printf Call Tree

Call Tree

   fprintf --\    putchar ------ putc --\
             |                          |
    printf --+-- vfprintf --+-- fputs --+-- write
                            |           |
                     puts --/  fwrite --/

printf()

printf() calls vfprintf() in order to convert the arguments into a list of arguments for use by the macros in varargs.h. It also uses this call to convert the printf(fmt) into fprintf(stdout,fmt).

vfprintf()

vfprintf() calls the common routine vsprintf() which is used for printf(), sprintf(), and fprintf(). This routine places the converted string into a buffer. The buffer is then passed to fputs() for printing.

fputs()

Since PMON does not implement buffered output, fputs() calls write() directly.

write()

write() calls _write(). This indirection is necessary to permit the library to be used for both building PMON and building clients.

_write()

Because PMON does not use interrupt driven I/O, it is necessary for PMON to poll the devices frequently looking for newly arrived characters. This is done by calling scandevs(). Having done this _write() then calls _chwrite().

scandevs()

This function scans all devices looking for newly-arrived input. It does this by calling the driver with OP_RXRDY. If the return status indicates that a character is ready, it calls with OP_RX to retreive that actual character. The characters are placed in the input queue via Qput(). Characters that need echoing are sent to the device via _chwrite().

_chwrite()

This function writes a single character to the designated device. It does this by repeatedly calling the driver with OP_TXRDY until ready. It then calls the driver with OP_TX to actually transmit the character.