C Struct values not being assigned - c

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.

Answer 2

See kernel docs for struct information
The reserved[] array for the v4l2_ext_control struct is only of length 2...so you're modifying memory that you shouldn't be with the line:
extControl.reserved2[2] = 0;