add errmsg API

buffer/errmsg_cvt.c

@@ -0,0 +1,29 @@
+#include <stdarg.h>
+#include <sys/uio.h>
+#include "errmsg.h"
+#include "str.h"
+
+/* the people who defined stdarg.h need to be taken behind the barn and shot */
+int errmsg_cvt(struct iovec* x,const char* message, va_list a) {
+  int i,j;
+  j=0;
+  if (argv0) {
+    x[0].iov_base=(char*)argv0;
+    x[0].iov_len=str_len(argv0);
+    x[1].iov_base=": ";
+    x[1].iov_len=2;
+    j=2;
+  }
+  x[j].iov_base=(char*)message; x[j].iov_len=str_len(message); ++j;
+
+  for (i=0; j<22; ++i) {
+    const char* t=va_arg(a,const char*);
+    if (!t) break;
+    x[j].iov_base=(char*)t;
+    x[j].iov_len=str_len(t);
+    ++j;
+  }
+  x[j].iov_base="\n";
+  x[j].iov_len=1;
+  return j+1;
+}