Turning on aled on beagleboard from user space

Linux kernel documentation here.
Using MMap

void TurnOnLED ()
{
   int fd = open("/dev/mem", O_RDWR);
    volatile ulong *A;
    if (fd < 0)
    {
        printf("Could not open /dev/mem\n");
        return;
    }
    A = (ulong*) mmap(NULL, 0x10000, PROT_READ | PROT_WRITE,
            MAP_SHARED, fd, 0x49050000);
    if (A == MAP_FAILED)
    {
        printf("Mapping IO failed\n");
        close(fd);
        return;
    }
    A[0x603C / 4] |= 0x600000;  
    munmap(A, 0x10000);
    close (fd);
}

the use of "volatile"  tells the compiler that each use of the variable A in the program must result in an actual load or store instruction.

Using sysfs
#Export the led pin:
echo 139 > /sys/class/gpio/export

for i in `seq 1 10`;
do
    #Turn on the led
    echo "high" >  /sys/class/gpio/gpio139/direction
    sleep 1
    #Turn off the led
    echo "low"  > /sys/class/gpio/gpio139/direction
    sleep 1
done
       
#unexport the led pin
echo 139 > /sys/class/gpio/unexport

calculating the pin io instruction can be found in the following video: