Prática de Shadowing: printf is Actually a Secret Virtual Machine - And a Giant Security Hole! - Aprenda a falar inglês com o YouTube

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Hey, I'm Dave.

Welcome to my shop.

Today I want to talk about one of the strangest little machines hiding inside almost every C runtime that's ever shipped.

It's a function so familiar that most programmers stop even seeing it.

They use it the way you would use a light switch in your garage.

Flip it, it does its job, and you move on.

But Printf is not really just a function that prints things.

It's a formatting engine.

It's a tiny interpreter.

It reads a miniature language,

walks through a stream of dynamic arguments that it cannot actually see in any type-safe way,

way, converts binary values into human readable text,

aligns them, pads them, truncates them,

switches bases, rounds off floating point numbers,

and then pours the result out into your console,

a file, a string buffer,

or whatever other destination the runtime has wired up behind the curtain.

And the funny thing is,

just like that made up old cliche about the brain,

most programmers use about 10% of it.

So today we're going to start with the usual %D, %S,

and %X, because that's the part that everybody thinks they already know.

But we're going to get there by going backwards first,

down into a raw x86 assembly version of a fast SprintF replacement written by my first manager at Microsoft, Ben Slivka.

Ben was the development manager for MS-DOS when I was there,

and his formatter is wonderfully revealing because there's nowhere for the magic to hide.

There's no template engine, no managed runtime,

no allocator ferry, and no dependency graph the size of a European Parliament.

It just registers pointers in a state machine and a format string being chewed one byte at a time.

And once you've seen printf at that level,

the modern version stops looking like a boring library function and starts looking like what it really is.

A tiny little text-rendering virtual machine with a surprisingly sharp toolkit hidden inside of it.

Ben's file describes itself as a fast replacement for C runtime sprintf,

written in 1987 and updated in 1988,

and it explicitly supports a compact subset of the C formatting language.

Flags, width, optional H or L size modifiers,

decimal, unsigned decimal, string, lowercase and uppercase hex,

plus percent percent for printing a literal percent sign.

That's not the whole modern printf language,

but it's enough to see the skeleton,

and the skeleton is the important part for now.

The formatter begins life with two pointers,

one pointing at the format string and one pointing at the destination buffer.

In assembly terms, that's essentially the DSSI pair walking through the source and the ESDI pair writing to the output.

Every iteration loads one byte from the format string.

If it's an ordinary character, it just stores it.

If it's a percent sign, the machine changes personality.

It stops being a copier at that point and becomes a parser.

Ben's code literally jumps through the states named start,

flag, width, size, and type,

which is exactly the grammar hiding inside of every printf call that you've ever written.

And that's the first mental shift.

A format string is not just a string, It's a program.

A very small, very weird program, but a program nonetheless.

Take something like this.

Now, to you and me,

that reads like a sentence with a few holes in it,

but to printf it's a stream of instructions.

Copy the letters E R R O R and then a space,

and then it sees percent.

Resets its little internal defaults,

sees zero, decides now that the zero fill character is to be used instead of spaces,

sees eight, so it builds a field width of 8,

sees X, fetches an integer argument,

converts it to uppercase hexadecimal,

pads it to 8 characters,

and then copies it out.

Then it goes back to plain copying until the next percent sign.

Ben's assembly makes that very literal.

When it encounters a percent sign,

it clears the width, sets the default fill character to be blank,

assumes right justification, assumes it is not dealing with a long value,

and then transitions into the flag parsing state.

That is the runtime winding up the little clockwork mouse and sending it through the format specifier maze.

Width parsing is particularly fun because it shows how simple the machinery can be.

When Ben's formatter sees a digit in the width state,

it converts the ASCII digit to binary,

multiplies the current width by 10,

adds the digit, and stays in the width state.

That's it.

The string %123d is not special.

If the first width digit is 0,

the fill character becomes 0,

which is why %08 behaves the way it always has.

And that also explains the first,

most basic trick that programmers really learn without thinking about it.

