\n", shm->scene);
+	}
+
+	if ( wait(&child_status) != -1 ) {
+		if ( child_status > 0 ) return child_status;
+	}
+	if ( wait(&child_status) != -1 ) {
+		if ( child_status > 0 ) return child_status;
+	}
+	return 0;
+}
+
+int worker_gl() {
+	int done=0;
+	while ( !done ) {
+		while ( sem_wait(&shm->worker_gl_can_render) == -1 ) {
+			switch(errno) {
+				case EINTR:
+				case EAGAIN:
+					break;
+				default:
+					return 10;
+			}
+		}
+		printf("worker_gl %i\n", shm->scene);
+		
+		shm->scene++;
+
+		if ( sem_post(&shm->parent_can_read_result) == -1 ) {
+			return 11;
+		}
+	}
+	return 0;
+}
+
+int worker_sdl() {
+	int done=0;
+	while ( !done ) {
+		while ( sem_wait(&shm->worker_sdl_can_render) == -1 ) {
+			switch(errno) {
+				case EINTR:
+				case EAGAIN:
+					break;
+				default:
+					return 20;
+			}
+		}
+
+		printf("worker_sdl %i\n", shm->scene);
+		shm->scene++;
+
+		if ( sem_post(&shm->parent_can_read_result) == -1 ) {
+			return 21;
+		}
+	}
+	return 0;
+}
-- 
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