Quick test to make sure your compiler knows trigraphs.

trigraphs.c

#include <unistd.h>

static char *program_name =

  "trigraphs.c";
/* Trinity BLAKE 2020-2022
 * Public domain */

char trigraphs[] =

/* From the C89 draft specification, section 2.2.1.1:
 *	All occurrences in a source file of the following sequences of
 *	three characters (called trigraph sequences[5]) are replaced
 *	with the corresponding single character. */
	"??="    /* #  */
	"??("    /* [  */
	"??/??/" /* \\  - escaped backslash; will print '\' */
	"??)"    /* ]  */
	"??'"    /* ^  */
	"??<"    /* {  */
	"??!"    /* |  */
	"??>"    /* }  */
	"??-"    /* ~  */
;

/*	No other trigraph sequences exist. */
int trigraphs_s = 10; /* (9 trigraphs + nul terminator) */
/* Each ? that does not begin one of the trigraphs listed above is not
 * changed. */

int main(int argc, char *argv[]){
	int i;
	char *argv0;

	if(sizeof(trigraphs) / sizeof(trigraphs[0]) != trigraphs_s){
		argv0 = argv[0] != NULL ? argv[0] : program_name;
		for(i = 0; argv[0][i] != '\0'; ++i);
		write(1, argv0, i);
		write(1,
			": Trigraphs were not parsed correctly by the"
			" compiler.\n", 55
		);
		i = 71; /* sysexits(3) EX_OSERR */
	}else{
		trigraphs[trigraphs_s - 1] = '\n';
		write(1, trigraphs, trigraphs_s);
		i = 0;
	}
	return i;
}