%AD means print a decimal integer in a field of at least eight characters wide,

padded on the left with spaces. %08d means the same thing,

but padded with zeros. %-8d means the left justified instead, padding on the right.

Not one little grammar gives you primitive table layout.

And yes, in the age of HTML,

JSON, CSS, and terminal UI libraries,

this can look pretty quaint,

but if you're writing logs,

diagnostics, status dumps, firmware tools,

kernel utilities, old-school command line apps,

or anything that needs to be read by a tired human at 2.17 in the morning,

aligned columns still matter.

A good log file is not just data,

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it's an emergency instrument panel and printf is one of the oldest examples that we have.

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The basic numeric conversions are the ones that everybody knows,

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at least roughly. %d and %i print signed decimal, %u prints unsigned decimal, %x prints hexadecimal with lowercase letters and %X uses uppercase. %O prints octal,

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which is mostly useful if you live in Unix permission land or PDP-11 land,

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or enjoy explaining to junior developers why 010 is not always

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10. %C prints a character, %S prints a string. %P prints a pointer in an implementation-defined form,

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which is standards committee language,

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for you'll get something pointer-looking,

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but don't build a religion around expecting the exact spelling.

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The size These modifiers are where the harmless looking stuff starts to matter.

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In Ben's version, H means short and L means long,

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and his code tracks that with a flag named format long.

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When it reaches the type state, %LD and %LU are handled differently from ordinary %D and %U because on that old 16-bit target,

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a long is not just naturally sitting in one register waiting to be printed.

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The formatter has to fetch two words and treat them as one 32-bit value.

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This is one of the reasons the printf has always been both powerful and dangerous.

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The format string is the only thing telling the runtime what to fetch next.

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If you say %d but pass a double,

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or say %s and then pass an integer,

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there's no object metadata coming to rescue you in Classic C.

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Printf will simply reach into the argument list,

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pull out the number of bytes it believes that should be there,

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and interpret them according to the little formatting program that you handed it.

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It's a bit like giving a blindfolded machinist a box of random parts

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and instructions and saying the third item is a carburetor.

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If the third item is actually a mousetrap,

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the machinist is still going to install it.

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That's why compilers now try hard to warn you about mismatched format strings and why you should let them.

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The machine is powerful, but it trusts you in that terrifying old C way,

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like handing my teenager the keys to my Corvette and saying I'm sure you'll make good decisions.

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But there's no stability control to save you when you make a mistake.

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Now let's talk about strings because %s is simple until it isn't.

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In the ordinary case of printf hello %s,

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printf expects a pointer to a null terminated character array.

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It starts at that address and copies characters until it finds a zero byte.

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Ben's version does this with a sterlin macro that uses repnyscansby,

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one of those lovely old x86 string instructions

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that feels like it came from a CPU designed by somebody who wanted assembly language programmers to have a fighting chance.

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It scans for the null terminator,

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computes the length, and then uses that length to decide how much fill is needed after or before the string.

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That gives you the basic string alignment tools.

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It also gives you right-aligned and left-aligned text fields.

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If you've ever seen a neatly aligned command table,

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maybe top or some kind of task manager,

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there's a decent chance that some descendant of the idea was involved.

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But modern printf goes further.

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Precision on a string means maximum string length.

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That surprises people because with floating points,

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precision means digits after the decimal or significant digits depending on the conversion.

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The real trick is the dynamic version where you can pass the length.

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It's one of the best printf tricks in all of C.

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It lets you print a substring or a buffer or anything else that is not already null-terminated for you.

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Suppose you just parse the packet or a file format or a slice of memory and you have a pointer plus length.

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The beginner copies it out to a temporary buffer,

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appends a null-byte, prints it,

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and then frees the buffer.

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The experience coder uses %.star-s and moves on with his life.

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That's the kind of idiom

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that makes you feel like you've just found a hidden door in a house that you've lived in for 20 years.

