CODEWITHSUNDEEP
cmacosopenglgccclang
user755921
user755921

Reputation:

C/OSX/clang confusion: "symbol(s) not found" happening at link time instead of compile time

I have a question, but I'm not sure if it's about C, clang or OSX.

When I compile a simple GLUT program like so:

clang test.c -framework OpenGL -framework GLUT

And I intentionally insert a function call that doesn't exist, like so:

thisFunctionIsNotDefinedAnywhere();

I get an error, as expected. But here's the rub -- I don't get the error until link time!

Undefined symbols for architecture x86_64:
    "thisFunctionIsNotDefinedAnywhere" referenced from:
    _main in test-e099d2.o
ld: symbol(s) not found for architecture x86_64

Why is this? Is this because pre-C99 there were implicit declarations? I've been programming for a long time and have never run into this before. Is it because I've been spoiled on GCC and MSVC which (I seem to remember) cause a compiler error in this situation? Or does it have to do with how framework linking works in OSX, which I am new to?

I appreciate any clarification!

Upvotes: 1

Views: 239

Answers (2)

Reto Koradi
Reto Koradi

Reputation: 54562

As expected, this gives a warning with a recent version of clang. I tried it with the version that comes with Xcode 6.1, and got this compiler output:

$ clang test.c
test.c:2:5: warning: implicit declaration of function 'thisFunctionIsNotDefinedAnywhere' is invalid in
      C99 [-Wimplicit-function-declaration]
    thisFunctionIsNotDefinedAnywhere();
    ^
1 warning generated.
Undefined symbols for architecture x86_64:
  "_thisFunctionIsNotDefinedAnywhere", referenced from:
      _main in test-376416.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Since using undeclared functions is legal in C, the compiler message can't be an error. But it does show a clear warning, without specifically enabling warnings with command line options.

Upvotes: 1

Iharob Al Asimi
Iharob Al Asimi

Reputation: 53006

Because of implicit function declaration, if you enable warnings then you would be warned that thisFunctionIsNotDefinedAnywhere(); is implicitly declared, returning int by default.

If you add a function prototype, then the warning will be gone, but then at the link stage, the compiler wont find the function definition, and issue the error.

Upvotes: 0

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