Re: where is the printf source for busybox?

[Mohan L <l.mohanphy@xxxxxxxxx>]

On Wed, Mar 12, 2014 at 7:14 AM, Kim Chan <ckim@xxxxxxxxxx> wrote:

Hi,

I've had this question for some time past and I would like to know the answer now..

I wanted to follow a printf in busybox but couldn't find the source of the printf function.

Then I throught probably the printf is provided by the system library where printf is connected to proper linux system call. (Is it printk?)

So I checkd LD_LIBRARY_PATH of my shell and found libc.so under /lib. I could see by 'nm libc.so' that printf is in the library.

But where can I see the printf source? Is it under gcc source? in GNU sdk, under glibc-2.16.0, I tried

# grep 'int printf(' * -r

but nothing comes up.

In linux kernel source doing the same gaves me

        arch/x86/boot/boot.h:int printf(const char *fmt, ...);
        arch/x86/boot/printf.c:int printf(const char *fmt, ...)
        arch/um/include/shared/kern.h:extern int printf(const char *fmt, ...);
        arch/um/os-Linux/user_syms.c:extern int printf(const char *, ...);
        arch/powerpc/boot/stdio.h:extern int printf(const char *fmt, ...) __attribute__((format(printf, 1, 2)));

so printf is defined only for x86 architecture?

I understand printf is for user program not kernel, so I want to see how the printf is connected to kernel print function. (the system call, I know in sparc, linux system call is trap 0x90, function 59 when using assembly)

Where can I find the printf (user program) source?

Thanks in advance.

Chan

Hi Chan,
 

Here is Glibc printf.c code:

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

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

  return done;
}

it in turn call vfprintf. You can see Glibc vfprintf.c (glibc-2.18/stdio-common/vfprintf.c)

Correct me If my understanding is wrong.

Thanks

Mohan L

 

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