Automatic random testing for printf() with regard to %-formatting.

rndfmt.c

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <time.h>
#include <ctype.h>

char rand_chr()
{
	char c;
	do {
		c = rand() % 0x80;
	} while (!isalnum(c));
	return c;
}

int rand_str(char *buf, int max_len)
{
	int len = rand() % (max_len + 1);
	char *s = buf;
	int i;
	for (i = 0; i < len; i++)
		*s++ = rand_chr();
	return len;
}

int main(int argc, char *argv[])
{
	char fmt[16];
	char str[16];
	char *s = fmt;
	char *flags = "-+ #0";
	char *specs = "diouxXcsp";
	char spec;
	int i;
	if (argc == 1)
		srand(time(NULL));
	else
		srand(atoi(argv[1]));
	*s++ = '%';
	for (i = strlen(flags); i; i--)
		if (rand() % 2)
			*s++ = flags[i-1];
	spec = specs[rand() % strlen(specs)];
	*s = '\0';
	if (rand() % 2)
		printf("printf(\"[%s%d", fmt, rand() % 12 + 4);
	else
		printf("printf(\"[%s", fmt);
#ifdef TEST_LENGTH_MODIFIERS
	if (rand() % 2)
		printf("%c", (rand() % 2) ? 'h' : 'l');
#endif
	printf("%c]\", ", spec);
	switch (spec) {
	case 'd': case 'i':
		switch (rand() % 3) {
		case 0:
			printf("0");
			break;
		case 1:
			printf("-");
		case 2:
			printf("%d", rand() / 2);
		}
		break;
	case 'c':
		printf("'%c'", rand_chr());
		break;
	case 's':
		rand_str(str, 12);
		printf("\"%s\"", str);
		break;
	default:
		if (rand() %2)
			printf("0");
		else
			printf("%d", rand());
	}
	printf(");\n");
	return 0;
}

smoke_fmt.c

#include <stdio.h>

int main()
{
	int a = -357;
	int b = 123;
	int n;
	/* no formatting */
	printf("hello\n");
	/* basic formatting */
	printf("%d\n", a);
	/* multiple */
	printf("%d + %d = %d\n", a, b, a+b);
	/* %n */
	printf("%d + %d =%n %d\n", a, b, &n, a+b);
	/* arg as width */
	printf("%*d\n", n, a);
	/* flag */
	printf("%0*d\n", n, a);
	/* multiple flags */
	printf("%+0*d\n", n, b);
	/* unordered flags */
	printf("%0+*d\n", n, b);
	/* length modifier */
	printf("%0+*ld\n", n, b);
	/* some complex combination */
	printf("%2c, %0+*ld, %#X\n", 'z', n/2, b, 0xBC);
	return 0;
}

test_fmt.sh

#!/bin/sh

# requirements:
#   * POSIX shell
#   * Tiny C Compiler (TCC) with any reference libc (e.g. musl)
#   * neatcc driver supporting -r (https://github.com/lecram/neatrun)
#   * md5sum(1)

# first do smoke testing
smk=smoke_fmt.c
otcc=$(tcc -run "$smk" | md5sum)
oncc=$(neatcc -r "$smk" | md5sum)
if [ "$otcc" != "$oncc" ]; then
	printf "smoke tests failed\n"
	exit
fi
printf "smoke tests passed\n"

# do random testing
src=tmpfmt.c
pre="#include <stdio.h>\nint main() {\n\t"
pos="\n\tfflush(stdout);\n\treturn 0;\n}\n"
if [ $# -eq 0 ]; then
	seed=$(date '+%s')
else
	seed=$1
fi
seed=$((seed%10000))
i=0
while true; do
	rndfmt=$(tcc -run rndfmt.c $seed)
	printf "$pre%s$pos" "$rndfmt" > "$src"
	otcc=$(tcc -run "$src")
	oncc=$(neatcc -r "$src")
	printf "%4d %s %s %s\n" "$seed" "$rndfmt" "$otcc" "$oncc"
	if [ "$otcc" != "$oncc" ]; then
		break
	fi
	seed=$((seed*(seed+i)/10%10000+7))
	i=$((i+1))
done