GCC installed. Mathematica still won't compile to C - c

Question

I'm running Mathematica 8 on a MacOSX, trying to compile even the simplest program to C. Anything having to do with C simply doesn't work in Mathematica. I have GCC 4.2 installed; I've even reinstalled it multiple times with XCode. Here's what I'm doing and the errors I'm getting:
First, I always evaluate the command
Needs["CCompilerDriver`"]
If I set the compilation target to C,
c = Compile[ {{x}}, x^2 + Sin[x^2], CompilationTarget -> "C"];
I get an error that reads: Compile::nogen : A library could not be created from the compiled function.
If I try to create a library,
demoFile = FileNameJoin[{$CCompilerDirectory,"SystemFiles","CSource","createDLL_demo.c"}];
lib = CreateLibrary[{demoFile},"testLibrary"]
I get an message $Failed. Wolfram says that this is because I don't have a C compiler installed. I find that hard to believe because when I run
CCompilers[]
It tells me that I've got GCC installed: {{"Name" -> "GCC",
"Compiler" -> CCompilerDriver'GCCCompiler`GCCCompiler,
"CompilerInstallation" -> "/usr/bin", "CompilerName" -> Automatic}}
What's more, terminal says I have GCC installed too!! Any help would be appreciated. I'd really like to compile Mathematica to C.

Answer 1

In this answer I'll collect some debugging steps for similar problems, for future reference. Feel free to edit/improve them.
If compiling to C code does not work from Mathematica 8,
Check that you have a supported C compiler installed and it works (the obvious).
Note that the compiler does not necessarily have to be in the PATH, at least on Windows/Visual Studio it doesn't.
Check that Mathematica recognizes the compiler
<< CCompilerDriver`
CCompilers[]
will list the compilers known to Mathematica.
Check what commands Mathematica executes to compile the generated C code:
Compiler`$CCompilerOptions = {"ShellCommandFunction" -> Print};
Compile[{{x}}, x^2, CompilationTarget -> "C"];
Note that with "ShellCommandFunction" -> Print the commands will not be executed, so you'll need to re-set Compiler`$CCompilerOptions to {} after this step is complete to allow command execution again.
Check the output/errors from the compiler:
Compiler`$CCompilerOptions = {"ShellOutputFunction" -> Print};
Compile[{{x}}, x^2, CompilationTarget -> "C"];
These last two steps will hopefully give you enough clues to proceed. With this information you can check if the correct library / include paths are passed to the compiler (in the case of gcc/icc, look at the -L option which specifies library paths and the -I option which specifies include paths). Then check if the required include and library files are present at those paths.

Answer 2

If you get Compile::nogen, you can see the compiler output by setting ShellOutputFunction->Print right in the Compile expression:
c = Compile[ {{x}}, x^2 + Sin[x^2],
CompilationTarget -> {"C", "ShellOutputFunction"->Print}];
In general, this is how you can pass options to the underlying CreateLibrary call, by changing CompilationTarget->"C" to CompilationTarget->{"C", options}. Setting Compiler`$CCompilerOptions works too, but this technique has the advantage of not setting a global variable.

Answer 3

It is a shame that the only error you are seeing is $Failed, that's not terribly helpful; I wonder if perhaps there are some file or directory permissions problems?
I'm running on linux not Mac so I am not sure if my setup is "close enough" or not. On my machine your Compile command succeeds and generates a file .Mathematica/ApplicationData/CCompilerDriver/BuildFolder/blackie-desktop-5077/compiledFunction1.so in my home directory. Is there any way you can find a .Mathematica directory associated with your userid, and see if it exists and is writeable by mathematica?
Also, you could check to see if "gcc" is or is not being accessed by checking the file access time of /usr/bin/gcc before and after your call to Compile. From an operating system shell you can do ls -lu /usr/bin/gcc or from Mathematica perhaps Import["!ls -lu /usr/bin/gcc", "Text"]