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Now before we get to the more exotic stuff,

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let's finish the basic numeric tour with floating point. %F prints fixed point decimal, %E prints scientific notation. %G chooses between fixed

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and scientific depending on the value and the requested precision.

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The uppercase forms of %E and %G use uppercase exponent markers.

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If you care about the number of digits, precision is your friend.

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The important thing to remember is that floating point formatting is doing real work.

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It's not simply moving bytes around.

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It's converting a binary floating point approximation into a decimal representation that satisfies rounding rules and precision requirements.

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Modern implementations have extremely clever algorithms for this because the problem is a lot harder than it looks.

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Converting double detect correctly is one of those jobs that seems pretty boring until you try to do it yourself accurately,

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at which point it becomes a swamp full of math crocodiles.

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And this brings us to one of the coolest floating point formats that most people never use, %A.

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This prints the floating point number in hexadecimal with a binary exponent.

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Now it's going to look weird, something like this.

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Certainly not how you would print an invoice,

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but it's how you print the truth.

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Decimal output is often trying to be friendly.

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Hex floating point output is showing what the actual binary structure of the value is.

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If you're debugging floating point weirdness, %A is your x-ray machine.

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The number 0.1 looks pretty innocent in decimal,

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like a little cherub sitting on a cloud,

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but in binary floating point it is a repeating fraction wearing a fake mustache and %A catches it at the border.

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Now let's bring in the finale tricks,

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because this is where printf stops being merely useful and starts being delightfully sneaky.

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The first is dynamic width and precision with star.

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Ben's formatter already supported star for width,

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fetching the field width from the next argument instead of from the format string,

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and modern printf does that too,

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and it does it for precision as well.

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It's cool because it allows your formatting to adapt at runtime.

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Maybe you scan a table first to discover the widest name and then print all the rows using that width.

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Maybe you let the user specify a precision.

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Maybe your diagnostic tool aligns values based on the longest label in the current dataset.

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Suddenly your format string is not a fixed stencil anymore,

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it's like a layout engine with parameters.

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The second trick is SNPrintf as a measuring tool.

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This is a civilized way to build formatted strings when you don't know how big the result will be.

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The first call tells you how many characters would have been written,

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not including the Null Terminator.

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Then you allocate enough space for exactly that to do the real formatting in.

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This is infinitely better than guessing that 256 bytes ought to be enough,

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because as we eventually all learn,

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the universe loves to punish the phrase, ought to be enough.

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And the third trick is the alternate format flag, or hash.

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Most people only know it from hex,

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printf, percent, hash, x, which gives you something like beef in this case,

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but it also affects floating point output.

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With %g trailing zeros are normally removed.

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With % number sign g they can be preserved.

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That can matter when you are generating text

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that will be read by another programmer when 42 and 42.0 are not visually communicating the same idea.

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The alternate form is printf saying,

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fine, I'll leave the decimal furniture where you put it.

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Then the fourth trick is positional arguments,

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which are common in POSIX style printf implementations and are absolutely vital for localization.

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Instead of consuming arguments strictly from the left to right,

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the format string can say use argument number two here and argument number one here.

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In English, you might say Dave has five messages,

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but in another language, the natural order might be five messages has Dave,

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except hopefully in better grammar than that.

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Without positional arguments, translators are trapped by the English argument order,

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but with them, the sentence can move around without changing the code.

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The fifth trick is correct integer size formatting.

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Not very flashy, but it's the difference between code

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that works on your machine and code that survives contact with a different architecture.

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For size T, use %ZU.

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For pointer def, use %TD.

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For maximum width integers, use %JD.

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And for fixed width integers like UN64T,

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use the macros from inttypes.h.

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It's not glamorous, but neither is torquing the lug nuts on your Camaro.

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You still do it because eventually the wheel comes off if you don't.

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Ask me how I know these things.

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And not just from C.

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The sixth trick is %N, which is both fascinating and a little radioactive.

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%N does not print anything.

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Instead, it stores the number of characters printed so far into an integer pointer that you provide.

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After this, the code contains 5.

