-extern int os_getloadavg(void);
-extern void os_restarting_signal(int, void (*)(int));
-extern void os_non_restarting_signal(int, void (*)(int));
-extern char *os_strexit(int); /* char to match os_strsignal */
-extern char *os_strsignal(int); /* char to match strsignal in some OS */
+/* If these exist as a macro, then they're overridden away from us and we
+rely upon the system headers to provide prototype declarations for us.
+Notably, strsignal() is not in the Single Unix Specification (v3) and
+predicting constness is awkward. */
+
+#ifndef os_getloadavg
+ extern int os_getloadavg(void);
+#endif
+#ifndef os_restarting_signal
+ extern void os_restarting_signal(int, void (*)(int));
+#endif
+#ifndef os_non_restarting_signal
+ extern void os_non_restarting_signal(int, void (*)(int));
+#endif
+#ifndef os_strexit
+ extern const char *os_strexit(int); /* char to match os_strsignal */
+#endif
+#ifndef os_strsignal
+ extern const char *os_strsignal(int); /* char to match strsignal in some OS */
+#endif
+#ifndef os_unsetenv
+ extern int os_unsetenv(const uschar *);
+#endif
+#ifndef os_getcwd
+ extern uschar *os_getcwd(uschar *, size_t);
+#endif
+
+#ifdef OS_PIPE_RW_EINTR
+ extern ssize_t os_pipe_read(int fd, void * buf, size_t count);
+ extern ssize_t os_pipe_write(int fd, void * buf, size_t count);
+ extern ssize_t os_pipe_writev(int fd, const struct iovec * iov, int iovcnt);
+#else
+# define os_pipe_read(fd, buf, count) read(fd, buf, count)
+# define os_pipe_write(fd, buf, count) write(fd, buf, count)
+# define os_pipe_writev(fd, buf, count) writev(fd, buf, count)
+#endif