The redirection of mmap through __syscall(2) is broken in multiple ways.
Just call mmap and other syscalls directly.

Index: lib/libv4lconvert/libv4lsyscall-priv.h
--- lib/libv4lconvert/libv4lsyscall-priv.h.orig
+++ lib/libv4lconvert/libv4lsyscall-priv.h
@@ -107,10 +107,7 @@
 	syscall(SYS_mmap, (void *)(addr), (size_t)(len), \
 			(int)(prot), (int)(flags), (int)(fd), (off_t)(off))
 #elif defined(__OpenBSD__)
-register_t __syscall(quad_t, ...);
-#define SYS_MMAP(addr, len, prot, flags, fd, offset) \
-	__syscall((quad_t)SYS_mmap, (void *)(addr), (size_t)(len), \
-			(int)(prot), (int)(flags), (int)(fd), 0, (off_t)(offset))
+#define SYS_MMAP mmap
 #else
 #define SYS_MMAP(addr, len, prot, flags, fd, off) \
 	syscall(SYS_mmap2, (void *)(addr), (size_t)(len), \
@@ -122,6 +119,18 @@ register_t __syscall(quad_t, ...);
 
 #else
 
+#if defined(__OpenBSD__)
+
+#define SYS_OPEN	open
+#define SYS_CLOSE	close
+#define SYS_IOCTL	ioctl
+#define SYS_READ	read
+#define SYS_WRITE	write
+#define SYS_MMAP	mmap
+#define SYS_MUNMAP	munmap
+
+#else
+
 int v4lx_open_wrapper(const char *, int, int);
 int v4lx_close_wrapper(int);
 int v4lx_ioctl_wrapper(int, unsigned long, void *);
@@ -137,6 +146,8 @@ int v4lx_munmap_wrapper(void *, size_t);
 #define SYS_WRITE(...) v4lx_write_wrapper(__VA_ARGS__)
 #define SYS_MMAP(...) v4lx_mmap_wrapper(__VA_ARGS__)
 #define SYS_MUNMAP(...) v4lx_munmap_wrapper(__VA_ARGS__)
+
+#endif
 
 #endif