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That sounds like a party trick until you realize it can be used to mark offsets while building complicated output.

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But %n is also infamous in security history

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because uncontrolled format strings can use it to write to memory

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and this is why printf of user input is not just bad style,

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it's an engraved invitation to chaos.

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So always write %f with a %s and then the user input.

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Let me state this even more plainly.

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If you pass user-provided text to the printf formatting engine,

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it will process it just as surely as if you had hand-coded that yourself.

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So never do that.

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That one extra %s is the difference between printing a string

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and letting the user hand your tiny formatting machine a bag of burglary tools.

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The seventh trick is binary output where supported.

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C23 adds %b, finally, for binary integer formatting,

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and some libraries had their own extensions before that.

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So when it's supported, this 42 will give you 101010.

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And depending on your implementation,

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you may get the 0b prefix.

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That's such an obvious debugging feature that it's almost funny it took that long and becomes standard in C.

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C had decimal, octal, and hex from the dawn of time,

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but binary, actual base the computer uses was apparently too exotic and risque for use in polite company.

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And then there are the platform-specific treats.

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On GNU systems, %m prints the string associated with the current error number,

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without you passing an argument.

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So instead of writing, you know,

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printf with open failed str error of the error number, you just write %m.

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It's not portable C, but it's kind of a good example of how printf has evolved into a diagnostic language,

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and not just a formatting function.

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Now here's the part that I think is worth taking pretty seriously.

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PrintF is old, but it's not primitive.

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It comes from an era when every byte mattered and every abstraction had to earn its keep.

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Ben's assembly formatter has dispatch tables for states,

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lookup tables for hex digits,

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a unit table for decimal long conversion,

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scratch buffers on the stack,

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and careful handling of signs,

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fill characters, width, and justification.

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You can almost see the runtime thinking.

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It's like looking at a pocket watch with the back removed where now every gear has a reason that it exists.

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And that's why this function is still worth studying.

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Not because we should go back to writing 16-bit assembly formatters by hand,

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although I admit there's a certain therapeutic quality to doing that.

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It's worth studying because Printf is a compact lesson in language design,

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parsing, calling conventions, data representation,

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integer conversion, floating-point weirdness, security,

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localization, and API ergonomics all crammed into one little function that

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most of us learned in chapter 2 and then never looked at it again.

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And so, the humble format string is doing a lot more than it lets on.

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It's a contract between your program and the runtime.

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It says, here's how to walk through my memory,

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here's how to interpret each value,

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and here's how to transform it,

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and here's how to lay it out for human consumption.

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Get that contract right, and you get clean diagnostics,

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readable logs, compact code, and zero copy tricks like percent, point, star, s.

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Get it wrong, and you get garbage outputs,

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stack confusion, security vulnerabilities, and the kind of debugging session where you start bargaining with inanimate objects.

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So the next time you type printf %d,

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remember that you're not just printing an integer.

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You're invoking a tiny machine that has been refined across decades of C runtimes,

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operating systems, compilers, and architectures.

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It may be old, but it's not dull.

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And if you learn the parts of it that most programmers never touch,

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you can make your code cleaner,

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safer, faster, and sometimes just a little more elegant.

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Elegance is when the code knows exactly what it's doing,

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uses exactly what it needs,

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and leaves nothing unnecessary behind.

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Which, come to think of it,

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is not a bad description of a good printf implementation either.

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If you found today's episode interesting or entertaining,

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remember that I'm mostly in this for the subs and likes,

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so I'd be honored if you considered leaving me one of each before you go today.

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And if you're already subscribed, thank you!

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And if you have a question or a suggestion,

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please leave it in the comments here,

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and we answer the best of them every Friday on ShopTalk on the Dave's Attic channel.

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Check out an episode at of the link I'll try to throw up here,

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and subscribe there if you enjoy it as well.

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In the meantime, and in between time,

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hope to see you next time,

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right here in Dave's Garage.

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Do it, Glenn!

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Do it!

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Do it!

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