c - 通过 C 中的函数传递指针变量

Question

我在这里遇到了一些奇怪的行为。任何帮助都会很棒。

我是这样开始的:

   int *event_positions = (int *) malloc(1 * sizeof(int)); // let us start with 1 and then add more within the method. This should continue until we have all the flags we want.  
   int number_of_flags = event_extractor(vocal_data, size, event_positions);


 // HERE I WOULD LIKE TO USE THE VALUES OF event_positions BUT THE ARE WEIRD I.E. THEY DON'T MATCH THE VALUES BEING PRINTED IN THE LAST METHOD. 

然后事件提取器将变量传递给另一个方法。简化后看起来像这样:

int event_extractor (int *audio_samples, unsigned int size_of_audio ,int *event_flags)
{

    int number_of_flags = apply_threshold (lopass_samples, length, event_flags);
    // PRINT ARRAY event_flags HERE 
    // VALUES ARE INCORRECT AND WEIRD

}

最后一个方法:

int apply_threshold (int *audio_samples, unsigned int size_of_audio, int *event_flags)
{
// DO SOME STUFF HERE.
// PRINT THE ARRAY WHICH SHOW THE  CORRECT VALUES.



}

我希望这是清楚的。基本上我有一个数组，我将其作为参数传递，并且在该方法完成后无法访问这些值。

---

编辑 1

第一个文件:

   int *event_positions = (int *) malloc(1 * sizeof(int)); // let us start with 1 and then add more within the method. This should continue until we have all the flags we want.  
   int number_of_flags = event_extractor(vocal_data, size, event_positions);

第二个文件:

  int apply_threshold (int *audio_samples, unsigned int size_of_audio, int *event_flags)
{


int flag = 0; // this will be the number of flags that I have 
bool run = true; // this will make sure that a minimum amount of time passes before I grab another flag. It's a guard.
int counter = 0; // this is the counter for the above guard. 





printf("\n\nCURRENT MINIMUM TIME:  20100 SAMPLES \n\n");

// event_flags[0] = 1; // this first one is a dud. within the loop we will automatically start adding flags


int threshold = calculate_threshold_value(audio_samples, size_of_audio);

printf("\n\n this is the threshold %d \n\n", threshold);

int length = (int)size_of_audio;

for (int i = 0; i < length; i++) 
{

    if (audio_samples[i] > threshold  && run) 
    {

        // ** is this realloc working ?
        event_flags = (int*)realloc(event_flags, sizeof(int) * (flag+1)); // reallocate the size of the array
        event_flags[flag] = i;
        // printf("FLAG CREATED! %i\n ", i);
        printf("EVENT FLAG %i  %i\n",flag, event_flags[flag] );
        flag++;
        run = false;

    }   

    if (!run) {
        counter++;
        if (counter > 20100) { // hardcode minimum size for now. 
            counter = 0;
            run=true;
        }
    }

}

printf("\n\n\n NUMBER OF EVENTS --- %d\n", flag);

for (int i = 0; i < flag; i++) {
    printf("FLAG %i  -- %d\n", i, event_flags[i]);
}



printf("\nFIVE samples before and after my second flag: \n 0 should indicate a reach in the threshold\n");

for (int i = 0; i <10 ; i++) {
    printf("VOCAL SAMPLE %i  %i \n", i-5,audio_samples[event_flags[1]+i-5] );
}


return flag;



}

---

编辑 2

我已经根据 Erik 的模型更新了我的代码。我的循环现在看起来像这样

    if (audio_samples[i] > threshold  && run) 
    {

        // ** is this realloc working ?
        // event_flags = (int*)realloc(event_flags, sizeof(int) * (flag+1));
        *event_flags = (int*)realloc(*event_flags, sizeof(int) * (flag+1)); // reallocate the size of the array
        *event_flags[flag] = i;
        // printf("FLAG CREATED! %i\n ", i);
        printf("EVENT FLAG %i  %i\n",flag, *event_flags[flag] );
        flag++;
        run = false;

    }   

现在我收到一个看起来像这样的错误。有任何想法吗？

Answer 1

您是否在 apply_threshold 中重新分配 event_flags？如果是，则需要让调用者取回更新后的指针。

类似于:

int apply_threshold (int *audio_samples, unsigned int size_of_audio, int **event_flags) {
    *event_flags = realloc ...
}

...

int number_of_flags = apply_threshold (lopass_samples, length, &event_flags);

编辑:回应更新的问题:

event_flags = (int*)realloc(event_flags, sizeof(int) * (flag+1)); // reallocate the size of the array

这会更改 event_flags 指针的本地副本。来电者将看不到更改。使用我上面描述的方法。

EDIT2:更详细的示例。

void foo(int * v) {
    v = 0; // The local copy of main's myvar is now 0. main's actual myvar is unchanged
}

void bar(int ** v) {
    *v = 0; // Main's myvar is now 0, we have a pointer to it and can modify it.
}

int main() {
    int * myvar = (int *) malloc(1); // Allocate 1 byte and make myvar point at this byte.
    foo(myvar); // Call foo, passing a *copy of* myvar, which also points at the allocated byte
    bar(&myvar); // Call bar, passing a *pointer to* myvar, which again points to the allocated byte
}

EDIT3:回应新问题。

您的“长度”是整数数 还是字节数？您将其视为整数数量，如果它确实是字节数，这可能会导致您的错误。