trigraph ??/ ignored, use -trigraphs to enable

Question

My compiler (gcc) is giving me this warning. Please help me to understand what it means:

warning: trigraph ??/ ignored, use -trigraphs to enable

Answer 1

You have "accidentally" written a trigraph somewhere in your source code (the compiler's warning would pinpoint the line). Since trigraphs were invented to solve a problem that doesn't come into play on modern systems, you don't actually want the trigraph ??/ to be replaced with the character \.

Therefore, this warning should probably be ignored (you can tell the compiler to shut up by adding -Wno-trigraphs after -Wall in your command line; see the docs). But it would be good to show your source code so we can be sure.

Answer 2

Instead of adding a compiler flag, you could just escape each question mark with \, i.e. \?\?-. This helped in my case.