[Xcb] [PATCH v3] Allow xcb_send_request with >MAX_IOV iovecs

[Peter Harris]

This allows an application to do a scatter/gather operation on a large
image buffer to avoid the extra memcpy.

Use autoconf to use UIO_MAXIOV where IOV_MAX is not available (and the
POSIX minimum of 16 where neither are available).

Signed-off-by: Peter Harris <pharris at opentext.com>
---
 configure.ac   |    7 +++++++
 src/xcb_conn.c |    7 ++++++-
 2 files changed, 13 insertions(+), 1 deletions(-)

diff --git a/configure.ac b/configure.ac
index e94e32c..d27e2fd 100644
--- a/configure.ac
+++ b/configure.ac
@@ -110,6 +110,13 @@ dnl check for support for Solaris Trusted Extensions
 AC_CHECK_HEADERS([tsol/label.h])
 AC_CHECK_FUNCS([is_system_labeled])
 
+dnl check for IOV_MAX, and fall back to UIO_MAXIOV on BSDish systems
+AC_CHECK_DECL([IOV_MAX], [],
+	      [AC_CHECK_DECL([UIO_MAXIOV], [AC_DEFINE([IOV_MAX], [UIO_MAXIOV])],
+					   [AC_DEFINE([IOV_MAX], [16], [Define if not provided by <limits.h>])],
+					   [[#include <sys/uio.h>]])],
+	      [[#include <limits.h>]])
+
 xcbincludedir='${includedir}/xcb'
 AC_SUBST(xcbincludedir)
 
diff --git a/src/xcb_conn.c b/src/xcb_conn.c
index 725502a..69f6580 100644
--- a/src/xcb_conn.c
+++ b/src/xcb_conn.c
@@ -32,6 +32,7 @@
 #include <stdlib.h>
 #include <fcntl.h>
 #include <errno.h>
+#include <limits.h>
 
 #include "xcb.h"
 #include "xcbint.h"
@@ -204,7 +205,11 @@ static int write_vec(xcb_connection_t *c, struct iovec **vector, int *count)
          i++;
     }
 #else
-    n = writev(c->fd, *vector, *count);
+    n = *count;
+    if (n > IOV_MAX)
+	n = IOV_MAX;
+
+    n = writev(c->fd, *vector, n);
     if(n < 0 && errno == EAGAIN)
         return 1;
 #endif /* _WIN32 */    
-- 
1.7.2.5