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Comments on Counting number of assignments that a `fscanf` format strings implies
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Counting number of assignments that a `fscanf` format strings implies
I'm writing a function that counts the number of assignments for a fscanf format string. I studied the documentation in C standard 7.21.6.2
It looks like it works. It passes all test cases I have written and yields no warnings with -Wall -Wextra -pedantic -std=c17. While I appreciate design advises, my main concern is if the code is correct or not, so I would be grateful if you found any test case that breaks it. Minor things like variable naming and such is not really interesting.
The function requires the format string to be valid. If not, the behavior is undefined.
#include <stdio.h>
#include <ctype.h>
#include <string.h>
#include <assert.h>
// Counts the number of assignments that should be made by scanf and alike.
//
// Assumes a valid format string. If it's not valid, behavior is undefined.
int count_assignments(const char *fmt) {
int ret = 0;
// Note that n is removed, because it suppresses assignments
static const char specifiers[] = "diouxaefgcspAEFGX";
static const char singlelength[] = "hljztL";
static const char doublelength[] = "hl";
while(*fmt) {
if(*fmt == '%') {
fmt++;
// Skip width modification
while(isdigit(*fmt)) fmt++;
// Check length modification
if(strchr(singlelength, *fmt)) {
fmt++;
if(strchr(doublelength, *fmt)) {
fmt++;
goto READ_SPECIFIER;
}
goto READ_SPECIFIER;
}
if(*fmt == '[') {
while(*fmt != ']') fmt++;
ret++;
goto END;
}
goto READ_SPECIFIER;
}
goto END;
READ_SPECIFIER:
if(strchr(specifiers, *fmt))
ret++;
END:
fmt++;
}
return ret;
}
int main(void)
{
struct test_case {
const char *fmt;
const int n;
} test_cases[] = {
{ "foo", 0 },
{ "%s", 1 },
{ "%d%d", 2 },
{ "%lld", 1 },
{ "%%", 0 },
{ "%d%%%d", 2 },
{ "%2333d%c%33f", 3 },
{ "%[bar]", 1 }
};
for(size_t i=0; i<sizeof test_cases/sizeof test_cases[0]; i++) {
struct test_case tc = test_cases[i];
printf("%s %d %d\n", tc.fmt, tc.n, count_assignments(tc.fmt));
assert(count_assignments(tc.fmt) == tc.n);
}
}
Post
Just looking at your code, I think you'll need to look at a few nasties in scan sets:
-
%[^]%a-z]stops at the second], not the first, for example, and I think your code would misconstrue the%aas a conversion rather than part of the scan set. - Likewise
%[]%[a-z]; the one scan set there stops at the second], but your code would count two scan sets, I believe.
You also need to know about %n (it's a conversion specifier, but it doesn't get counted). You've removed it from the list of conversion specifiers, so you may be OK with that (but your testing should test it).
You'll need to worry about suppressed conversions %23*f (which don't get counted either).
I think your code would accept %hld or %lhu as valid (whereas %hhd and %llu are valid — it should be two of the same length modifier). That's one specific case of the more general comment:
You are not validating the format — which is OK, as long as you know that's what you're (not) doing.
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