Homeport selfupdate error,please help me to resolve

[jinleicrazysweet@…]

---> Updating MacPorts base sources using rsync receiving file list ... done

sent 36 bytes received 69 bytes 14.00 bytes/sec total size is 3594240 speedup is 34230.86 receiving file list ... done

sent 36 bytes received 76 bytes 5.74 bytes/sec total size is 512 speedup is 4.57 MacPorts base version 2.1.1 installed, MacPorts base version 2.1.3 downloaded. ---> Updating the ports tree Synchronizing local ports tree from rsync://rsync.macports.org/release/tarballs/ports.tar receiving file list ... done

sent 36 bytes received 70 bytes 30.29 bytes/sec total size is 54753280 speedup is 516540.38 receiving file list ... done

sent 36 bytes received 77 bytes 45.20 bytes/sec total size is 512 speedup is 4.53 receiving file list ... done

sent 36 bytes received 70 bytes 30.29 bytes/sec total size is 9346970 speedup is 88178.96 receiving file list ... done

sent 36 bytes received 77 bytes 32.29 bytes/sec total size is 512 speedup is 4.53 ---> MacPorts base is outdated, installing new version 2.1.3 Installing new MacPorts release in /opt/local as root:admin; permissions 0755; Tcl-Package in /Library/Tcl

checking build system type... i386-apple-darwin11.4.2 checking host system type... i386-apple-darwin11.4.2 checking target system type... i386-apple-darwin11.4.2 checking MacPorts version... 2.1.3 checking for sw_vers... /usr/bin/sw_vers checking for defaults... /usr/bin/defaults checking for xcode-select... /usr/bin/xcode-select checking Mac OS X version... 10.7.5 checking Xcode location... /Applications/Xcode.app/Contents/Developer checking Xcode version... 4.6 checking for gcc... /usr/bin/cc checking whether the C compiler works... no configure: error: in `/opt/local/var/macports/sources/rsync.macports.org/release/tarballs/base': configure: error: C compiler cannot create executables See `config.log' for more details Command failed: cd /opt/local/var/macports/sources/rsync.macports.org/release/tarballs/base && CC=/usr/bin/cc ./configure --prefix=/opt/local --with-tclpackage=/Library/Tcl --with-install-user=root --with-install-group=admin --with-directory-mode=0755 --enable-readline && make && make install SELFUPDATING=1 Exit code: 77 Error: /opt/local/bin/port: port selfupdate failed: Error installing new MacPorts base: command execution failed

I installed Xcode,so I want know the reason for the error