How does printf work internally?

Question

I am curious as to how printf works internally within Linux. I don't understand how it writes data to STDOUT.

After a bit of searching for the internals, I downloaded glibc and took a look at the source code:

__printf (const char *format, ...)
{
   va_list arg;
   int done;

   va_start (arg, format);
   done = vfprintf (stdout, format, arg);
   va_end (arg);

   return done;
}

After finding this, I went deeper into the vfprintf function - but the file is about 2500 lines of unfamiliar C code. I'm looking for an explanation from 10,000 feet of how printf works with a computer's memory and output to display characters on screen.

If I were a piece of assembly code what would I have to do to accomplish the same task? Is it operating system dependent?

It all boils down to writing to a device (almost as if it was a file). The device driver takes over and interprets the ASCII values and tells the monitor how to display them. Same with disk, mice, etc. — Jiminion, Aug 16 '13 at 18:35
no, he wants to understand vfprintf too, not just the underlying write. The other thread contains the rest, though. — Antti Haapala -- Слава Україні, Aug 17 '13 at 06:59

score 9 · Accepted Answer · answered Aug 16 '13 at 18:34

9

I think you're looking at the wrong layer. The logic in vfprintf is responsible for formatting its arguments and writing them through the underlying stdio functions, usually into a buffer in the FILE object it's targetting. The actual logic for getting this output to the file descriptor (or on other non-POSIX-like systems, the underlying device/file representation) is probably in fwrite, fputc, and/or some __-prefixed internal functions (perhaps __overflow).

answered Aug 16 '13 at 18:34

R.. GitHub STOP HELPING ICE

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Thank you - between this answer and http://stackoverflow.com/questions/2442966/c-c-function-definitions-without-assembly I have a better understanding of what's going on. – sdasdadas Aug 16 '13 at 18:44

How does printf work internally?

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