err: Add err(), warn(), errx() and warnx() familiy of functions

Some systems such as Windows or musl-libc based ones do not have these
BSD extensions. In addition libbsd itself is making use of the warnx()
functions, so we better provide these interfaces in case they are
missing.

include/bsd/err.h

@@ -29,11 +29,15 @@
 #include <sys/cdefs.h>
 #if __has_include_next(<err.h>)
 #include_next <err.h>
+#else
+#define LIBBSD_NEED_ERR_H_FUNCS
 #endif
 #else
 #include <bsd/sys/cdefs.h>
 #if __has_include(<err.h>)
 #include <err.h>
+#else
+#define LIBBSD_NEED_ERR_H_FUNCS
 #endif
 #endif
 
@@ -52,6 +56,26 @@ void verrc(int status, int code, const char *format, va_list ap)
 	__printflike(3, 0) __dead2;
 void errc(int status, int code, const char *format, ...)
 	__printflike(3, 4) __dead2;
+
+#ifdef LIBBSD_NEED_ERR_H_FUNCS
+void vwarn(const char *format, va_list ap)
+	__printflike(1, 0);
+void vwarnx(const char *format, va_list ap)
+	__printflike(1, 0);
+void warn(const char *format, ...)
+	__printflike(1, 2);
+void warnx(const char *format, ...)
+	__printflike(1, 2);
+
+void verr(int status, const char *format, va_list ap)
+	__printflike(2, 0) __dead2;
+void verrx(int status, const char *format, va_list ap)
+	__printflike(2, 0) __dead2;
+void err(int status, const char *format, ...)
+	__printflike(2, 3) __dead2;
+void errx(int status, const char *format, ...)
+	__printflike(2, 3) __dead2;
+#endif
 __END_DECLS
 
 #endif

src/err.c

@@ -26,6 +26,9 @@
  */
 
 #include <err.h>
+#ifdef LIBBSD_NEED_ERR_H_FUNCS
+#include <errno.h>
+#endif
 #include <string.h>
 #include <stdarg.h>
 #include <stdlib.h>
@@ -73,3 +76,76 @@ errc(int status, int code, const char *format, ...)
 	verrc(status, code, format, ap);
 	va_end(ap);
 }
+
+#ifdef LIBBSD_NEED_ERR_H_FUNCS
+void
+vwarn(const char *format, va_list ap)
+{
+	vwarnc(errno, format, ap);
+}
+
+void
+warn(const char *format, ...)
+{
+	va_list ap;
+
+	va_start(ap, format);
+	vwarnc(errno, format, ap);
+	va_end(ap);
+}
+
+void
+vwarnx(const char *format, va_list ap)
+{
+	fprintf(stderr, "%s: ", getprogname());
+	if (format)
+		vfprintf(stderr, format, ap);
+	fprintf(stderr, "\n");
+}
+
+void
+warnx(const char *format, ...)
+{
+	va_list ap;
+
+	va_start(ap, format);
+	vwarnx(format, ap);
+	va_end(ap);
+}
+
+void
+verr(int status, const char *format, va_list ap)
+{
+	verrc(status, errno, format, ap);
+}
+
+void
+err(int status, const char *format, ...)
+{
+	va_list ap;
+
+	va_start(ap, format);
+	verrc(status, errno, format, ap);
+	va_end(ap);
+}
+
+void
+verrx(int eval, const char *format, va_list ap)
+{
+	fprintf(stderr, "%s: ", getprogname());
+	if (format)
+		vfprintf(stderr, format, ap);
+	fprintf(stderr, "\n");
+	exit(eval);
+}
+
+void
+errx(int eval, const char *format, ...)
+{
+	va_list ap;
+
+	va_start(ap, format);
+	verrx(eval, format, ap);
+	va_end(ap);
+}
+#endif

src/libbsd.map

@@ -168,3 +168,16 @@ LIBBSD_0.9.1 {
     strnvis_netbsd;
     strnunvis_netbsd;
 } LIBBSD_0.9;
+
+LIBBSD_0.10.0 {
+    /* These BSD extensions are available on GNU systems, but not on other
+     * systems such as Windows or musl libc based ones. */
+    vwarn;
+    vwarnx;
+    warn;
+    warnx;
+    verr;
+    verrx;
+    err;
+    errx;
+} LIBBSD_0.9.1;