devctl(),devctlv()

Control a device

Synopsis:

#include <sys/types.h>
#include <unistd.h>
#include <devctl.h>

int devctl( int filedes,
            int dcmd,
            void * dev_data_ptr,
            size_t n_bytes,
            int * dev_info_ptr );

int devctlv( int filedes,
             int dcmd,
             int sparts,
             int rparts,
             const iov_t *sv,
             const iov_t *rv,
             int *dev_info_ptr);

Arguments:

filedes
A file descriptor that you obtained by opening the device.
dcmd
A device-specific command for the process managing the open device. The set of valid device-control commands, the associated data interpretation, the returned dev_info_ptr values, and the effect of the command on the device all depend on the device driver.
For: See:
General information Device-control commands,” below
Commands for manipulating processes Controlling processes via the /proc filesystem in the Processes chapter of the QNX Neutrino Programmer's Guide
Specific commands Devctl and Ioctl Commands and the <sys/dcmd_*.h> header files
dev_data_ptr
(devctl() only) Depending on the command, this argument is one of:
  • a pointer to a buffer containing data to be passed to the driver
  • a receiving area for data coming from the driver
  • both of the above
  • NULL
n_bytes
(devctl() only) The size of the data to be sent to the driver, or the maximum size of the data to be received from the driver. If the size is greater than SSIZE_MAX - sizeof(io_devctl_t) (see <limits.h>), the function fails and sets errno to EOVERFLOW.
sparts
(devctlv() only) The number of elements in the sv array. The maximum number of entries is UIO_MAXIOV, and the sum of sparts and rparts must not exceed 524288, or the function fails and sets errno to EOVERFLOW.
rparts
(devctlv() only) The number of elements in the rv array. The maximum number of entries is UIO_MAXIOV, and the sum of sparts and rparts must not exceed 524288, or the function fails and sets errno to EOVERFLOW.
sv
(devctlv() only) An array of buffers that hold the data to send to the driver. The combined length of these buffers and those in rv must not exceed SSIZE_MAX, or the function fails and sets errno to EOVERFLOW.
rv
(devctlv() only) An array of buffers in which the driver can store data to return to the caller. The combined length of these buffers and those in sv must not exceed SSIZE_MAX, or the function fails and sets errno to EOVERFLOW.
dev_info_ptr
NULL, or a pointer to a location that the device can use to return additional status information instead of just success or failure. The data returned via dev_info_ptr depends on the device driver.

Library:

libc

Use the -l c option to qcc to link against this library. This library is usually included automatically.

Description:

The devctl() function sends the device-specific command dcmd to the process managing the device opened as filedes. For example, you can send commands to specify properties for devices such as keyboards, sound cards or serial ports.

The devctlv() function is similar to devctl(), but uses I/O vectors for passing data to and from the driver.

Device-control commands

Use these macros to set up the device-control commands:
__DIOF(class, cmd, data)
Get information from the device.
__DION(class, cmd)
Send a command with no associated data.
__DIOT(class, cmd, data)
Pass information to the device.
__DIOTF(class, cmd, data)
Pass some information to the device, and get some from it.
The arguments to these macros are:
class
The major category for the command. The device-control commands are divided into the following classes to make organization easier:
  • _DCMD_ALL — Common (all I/O servers).
  • _DCMD_CAM — Low-level (Common Access Method) devices, such as disks or CD-ROMs.
  • _DCMD_CHR — Character devices.
  • _DCMD_FSYS, _DCMD_BLK — Filesystem/block I/O managers.
  • _DCMD_INPUT — Input devices.
  • _DCMD_IP — Internet Protocol.
  • _DCMD_MEM — Memory card.
  • _DCMD_MISC — Miscellaneous commands.
  • _DCMD_MIXER — Mixer (Audio).
  • _DCMD_NET — Network devices.
  • _DCMD_PROC — Process manager.
cmd
The specific command in the class.
data
The type of data to pass to and/or from the device. The dev_data_ptr argument to devctl() must be a pointer to this type of data, and n_bytes is usually the size of this type of data.
Note: The size of the structure that's passed as the last field to the __DIO* macros must be less than 2^14 == 16 KB. Anything larger than this interferes with the upper two directional bits.
Resource managers can use the following macros, which are defined in <devctl.h>, when handling commands:
get_device_command(cmd)
Extract the class and the specific device command from cmd (i.e., strip off the data type and the direction).
get_device_direction(cmd)
Get the direction of the command (DEVDIR_TO, DEVDIR_FROM, DEVDIR_TOFROM, or DEVDIR_NONE).
Note: If you're writing a resource manager, note that the iofunc_devctl_default() function handles the DCMD_ALL_GETFLAGS and DCMD_ALL_SETFLAGS commands; your resource manager will need to handle all other devctl commands that apply to it.

