Mazda cars used to have a bug where they used printf(str) instead of printf("%s", str) and their media system would crash if you tried to play the "99% Invisible" podcast in them. All because the "% In" was parsed as a "%n" with some extra modifiers. https://99percentinvisible.org/episode/the-roman-mars-mazda-...
-Werror=format-security> The ISO C90 functions abort, abs, acos, asin, atan2, atan, calloc, ceil, cosh, cos, exit, exp, fabs, floor, fmod, fprintf, fputs, free, frexp, fscanf, isalnum, isalpha, iscntrl, isdigit, isgraph, islower, isprint, ispunct, isspace, isupper, isxdigit, tolower, toupper, labs, ldexp, log10, log, malloc, memchr, memcmp, memcpy, memset, modf, pow, printf, putchar, puts, realloc, scanf, sinh, sin, snprintf, sprintf, sqrt, sscanf, strcat, strchr, strcmp, strcpy, strcspn, strlen, strncat, strncmp, strncpy, strpbrk, strrchr, strspn, strstr, tanh, tan, vfprintf, vprintf and vsprintf are all recognized as built-in functions unless -fno-builtin is specified (or -fno-builtin-function is specified for an individual function).
Builtin here doesn't mean that GCC won't ever emit calls to library functions, only that it reserves not to and allows itselfs to make assumptions about how the functions work, including diagnosing misuse.
The library functions themselves might also be marked with __attribute__(format(...)) as the sibling comment notes but that is not necessarily required for GCC to check the format strings.
Also, in personal projects, I almost always used custom wrapper functions for printf/fprintf/sprintf for various reasons, so that default wouldn’t be of much use, unless maybe I could enable it for the custom functions.
For loading translated strings, I'm missing some library function to verify whether two format strings are argument-compatible.
Aren't those usually resolved to string literals by preprocessor such that the compiler still could emit a warning?
(I have no recommendations. When I've seen this stuff done properly, on the occasions I've managed not to avoid doing it, it's always been using some in-house system.)
You can with the $ syntax. Never seen it used though. Maybe it isn't very portable.
I think the real reason you rarely see it, is it is only used with internationalisation–the idea being if you translate the format string, the translator may need to reorder the parameters for a natural translation, given differences in word order in different languages. However, a lot of software isn't internationalised, or if it is, the internationalisation is in end-user facing text, which nowadays usually ends up in a GUI or web UI, so printf has less to do with it. And the kind of lower-level tools/components for which people still often use C are less likely to be internationalised, since they are targeted at a technical audience who are expected to be able to read some level of English.
I like printf format strings, but as a way of handling localizable strings I don't think they are the best.
A good example of this is pluralisation. We've all done things like:
printf("%d file(s) copied\n", count);
printf("%d file%s copied\n", count, count != 1 ? "s" : "");
printf() has zero hope of coping with this complexity. gettext provides a special function to handle this, ngettext(), which is passed the number as a separate argument, so it can select which plural form to use. And then the translated message files contain a header defining how many plural forms that language has, and the rules to choose which one to use. And for some languages it is crazy complex. Arabic is the most extreme, for which the manual gives this plural rule:
Plural-Forms: nplurals=6; \
plural=n==0 ? 0 : n==1 ? 1 : n==2 ? 2 : n%100>=3 && n%100<=10 ? 3 \
: n%100>=11 ? 4 : 5;One practical reason I can think of is because not everyone compiles their own code.
You must most definitely look for and enable such flags as they become available in your own projects. (eg I was rooting for -Wlifetime but it did not land for various reasons)
But when you compile other people's code, your breaking your local build doesn't help anyone. Best you can do is to submit a bug report, which may or may not be ignored.
std::format specifically only works for constant† format strings. Not because they can't make it work with a dynamic format, std::vformat is exactly that, but most of the time you don't want and shouldn't use a dynamic format and the choice to refuse dynamic formats in std::format means fewer people are going to end up shooting themselves in the foot.
Because it requires constant formats, std::format also gets to guarantee compile time errors. Too many or not enough arguments? Program won't build. Wrong types? Program won't build. This shifts some nasty errors hard left.
† Not necessarily a literal, any constant expression, so it just needs to have some concrete value when it's compiled.
The %n functionality also makes printf accidentally Turing-complete even with a well-formed set of arguments. A game of tic-tac-toe written in the format string is a winner of the 27th IOCCC.
- sez wiki.
A not so fun fact:
Because the %n format is inherently insecure, it's disabled by default.
- MSVC reference.
No it doesn't. Printf has no way to loop so it's not Turing complete. Even if you did what the IOCCC entry did with putting it into a loop it still wouldn't be Turing complete as it would not have an infinite memory.
It's definitely a bit fuzzy. I'm sure lots of philosophy papers have been written on when exactly it is or isn't appropriate to consider a finite computational system as a finite approximation to a Turing-complete system. In realistic everyday cases, however, it's usually clear enough what should and shouldn't count as such.
Adding another state to a FSA adds more memory.
There is no difference between a hobbled Turing machine and a FSA. Turing machines aren't a useful concept in the real world and that is okay.
Yes, but it also changes the state transition logic. You can't just 'add 100 more states' to an FSA in the same way that you can 'add 100 more stack slots' to a bounded pushdown automaton.
As I said previously, these are somewhat fuzzy distinctions, and I'm not saying that they're easy to make mathematically precise. They do however seem clear enough in most cases of practical interest. There are many real-world computing systems that would be Turing-complete if they had unbounded memory. There are others that are not Turing-complete for more fundamental reasons than memory limitations. Again, I acknowledge that 'more fundamental' is not a mathematically precise concept.