Question

I’m trying to use the Video4Linux2 API, but something peculiar is happening with the structs that I’m supposed to use to change the various controls on a given camera. For some reason, some of the members aren’t reporting back as changed on assignment. I wrote the following code to simplify the problem:

#include <getopt.h>
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <sys/select.h>
#include <linux/videodev2.h>
#include <stdlib.h>
#include <errno.h>
#include <stdbool.h>

int setExtendedControl(int id, int value);
int fd = 0;

int main()
{
    setExtendedControl(20, 30);
    return 0;
}

int setExtendedControl(int id, int value)
{
    struct v4l2_ext_control extControl;
    struct v4l2_ext_controls extControls;

    extControl.id = id;
    extControl.value = value;
    extControl.value64 = value;
    extControl.reserved2[0] = 0;
    extControl.reserved2[1] = 0;
    extControl.reserved2[2] = 0;

    //Put the individual control structure into the
    //multi-control container structure and initialize the container
    //as well
    extControls.ctrl_class = V4L2_CTRL_CLASS_MPEG;
    extControls.count = 1;
    extControls.controls = &extControl;
    printf("extControls.controls = %i, extControl = %i\n", (extControls).controls, &extControl);



    extControls.reserved[0] = 0;
    extControls.reserved[1] = 0;
    extControls.reserved[2] = 0;
    extControls.error_idx = 0;

    printf("Verifying settings:\n");
    printf("extControl.id = %i, id = %i\n", extControl.id, id);
    printf("extControl.value = %i, value = %i\n", extControl.value, value);
    printf("extControl.value64 = %i, value = %i\n", extControl.value64, value);
    printf("extControl.reserved2[0] = %i, set to 0\n", extControl.reserved2[0]);
    printf("extControl.reserved2[1] = %i, set to 0\n", extControl.reserved2[1]);
    printf("extControl.reserved2[2] = %i, set to 0\n\n", extControl.reserved2[2]);

    printf("extControls.ctrl_class = %i, V4L2_CTRL_CLASS_MPEG = %i\n", extControls.ctrl_class,
            V4L2_CTRL_CLASS_MPEG);
    printf("extControls.count = %i, set to 1\n", extControls.count);
    printf("extControls.reserved[0] = %i, set to 0\n", extControls.reserved[0]);
    printf("extControls.reserved[1] = %i, set to 0\n", extControls.reserved[1]);
    printf("extControls.reserved[2] = %i, set to 0\n", extControls.reserved[2]);
    printf("extControls.error_idx = %i, set to 0\n", extControls.error_idx);

    printf ("\nRunning secondary check..\n\n");


    int rval;

    //Set up the individual control structure



    //Try to change the control and return the
    //value reporting the outcome.
    rval = ioctl(fd, VIDIOC_S_EXT_CTRLS, &extControls);

    if (extControls.controls != &extControl)
    {
        printf("\n\nLost the pointer on initial set!\n");
    }
    //printf("error_idx after initial set %i\n", extControls.error_idx);

    //printf("extControl = %i, extControls = %i\n");

    //freeStructs(extControl, extControls);

    return rval;
}

When I run this, the “value” and “value64” members aren’t set. The following is the printf outputs:

extControls.controls = -1954893344, extControl = -1954893344
Verifying settings:
extControl.id = 20, id = 20
extControl.value = 0, value = 30
extControl.value64 = 0, value = 30
extControl.reserved2[0] = 0, set to 0
extControl.reserved2[1] = 0, set to 0
extControl.reserved2[2] = 0, set to 0

extControls.ctrl_class = 10027008, V4L2_CTRL_CLASS_MPEG = 10027008
extControls.count = 1, set to 1
extControls.reserved[0] = 0, set to 0
extControls.reserved[1] = 0, set to 0
extControls.reserved[2] = 0, set to 0
extControls.error_idx = 0, set to 0

And here is a small snippet from gdb:

(gdb) step
printf (__fmt=0x400880 "extControls.controls = %i, extControl = %i\n") at /usr/include/x86_64-linux-gnu/bits/stdio2.h:105
105       return __printf_chk (__USE_FORTIFY_LEVEL - 1, __fmt, __va_arg_pack ());
(gdb) step
setExtendedControl (id=20, value=30) at ioctlTest.c:30
30          extControl.reserved2[0] = 0;
(gdb) step
Hardware watchpoint 2: extControl

Old value =
    {id = 4294960502, size = 32767, reserved2 = {4196309}, {value = 0, value64 = 67750802696962048, string = 0xf0b2ff00000000 <Address 0xf0b2ff00000000 out of bounds>}}
New value =
    {id = 20, size = 32767, reserved2 = {0}, {value = 0, value64 = 67750802696962048, string = 0xf0b2ff00000000 <Address 0xf0b2ff00000000 out of bounds>}}

It’s not happening here, but there have been other instances in a beefier version of this code where the pointer assignment for extControls.controls and int assignment of member extControl.id both fail. What would cause a struct member assignment failure of this nature?

Thanks in advance!

Answer 1

Can’t check this myself, but looking at this spec of struct v4l2_ext_control (Table 1), it says:

Table 1. struct v4l2_ext_control

__u32   id      Identifies the control, set by the application.
__u32   reserved2[2]        Reserved for future extensions. Drivers and applications must set the array to zero.

union   (anonymous)      
    __s32   value       New value or current value.
    __s64   value64     New value or current value.
    void *  reserved    Reserved for future pointer-type controls. Currently unused.

Note three things here:

• value and value64 are part of a union, they occupy the same location in memory
• reserved2 has size two, so only, reserved2[0], reserved2[1], no reserved2[2] !
• according to this spec, reserved memory address precedes the union within the struct

This means that when you set

extControl.reserved2[2] = 0;

you are actually writing outside the memory assigned to that array, overwriting the data of the union, thus altering “both” value and value64 members at the same time.