Can't interrupt Arduino Uno's bootloader from C - arduino

Question

I'm trying to get the Arduino into programming mode. I want to examine the flash to see if my software is already installed and then upload one of several binaries depending on its need. For testing, I have a simple sketch that does nothing in the loop but sends "In Regular Mode" over the serial port on setup.
Toggling DTR/RTS does indeed reset the Arduino but sending a sync request does nothing and the bootloader times out and passes control to the sketch.
#include <stdio.h>
#include <unistd.h>
#include <stdint.h>
#include <fcntl.h>
#include <sys/io.h>
#include <sys/ioctl.h>
#include <termios.h>
int main()
{
static const uint8_t bSyncString [] = {0x30, 0x20}; // {CMD_STK_GET_SYNC, SYNC_CRC_EOP};
uint8_t bBuffer [256];
unsigned int ctl;
struct termios tty;
int iFd = open ("/dev/ttyACM0", O_RDWR | O_NOCTTY);
if (iFd >= 0)
{
// From STK500 spec: 115.2kbps, 8 data bits, 1stop bits, no parity
if (tcgetattr (iFd, &tty) == 0)
{
cfsetospeed(&tty, B115200);
cfsetispeed(&tty, B115200);
tty.c_cflag |= CS8|CSTOPB;
tty.c_cflag &= ~(PARENB|PARODD);
if (tcsetattr(iFd, TCSANOW|TCSADRAIN, &tty) == 0)
printf ("Attributes set\n");
}
if (ioctl (iFd, TIOCMGET, &ctl) >= 0) // Get current line states
{
ctl &= ~(TIOCM_DTR|TIOCM_RTS);
if (ioctl(iFd, TIOCMSET, &ctl) >= 0) // DTR and RTS low
{
usleep (50 * 1000);
ctl |= TIOCM_DTR|TIOCM_RTS;
if (ioctl(iFd, TIOCMSET, &ctl) >= 0) // DTR and RTS high
{
usleep (50 * 1000);
if (write (iFd, bSyncString, 2) == 2)
if (read (iFd, bBuffer, sizeof (bBuffer)) > 0)
printf ("Received %s\n", bBuffer);
}
}
}
close (iFd);
}
}

Answer 1

Got it. I took the source of avrdude, extracted the relevant bits and then cut them down to the bone, pasting functionality back and forth until it worked.
Some of my serial settings were off and the delay before sending the write was too short so a minimal setup is as follows:
int main()
{
static const uint8_t bSyncString [] = {0x30, 0x20}; // {CMD_STK_GET_SYNC, SYNC_CRC_EOP};
uint8_t bBuffer [256];
unsigned int ctl;
struct termios tty;
int iFd = open ("/dev/ttyACM0", O_RDWR | O_NOCTTY);
if (iFd >= 0)
{
// From STK500 spec: 115.2kbps, 8 data bits, 1stop bits, no parity
if (tcgetattr (iFd, &tty) == 0)
{
tty.c_iflag = IGNBRK;
tty.c_oflag = 0;
tty.c_lflag = 0;
tty.c_cflag = (CS8 | CREAD | CLOCAL);
tty.c_cc[VMIN] = 1;
tty.c_cc[VTIME] = 0;
cfsetospeed(&tty, B115200);
cfsetispeed(&tty, B115200);
if (tcsetattr(iFd, TCSANOW, &tty) == 0)
printf ("Attributes set\n");
}
if (ioctl (iFd, TIOCMGET, &ctl) >= 0) // Get current line states
{
ctl &= ~(TIOCM_DTR|TIOCM_RTS);
if (ioctl(iFd, TIOCMSET, &ctl) >= 0) // DTR and RTS low
{
usleep (50 * 1000);
ctl |= TIOCM_DTR|TIOCM_RTS;
if (ioctl(iFd, TIOCMSET, &ctl) >= 0) // DTR and RTS high
usleep (250 * 1000);
}
}
if (write (iFd, bSyncString, 2) == 2)
{
int iRead = read (iFd, bBuffer, sizeof (bBuffer));
if (iRead > 0)
{
printf ("Received %s\n", bBuffer);
for (int i = 0; i < iRead; i++)
printf ("%02x ", bBuffer [i]);
printf ("\n");
}
}
close (iFd);
}
}