diff --git a/user/user.h b/user/user.h
index 8b3a1b6..1e56f3d 100644
--- a/user/user.h
+++ b/user/user.h
@@ -4,43 +4,121 @@
 
 struct stat;
 
-// system calls
-int   fork(void);
-int   exit(int) __attribute__((noreturn));
-int   wait(int *);
-int   pipe(int *);
-int   write(int, const void *, int);
-int   read(int, void *, int);
-int   close(int);
-int   kill(int);
-int   exec(const char *, char **);
-int   open(const char *, int);
-int   mknod(const char *, short, short);
-int   unlink(const char *);
-int   fstat(int fd, struct stat *);
-int   link(const char *, const char *);
-int   mkdir(const char *);
-int   chdir(const char *);
-int   dup(int);
-int   getpid(void);
-char *sbrk(int);
-int   sleep(int);
-int   uptime(void);
-int   trace(int);
+/**
+ * System calls
+ */
 
-// ulib.c
-int   stat(const char *, struct stat *);
+/** Create a child process */
+int fork(void);
+
+/** Exit (terminate) the calling process */
+int exit(int) __attribute__((noreturn));
+
+/** Blocks the calling process until child process terminates. */
+int wait(int *);
+
+/** pipe */
+int pipe(int *);
+
+/** write */
+int write(int, const void *, int);
+
+/** read */
+int read(int, void *, int);
+
+/** close */
+int close(int);
+
+/** kill */
+int kill(int);
+
+/** exec */
+int exec(const char *, char **);
+
+/** open */
+int open(const char *, int);
+
+/** mknod */
+int mknod(const char *, short, short);
+
+/** unlink */
+int unlink(const char *);
+
+/** fstat */
+int fstat(int fd, struct stat *);
+
+/** link */
+int link(const char *, const char *);
+
+/** mkdir */
+int mkdir(const char *);
+
+/** chdir */
+int chdir(const char *);
+
+/** dup */
+int dup(int);
+
+/** getpid */
+int getpid(void);
+
+/** sbrk */
+char *sbrk(int);
+
+/** sleep */
+int sleep(int);
+
+/** uptime */
+int uptime(void);
+
+/** trace */
+int trace(int);
+
+/**
+ * Library functions (ulib.c)
+ */
+
+/** stat */
+int stat(const char *, struct stat *);
+
+/** strcpy */
 char *strcpy(char *, const char *);
+
+/** memmove */
 void *memmove(void *, const void *, int);
+
+/** strchr */
 char *strchr(const char *, char c);
-int   strcmp(const char *, const char *);
-void  fprintf(int, const char *, ...);
-void  printf(const char *, ...);
+
+/** strcmp */
+int strcmp(const char *, const char *);
+
+/** fprintf */
+void fprintf(int, const char *, ...);
+
+/** printf */
+void printf(const char *, ...);
+
+/** gets */
 char *gets(char *, int max);
-u32   strlen(const char *);
+
+/** strlen */
+u32 strlen(const char *);
+
+/** memset */
 void *memset(void *, int, u32);
+
+/** malloc */
 void *malloc(u32);
-void  free(void *);
-int   atoi(const char *);
-int   memcmp(const void *, const void *, u32);
+
+/** free */
+void free(void *);
+
+/** atoi */
+int atoi(const char *);
+
+/** memcmp */
+int memcmp(const void *, const void *, u32);
+
+/** memcpy */
 void *memcpy(void *, const void *, u32);