I use Xcode2 on Mac OS X 10.4, and program c++ with GSL library. The GSL header files are in /opt/local/include/gsl/gsl_xxx.h I also edit the settings of "Header Search Path" to be /opt/local/include and type # include <gsl/gsl_xxx.h> in my program. But Xcode2 still cannot locate the file. I am very sure the file is there and I didn't type it wrong. Sometimes Xcode2 can find it and build my program successfully, sometimes not. Looks completely random. I am so tired of Xcode2. Can someone save me?