diff --git a/kernel/src/sys/mem.cpp b/kernel/src/sys/mem.cpp
index 0950b6ad..1d046d00 100644
--- a/kernel/src/sys/mem.cpp
+++ b/kernel/src/sys/mem.cpp
@@ -7,6 +7,8 @@
 #include "memory/VMM.h"
 #include <stddef.h>
 
+// FIXME: Round size up instead of down. (use get_blocks_from_size)
+
 #define MAP_FAIL(errno) 0xffffffffffffff00 | (unsigned char)(errno)
 
 void sys_mmap(Context* context, void* address, size_t size, int flags)
diff --git a/libs/libc/include/sys/mman.h b/libs/libc/include/sys/mman.h
index ed7d4173..91e01264 100644
--- a/libs/libc/include/sys/mman.h
+++ b/libs/libc/include/sys/mman.h
@@ -5,6 +5,7 @@
 
 typedef unsigned long off_t;
 
+/* Address returned by mmap when it fails. */
 #define MAP_FAILED (void*)-1
 
 #define PROT_READ_WRITE 1
@@ -16,8 +17,12 @@ extern "C"
 {
 #endif
 
-    void* mmap(void*, size_t, int, int, int, off_t);
-    int munmap(void* addr, size_t len);
+    /* Maps size bytes of memory (rounded down to the nearest page-aligned size) into the current process's address
+     * space at addr. If addr is null, the kernel will choose an address. */
+    void* mmap(void* addr, size_t size, int prot, int flags, int fd, off_t offset);
+    /* Unmaps size bytes of memory (rounded down to the nearest page-aligned size) at addr from the current process's
+     * address space. */
+    int munmap(void* addr, size_t size);
 
 #ifdef __cplusplus
 }