utils: add helper functions useful for allocating a ring buffer

This creates a mapping with twice the size of the allocated memory. The
second half of that mapping points at the same memory as the first half.

This is useful for ring buffers, because any read starting in the first
half can overflow into the second half as long as the read size is
smaller than the size of the memory area.

Signed-off-by: Felix Fietkau <nbd@nbd.name>

1 files changed, 47 insertions, 1 deletions

diff --git a/utils.c b/utils.c
index 91dd71e..e59002e 100644
--- a/utils.c
+++ b/utils.c
@@ -16,10 +16,11 @@
  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 
-#include "utils.h"
+#include <sys/mman.h>
 #include <stdarg.h>
 #include <stdlib.h>
 #include <stdio.h>
+#include "utils.h"
 
 #define foreach_arg(_arg, _addr, _len, _first_addr, _first_len) \
 	for (_addr = (_first_addr), _len = (_first_len); \
@@ -103,3 +104,48 @@ out:
 }
 
 #endif
+
+void *cbuf_alloc(unsigned int order)
+{
+	char path[] = "/tmp/cbuf-XXXXXX";
+	unsigned long size = cbuf_size(order);
+	void *ret = NULL;
+	int fd;
+
+	fd = mkstemp(path);
+	if (fd < 0)
+		return NULL;
+
+	if (unlink(path))
+		goto close;
+
+	if (ftruncate(fd, cbuf_size(order)))
+		goto close;
+
+#ifndef MAP_ANONYMOUS
+#define MAP_ANONYMOUS MAP_ANON
+#endif
+
+	ret = mmap(NULL, size * 2, PROT_NONE, MAP_ANON | MAP_PRIVATE, -1, 0);
+	if (ret == MAP_FAILED) {
+		ret = NULL;
+		goto close;
+	}
+
+	if (mmap(ret, size, PROT_READ | PROT_WRITE, MAP_FIXED | MAP_SHARED,
+		 fd, 0) != ret ||
+	    mmap(ret + size, size, PROT_READ | PROT_WRITE,
+		 MAP_FIXED | MAP_SHARED, fd, 0) != ret + size) {
+		munmap(ret, size * 2);
+		ret = NULL;
+	}
+
+close:
+	close(fd);
+	return ret;
+}
+
+void cbuf_free(void *ptr, unsigned int order)
+{
+	munmap(ptr, cbuf_size(order) * 2);
+}