Returns:

EOK
Success.
EAGAIN
The devctl() command couldn't be completed because the device driver was in use by another process, or the driver was unable to carry out the request due to an outstanding command in progress.
EBADF
Invalid open file descriptor, filedes.
EINTR
The devctl() function was interrupted by a signal.
EINVAL
The device driver detected an error in dev_data_ptr or n_bytes.
EIO
The devctl() function couldn't complete because of a hardware error.
EFAULT
A fault occurred trying to access the buffers provided.
ENOTTY
The device doesn't support the dcmd command.
ENXIO
No such device or address. For example, there's no media available to be loaded in the device represented by the file descriptor.
EOVERFLOW
One of the following occurred:
  • An attempt was made to send or receive an amount of data that when added to the size of the device control structure exceeds the allowable limit.
  • (devctlv() only) The sum of the IOV lengths for the send and receive buffers combined exceeds SSIZE_MAX, or the number of parts for the send and receive buffers combined exceeds 524288.
EPERM
The process doesn't have sufficient permission to carry out the requested command.

Specific devctl() commands might return other error codes; see Devctl and Ioctl Commands.

Examples:

Example 1: Setting RTS on a serial port

Here's a quick example of setting and unsetting RTS (Request to Send) on a serial port:
/* For "devctl()" */
#include <devctl.h>
#include <sys/dcmd_chr.h>

/* For "open()" */
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

/* For "strerror()" */
#include <string.h>

/* For Errors */
#include <stdlib.h>
#include <stdio.h>

int check_RTS(int fd);

int main(void)
{
    int data = 0, fd, error;

    if((fd = open ("/dev/ser2", O_RDONLY)) == -1)
    {
     fprintf(stderr,
        "Error with open() on /dev/ser2.  Make sure it exists.\n");
     perror (NULL);
     exit(EXIT_FAILURE);
    }

    check_RTS(fd);

    /* Let's turn ON RTS now. */
    data = _CTL_RTS_CHG | _CTL_RTS;

    if (error = devctl (fd, DCMD_CHR_SERCTL, &data,
                   sizeof(data), NULL))
    {
    fprintf(stderr, "Error setting RTS: %s\n",
        strerror ( error ));
        exit(EXIT_FAILURE);
    }
    /* RTS should now be ON. */

    check_RTS(fd);

    sleep (2);


    /* Now let's turn RTS OFF. */
    data = _CTL_RTS_CHG | 0;

    if (error = devctl (fd, DCMD_CHR_SERCTL, &data,
                   sizeof(data), NULL))
    {
        fprintf(stderr, "Error setting RTS: %s\n",
            strerror ( error ));
        exit(EXIT_FAILURE);
    }
    /* RTS should now be OFF. */

    check_RTS(fd);

    close(fd);

    return (1);
}


int check_RTS(int fd)
{
    int data = 0, error;

    /*
     Let's see if RTS is set by using devctl() to get
     line status information.
    */
    if (error = devctl (fd, DCMD_CHR_LINESTATUS, &data,
                        sizeof(data), NULL))
    {
       fprintf(stderr, "Error getting RTS: %s\n",
           strerror ( error ));
       exit(EXIT_FAILURE);
    }

    if (data & _LINESTATUS_SER_RTS)
        printf("RTS is SET!\n");
    else
        printf("RTS is NOT set\n");

    return(1);
}

The two main areas of interest are the setting of data and the devctl() call. The data variable is used for both sending and receiving data.

When setting RTS, data is assigned a value that's sent to the device via devctl().
If data equals: RTS is turned:
_CTL_RTS_CHG | _CTL_RTS ON
_CTL_RTS_CHG OFF

When checking to see if RTS is set, we call devctl() with dcmd set to the DCMD_CHR_LINESTATUS macro and data containing any value (zero is clean). The devctl() function returns with data containing the Line Status value. This then can be used to determine what lines are set on that device. In our example, we check against _LINESTATUS_SER_RTS.

To find out what values to use with different DCMD_* commands, look in the appropriate <sys/dcmd_*.h> header file. For example, you'll find macros for the following values under DCMD_CHR_LINESTATUS in <sys/dcmd_chr.h>:

The value that's in the header is a bitwise AND with the value in data to see if the value is high for that line.

