diff --git i/atomicio.c w/atomicio.c
index 3930a07..81a14a4 100644
--- i/atomicio.c
+++ w/atomicio.c
@@ -37,11 +37,7 @@
  * ensure all of data on socket comes through. f==read || f==write
  */
 ssize_t
-atomicio(f, fd, _s, n)
-	ssize_t (*f) ();
-	int fd;
-	void *_s;
-	size_t n;
+atomicio(ssize_t (*f)(int, const void *, size_t), int fd, const void *_s, size_t n)
 {
 	char *s = _s;
 	ssize_t res, pos = 0;
diff --git i/util.h w/util.h
index b00059c..f24d0c3 100644
--- i/util.h
+++ w/util.h
@@ -41,7 +41,7 @@
 #define MAX(a, b) ((a) > (b) ? (a) : (b))
 #define MIN(a, b) ((a) < (b) ? (a) : (b))
 
-ssize_t    atomicio(ssize_t (*)(), int, void *, size_t);
+ssize_t    atomicio(ssize_t (*f)(int, const void *, size_t), int fd, const void *s, size_t n);
 char      *get_progname(char *);