Bonus: Plugging `_putchar()` Into a Custom `printf`¶

Tools: GCC, Make, strace

Goal¶

Connect a complete, self-contained printf implementation to the operating system by supplying the single function it needs from you.

Background¶

The utils/printf/ directory holds a portable printf implementation, mpaland/printf, designed for embedded systems that have no libc at all. It knows how to format everything — integers, padding, floats — but it has no idea how to get a character out of the process.

For that it delegates to one hook that you must provide:

void _putchar(char character);

The layering is the same idea as the rest of the session, with the formatting done by somebody else's library:

printf() / sprintf() / ...
        |
        v
   _putchar(c)          <-- you implement this
        |
        v
  write(1, &c, 1)       <-- the write syscall, fd 1 = stdout
        |
        v
      kernel

Your Task¶

Open _putchar.c and fill in the TODO so that the character is written to standard output using write(2):

ssize_t write(int fd, const void *buf, size_t count);

fd — use 1 for stdout.
buf — a pointer to the bytes to write. You have a char, not a pointer; take its address.
count — exactly one byte here.

You may use libc's write() directly for this exercise, or your own my_write() from the demo if you would rather keep libc out of it entirely.

Build & Run¶

make
./printfdemo

Check Your Work¶

Every line should be formatted exactly as the format strings in main.c ask: right-aligned and left-aligned columns should line up, the zero-padded field should carry leading zeros, and the float should show four decimals. Garbled or missing output usually means the address of the wrong thing was passed to write.
Look at what actually reaches the kernel:
```
strace -e trace=write ./printfdemo
```
Count the write calls against the number of characters printed. The ratio should tell you something specific about this design; be ready to say what, and how it compares with what libc's printf did in the session 01 demo.
Nothing in the output path should be libc's stdio. Check that strace shows no other output-related system call.

Bonus: Plugging _putchar() Into a Custom printf¶