how can the gcc 6 option "-Wmisleading-indentation" work reliable? Reading the GCC 6 Release Series Changes, New Features, and Fixes I find very interesting the new option `-Wmisleading-indentation`: -Wmisleading-indentation warns about places where the indentation of the code gives a misleading idea of the block structure of the code to a human reader. For example, given CVE-2014-1266: sslKeyExchange.c: In function 'SSLVerifySignedServerKeyExchange': sslKeyExchange.c:631:8: warning: statement is indented as if it were guarded by... [-Wmisleading-indentation] goto fail; ^~~~ sslKeyExchange.c:629:4: note: ...this 'if' clause, but it is not if ((err = SSLHashSHA1.update(&hashCtx, &signedParams)) != 0) ^~ I am just wondering how the option can recognize reliable these kind of misleading indentation when the `tab space` is "unknown" to `gcc` due to the fact it can be 4 or 8 or even what ever...

It can't. The developers state explicitly that it is based on a set of heuristics tuned to catch _most_ misleading indentation without too much noise. As with other compiler warnings, there will be false positives and false negatives. Let _current_ GCC loose on most any code with `-Wall` and weep.