another try with the shmen trick...this time use MAP_ANONYMOUS ... as GNU extension.

include/cbuf.h

@@ -23,7 +23,6 @@ CLASS(Cbuf) {
 	char * shm_name;  // shared memory identifier
 
 	char * data;
-	char * mirror;
 
 	size_t bsize;
 	size_t bused;

src/cbuf.c

@@ -1,5 +1,6 @@
 #define _POSIX_SOURCE
 #define _POSIX_C_SOURCE 200112L
+#define _GNU_SOURCE
 
 #include <sys/types.h>
 #include <sys/stat.h>
@@ -28,6 +29,7 @@ ctor(void * _this, va_list * params)
 	long   psize    = sysconf(_SC_PAGESIZE);
 	size_t size;
 	int    shm;
+	char * data;
 
 	this->shm_name = malloc(strlen(shm_name) + 7 + 2);
 	sprintf(this->shm_name, "/%06d_%s", getpid(), shm_name);
@@ -51,25 +53,22 @@ ctor(void * _this, va_list * params)
 		}
 
 		this->data = mmap (0, this->bsize << 1,
-				PROT_READ|PROT_WRITE, MAP_SHARED, shm, 0);
+				PROT_NONE, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
 		if (this->data == MAP_FAILED) {
 			this->data = NULL;
 			break;
 		}   
 
-		munmap(this->data + this->bsize, this->bsize);
-
-		this->mirror = mmap (this->data + this->bsize, this->bsize,
-				PROT_READ|PROT_WRITE, MAP_SHARED, shm, 0);
-		if (this->mirror != this->data + this->bsize) {
-			if (this->mirror == this->data - this->bsize) {
-				this->data    = this->mirror;
-				this->mirror += this->bsize;
-			}
-			else {
-				this->mirror = NULL;
-				break;
-			}
+		data = mmap (this->data, this->bsize,
+				PROT_READ|PROT_WRITE, MAP_FIXED|MAP_SHARED, shm, 0);
+		if (data != this->data) {
+			break;
+		}   
+
+		data = mmap (this->data + this->bsize, this->bsize,
+				PROT_READ|PROT_WRITE, MAP_FIXED|MAP_SHARED, shm, 0);
+		if (data != this->data + this->bsize) {
+			break;
 		}   
 
 		state = 1;
@@ -93,18 +92,14 @@ dtor(void * _this)
 
 	if (NULL != this->shm_name) {
 		free(this->shm_name);
-		this->shm_name = NULL;
 	}
 
-	if (NULL != this->data) {
-		munmap(this->data, this->bsize);
-		this->data = NULL;
+	if (NULL != this->data && MAP_FAILED != this->data) {
+		munmap(this->data, this->bsize << 1);
 	}
 
-	if (NULL != this->mirror) {
-		munmap(this->mirror, this->bsize);
-		this->mirror = NULL;
-	}
+	this->shm_name = NULL;
+	this->data     = NULL;
 }
 
 INIT_IFACE(Class, ctor, dtor, NULL);