寫C語言這麼久了,從來沒發現過這個東西的存在.

因為V同事的發現,以及K同事找到解答.

才發現原來這東西很久就存在了.

應該不容易遇到這個問題.

但是可能會有個大問題.
for example:
// xxxxx ??/
printf("xxxxn"); // this will be skiped.
會造成底下那一行也被當成註解.

至於gcc對於 trigraphs 預設是關閉的.
如果要用trigraphs需要下 gcc -trigraphs test.c
如果需要對trigraphs有warnings,要下 gcc -Wtrigraphs test.c
而VC 6.0預設是會做轉換,目前還找不到可以關掉它的方法.

以下是摘錄自 http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vclang/html/_clang_trigraphs.asp

C Languae Reference

Trigraphs

The source character set of C source programs is contained within the 7-bit ASCII character set but is a superset of the ISO 646-1983 Invariant Code Set. Trigraph sequences allow C programs to be written using only the ISO (International Standards Organization) Invariant Code Set. Trigraphs are sequences of three characters (introduced by two consecutive question marks) that the compiler replaces with their corresponding punctuation characters. You can use trigraphs in C source files with a character set that does not contain convenient graphic representations for some punctuation characters.

The following table shows the nine trigraph sequences. All occurrences in a source file of the punctuation characters in the first column are replaced with the corresponding character in the second column.

Trigraph Sequences

A trigraph is always treated as a single source character. The translation of trigraphs takes place in the first translation phase, before the recognition of escape characters in string literals and character constants. Only the nine trigraphs shown in the above table are recognized. All other character sequences are left untranslated.

The character escape sequence, ?, prevents the misinterpretation of trigraph-like character sequences. (For information about escape sequences, see Escape Sequences.) For example, if you attempt to print the string What??! with this printf statement

printf( "What??!n" );

the string printed is What| because ??! is a trigraph sequence that is replaced with the | character. Write the statement as follows to correctly print the string:

printf( "What??!n" );

In this printf statement, a backslash escape character in front of the second question mark prevents the misinterpretation of ??! as a trigraph.