forgot to check in the actual code m(

socket/socket_fastopen.c

@@ -0,0 +1,18 @@
+#include "socket.h"
+#include <sys/types.h>
+#include <sys/socket.h>
+#include <netinet/tcp.h>
+#include <errno.h>
+
+int socket_fastopen(int s) {
+#ifdef TCP_FASTOPEN
+  return setsockopt(s,SOL_TCP,TCP_FASTOPEN,(int[]){ 5 }, sizeof(int));
+#else
+#ifdef ENOPROTOOPT
+  errno=ENOPROTOOPT;
+#else
+  errno=ENOSYS;
+#endif
+  return -1;
+#endif
+}

socket/socket_fastopen_connect4.c

@@ -0,0 +1,20 @@
+#include "socket.h"
+#include <sys/types.h>
+#include <unistd.h>
+#include <sys/socket.h>
+#include <errno.h>
+
+ssize_t socket_fastopen_connect4(int s,const char* ip,uint16 port,const char* buf,size_t len) {
+#ifndef MSG_FASTOPEN
+  int r;
+  {
+#else
+  int r=socket_send4_flag(s,buf,len,ip,port,MSG_FASTOPEN);
+  if (r==-1 && errno==ENOTCONN) {
+#endif
+    /* apparently the kernel does not support TCP fast open */
+    r=socket_connect4(s,ip,port);
+    if (r==0) return write(s,buf,len);
+  }
+  return r;
+}

socket/socket_fastopen_connect6.c

@@ -0,0 +1,20 @@
+#include "socket.h"
+#include <sys/types.h>
+#include <sys/socket.h>
+#include <unistd.h>
+#include <errno.h>
+
+ssize_t socket_fastopen_connect6(int s,const char* ip,uint16 port,uint32_t scope_id,const char* buf,size_t len) {
+#ifndef MSG_FASTOPEN
+  int r;
+  {
+#else
+  int r=socket_send6_flag(s,buf,len,ip,port,scope_id,MSG_FASTOPEN);
+  if (r==-1 && errno==ENOTCONN) {
+#endif
+    /* apparently the kernel does not support TCP fast open */
+    r=socket_connect6(s,ip,port,scope_id);
+    if (r==0) return write(s,buf,len);
+  }
+  return r;
+}