C 标准是否定义了在 printf 中调用哪些函数？

Question

I'm working on an embedded project. I want printf to work with UART port.

I did some google and some people suggest that printf calls fputc and I needs to supply function definition of fputc to work with UART port. Other people suggest _write instead.

I am assuming that printf calls fputc , which then calls _write ?

What I want to ask is, does the C standard define anything about this? (Is it guaranteed that printf calls fputc ?)

Answer 1

C spec doesn't define anything about this aside from printf() functions as if it called fputc() multiple times.

It is not guaranteed that printf calls fputc .

---

I want printf to work with UART port.

Consider instead writing your own UART_printf() that calls vsprintf() and then sends the string out to the UART.

Answer 2

There is no guarantee that printf calls fputc .

But you can write your own uprintf function using variadic parameters that internally calls sprintf and then sends the characters to the UART.

Something like this (untested code):

int uprintf(const char *format, ...)
{
  va_list argList;
  va_start(argList, format);
  char buffer[1000];
  int retval = vsprintf(buffer, format, argList);
  va_end(argList);

  // write chars in buffer to uart here

  return retval;
}