Question

我正在尝试使用Video4Linux2 API，但是我应该用来改变给定摄像机上的各种控件的结构会发生一些奇怪的事情。出于某种原因，一些成员在作业时没有报告变更。我编写了以下代码来简化问题：

#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;
}

当我运行它时，未设置“value”和“value64”成员。以下是printf输出：

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

这是gdb的一小段代码：

(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>}}

这里没有发生，但是在这个代码的更强大版本中还有其他实例，其中extControls.controls的指针赋值和成员extControl.id的int赋值都失败了。什么会导致这种性质的结构成员分配失败？

提前致谢！

Answer 1

有关结构信息，请参阅kernel docs

reserved[]结构的v4l2_ext_control数组只有长度为2 ...所以你要修改的内存不应该是这行：

extControl.reserved2[2] = 0;

Answer 2

我自己无法检查，但是看看spec of struct v4l2_ext_control（表1），它说：

表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.

请注意以下三点：

value和value64是联盟的一部分，它们占据内存中的相同位置
reserved2的大小为2，仅限reserved2[0]，reserved2[1]，无reserved2[2]！
根据此规范，reserved内存地址位于struct

这意味着当你设置

extControl.reserved2[2] = 0;

实际上是在分配给该数组的内存之外写入，覆盖了union的数据，从而同时改变了“value和value64成员”。

C Struct值未分配

2 个答案: