http://devdot.wikispaces.com/Iphone+Serial+Port+Tutorial
Requirement:
1.Jailbroken Iphone with BSD Subsystem installed (Search the web for jailbreak tutorials)
2.Iphone development toolchain for Mac OSX, Linux, Windows (Cygwin) or other OS. I use this http://code.google.com/p/winchain/ in Windows.
3.Sparkfun Ipod connector or breakout board see http://www.sparkfun.com/commerce/product_info.php?products_id=633 or http://www.sparkfun.com/commerce/product_info.php?products_id=8295
4.PC USB UART – optional, but used in this tutorial. You will need either a 3.3V level UART for PC (see http://www.sparkfun.com/commerce/product_info.php?products_id=718) or you will need an old style 12V level serial cable with a level converter like this one http://www.compsys1.com/workbench/On_top_of_the_Bench/Max233_Adapter/max233_adapter.html (see warning below)
5.A soldering iron will help you out 
6.Microsoft Visual Studio – optional, but used in the source code examples for the PC serial port communication.
WARNING! Do not try to use a 12V level RS232 port for this without a level converter to ~3V, it will severely damage your Iphone or render it non-functional.
Hardware:
The Dock Port
In the Ipod/Iphone dock port, the pins we are concerned with are as follows
Pin 1
Ground
Pin 18
3.3V Power (+)
Pin 12
TX also known as Serial Transmit
Pin 13
RX also known as Serial Receive
To see a full description of all of the pins in the dock connector, see here: http://pinouts.ru/Devices/ipod_pinout.shtml
Connections: iPhone/iPod Touch RX should connect to TX of the connected device, TX to RX of the connected device, and Ground to the Ground of the connected device. If your device can be powered by a low amperage, 3.3V power source you may chose to connect PIN 18 as well and power your device directly from the iPhone/iPod Touch.
Sample Codes:
#include <stdio.h> /* Standard input/output definitions */
#include <string.h> /* String function definitions */
#include <unistd.h> /* UNIX standard function definitions */
#include <fcntl.h> /* File control definitions */
#include <errno.h> /* Error number definitions */
#include <termios.h> /* POSIX terminal control definitions */
static struct termios gOriginalTTYAttrs;
static int OpenSerialPort()
{
int fileDescriptor = -1;
int handshake;
struct termios options;
// Open the serial port read/write, with no controlling terminal, and don't wait for a connection.
// The O_NONBLOCK flag also causes subsequent I/O on the device to be non-blocking.
// See open(2) ("man 2 open") for details.
fileDescriptor = open("/dev/tty.iap", O_RDWR | O_NOCTTY | O_NONBLOCK);
if (fileDescriptor == -1)
{
printf("Error opening serial port %s - %s(%d).\n",
"/dev/tty.iap", strerror(errno), errno);
goto error;
}
// Note that open() follows POSIX semantics: multiple open() calls to the same file will succeed
// unless the TIOCEXCL ioctl is issued. This will prevent additional opens except by root-owned
// processes.
// See tty(4) ("man 4 tty") and ioctl(2) ("man 2 ioctl") for details.
if (ioctl(fileDescriptor, TIOCEXCL) == -1)
{
printf("Error setting TIOCEXCL on %s - %s(%d).\n",
"/dev/tty.iap", strerror(errno), errno);
goto error;
}
// Now that the device is open, clear the O_NONBLOCK flag so subsequent I/O will block.
// See fcntl(2) ("man 2 fcntl") for details.
if (fcntl(fileDescriptor, F_SETFL, 0) == -1)
{
printf("Error clearing O_NONBLOCK %s - %s(%d).\n",
"/dev/tty.iap", strerror(errno), errno);
goto error;
}
// Get the current options and save them so we can restore the default settings later.
if (tcgetattr(fileDescriptor, &gOriginalTTYAttrs) == -1)
{
printf("Error getting tty attributes %s - %s(%d).\n",
"/dev/tty.iap", strerror(errno), errno);
goto error;
}
// The serial port attributes such as timeouts and baud rate are set by modifying the termios
// structure and then calling tcsetattr() to cause the changes to take effect. Note that the
// changes will not become effective without the tcsetattr() call.
// See tcsetattr(4) ("man 4 tcsetattr") for details.
options = gOriginalTTYAttrs;
// Print the current input and output baud rates.
// See tcsetattr(4) ("man 4 tcsetattr") for details.
printf("Current input baud rate is %d\n", (int) cfgetispeed(&options));
printf("Current output baud rate is %d\n", (int) cfgetospeed(&options));
// Set raw input (non-canonical) mode, with reads blocking until either a single character
// has been received or a one second timeout expires.
// See tcsetattr(4) ("man 4 tcsetattr") and termios(4) ("man 4 termios") for details.
cfmakeraw(&options);
options.c_cc[VMIN] = 1;
options.c_cc[VTIME] = 10;
// The baud rate, word length, and handshake options can be set as follows:
cfsetspeed(&options, B19200); // Set 19200 baud
options.c_cflag |= (CS8); // RTS flow control of input
printf("Input baud rate changed to %d\n", (int) cfgetispeed(&options));
printf("Output baud rate changed to %d\n", (int) cfgetospeed(&options));
// Cause the new options to take effect immediately.
if (tcsetattr(fileDescriptor, TCSANOW, &options) == -1)
{
printf("Error setting tty attributes %s - %s(%d).\n",
"/dev/tty.iap", strerror(errno), errno);
goto error;
}
// Success
return fileDescriptor;
// Failure "/dev/tty.iap"
error:
if (fileDescriptor != -1)
{
close(fileDescriptor);
}
return -1;
}
