C Language Trigraphs


The symbols [ ] { } ^ \ | ~ # are frequently used in C programs, but in the late 1980s, there were code sets in use (ISO 646 variants, for example, in Scandinavian countries) where the ASCII character positions for these were used for national language variant characters (e.g. £ for # in the UK; Æ Å æ å ø Ø for { } { } | \ in Denmark; there was no ~ in EBCDIC). This meant that it was hard to write C code on machines that used these sets.

To solve this problem, the C standard suggested the use of combinations of three characters to produce a single character called a trigraph. A trigraph is a sequence of three characters, the first two of which are question marks.

The following is a simple example that uses trigraph sequences instead of #, { and }:

??=include <stdio.h>

int main()
    printf("Hello World!\n");

This will be changed by the C preprocessor by replacing the trigraphs with their single-character equivalents as if the code had been written:

#include <stdio.h>

int main()
    printf("Hello World!\n");

Note that trigraphs are problematic because, for example, ??/ is a backslash and can affect the meaning of continuation lines in comments, and have to be recognized inside strings and character literals (e.g. '??/??/' is a single character, a backslash).