Example 2: Cycling through Caps Lock, Num Lock, and Scroll Lock

In the following example, we open the device /dev/kbd and we start applying changes to the Caps Lock, Scroll Lock, and Num Lock properties.

The key lines in this example are the same as in the last example; they focus around the data variable. This value is just a simple integer value that's passed into the devctl() function. The data variable is assigned its values by simply performing a bitwise OR to the predefined values in the </usr/include/sys/dcmd_chr.h> header. Note the values used in the bitwise OR:
  • _CONCTL_NUM_CHG (Console Control Num Lock Change) ORed together with _CONCTL_NUM (Console Control Num Lock) turns on Num Lock.
  • _CONCTL_NUM_CHG on its own turns off Num Lock.
If data equals: Num Lock is turned:
_CONCTL_NUM_CHG | _CONCTL_NUM ON
_CONCTL_NUM_CHG OFF
This also applies for the other either/or values in the <dcmd_chr.h> header.
/* For "devctl()" */
#include <devctl.h>
#include <sys/dcmd_chr.h>

/* For "open()" */
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

/* For "strerror()" */
#include <string.h>

/* For Errors */
#include <stdlib.h>
#include <stdio.h>


int main(void)
{
    int data, fd, toggle = 1, error;

    /* Open the device we wish to manipulate. */
    if((fd = open ("/dev/kbd", O_RDONLY)) == -1)
    {
     fprintf(stderr,
        "Error with open() on /dev/kbd.  Make sure it exists.\n");
     perror (NULL);
     exit(EXIT_FAILURE);
    }

    while(1)
    {
    switch(toggle)
    {
        case 1:
        {
          /*
           Turn on Num Lock and make sure that 
           Caps and Scroll lock are turned off.
          */
          data = (_CONCTL_NUM_CHG | _CONCTL_NUM) | _CONCTL_CAPS_CHG |
                 _CONCTL_SCROLL_CHG;
          break;
        }
        case 2:
        {
          /*
           Turn off Num Lock and now turn on Caps Lock 
           (Scroll lock is already off).
          */
          data = _CONCTL_NUM_CHG | (_CONCTL_CAPS_CHG | _CONCTL_CAPS);
          break;
        }
        case 3:
        {
          /*
           Turn off Caps lock and turn on Scroll lock 
           (Num lock is already off).
          */
          data = _CONCTL_CAPS_CHG | (_CONCTL_SCROLL_CHG | _CONCTL_SCROLL);
          toggle = 0;
          break;
        }
    }

    /* Explanation below. */
    if (error = devctl (fd, DCMD_CHR_SERCTL, &data,
                        sizeof(data), NULL))
    {
        fprintf(stderr, "Error setting KBD: %s\n",
            strerror ( error ));
        exit(EXIT_FAILURE);
    }

    sleep(1);
    toggle++;
    }

    return (1);
}
Here's a quick explanation of the above devctl() call:
devctl (fd, DCMD_CHR_SERCTL, &data, sizeof(data), NULL)

The first parameter, fd, is the file descriptor of the device that's being changed. The second parameter is the device class that's being changed. In this case, it's a character device DCMD_CHR, with a “subclass” of _SERCTL. The third parameter is the data variable; this is the ORed value.

Example 3: Duration example

In this code, tcdropline(), which is used to disconnect a communications line, uses devctl() (this is the actual source code, tcdropline() is a standard library function):
#include <termios.h>
#include <devctl.h>
#include <errno.h>
#include <sys/dcmd_chr.h>

int tcdropline(int fd, int duration) {
    int error; 

    duration = ((duration ? duration : 300) << 16) |
      _SERCTL_DTR_CHG | 0;

    if(error = devctl(fd, DCMD_CHR_SERCTL, &duration,
                  sizeof duration, 0) == -1) {
       if(error == ENOSYS) {
          errno = ENOTTY;
       }
       return -1;
    }
    return 0;
}

Classification:

QNX Neutrino

Safety:  
Cancellation point Yes
Interrupt handler No
Signal handler Yes
Thread Yes

Caveats:

When devctl() fails, the effect of the failed command depends on the device driver. The corresponding data might be transferred, partially transferred, or not transferred at all.

The devctl() function was originally part of the POSIX 1003.1d draft standard; but it was deprecated in the IEEE Approved Draft 10 standard.