C/C++ 中带有“cout”的函数返回的 char 指针和 char 指针的奇怪行为

Question

I have a strange behavior with a char pointer initialized by the value of a return function and with the cout.

All my code is for an Arduino application, this is why I use char pointer, char array and string.h. I created a class named FrameManager , with a function getDataFromFrame to extract data from a string (in fact a char array). See above:

`char * FrameManager::getDataFromFrame ( const char frame[], char key[] )
{
  char *pValue = nullptr;
  int frameLength = strlen ( frame );
  int previousStartIndex = 0;
  for ( int i=0; i<frameLength; i++ ) {
    char c = frame[i];
    if ( c == ',' ) {
        int buffSize = i-previousStartIndex+1;
        char subbuff[buffSize];
        memset ( subbuff, 0, buffSize ); //clear buffer
        memcpy ( subbuff, &frame[previousStartIndex], i-previousStartIndex );
        subbuff[buffSize]='\0';
        previousStartIndex = i+1;

        int buffLength = strlen ( subbuff );
        const char *ptr = strchr ( subbuff, ':' );
        if ( ptr ) {
            int index = ptr-subbuff;
            char buffKey[index+1];
            memset ( buffKey, 0, index+1 );
            memcpy ( buffKey, &subbuff[0], index );
            buffKey[index+1]='\0';
            char buffValue[buffLength-index];
            memset ( buffValue, 0, buffLength-index );
            memcpy ( buffValue, &subbuff[index+1], buffLength-index );
            buffValue[buffLength-index]='\0';

            if ( strcmp ( key,buffKey ) == 0 ) {
                pValue = &buffValue[0];
                break;
            }
        }
    } else if ( i+1 == frameLength ) {
        int buffSize = i-previousStartIndex+1;
        char subbuff[buffSize];
        memcpy ( subbuff, &frame[previousStartIndex], frameLength-1 );
        subbuff[buffSize]='\0';
        int buffLength = strlen ( subbuff );
        const char *ptr = strchr ( subbuff, ':' );
        if ( ptr ) {
            int index = ptr-subbuff;
            char buffKey[index+1];
            memset ( buffKey, 0, index+1 );
            memcpy ( buffKey, &subbuff[0], index );
            buffKey[index+1]='\0';
            char buffValue[buffLength-index];
            memset ( buffValue, 0, buffLength-index );
            memcpy ( buffValue, &subbuff[index+1], buffLength-index );
            buffValue[buffLength-index]='\0';

            if ( strcmp ( key,buffKey ) == 0 ) {
                pValue = &buffValue[0];
                break;
            }
        }
    }
}

return pValue;

}`

In the main(), I created juste a little code to test the returned value:

int main(int argc, char **argv) {

  const char frame[] = "DEVICE:ARM,FUNC:MOVE_F,PARAM:12,SERVO_S:1";
  FrameManager frameManager;
  char key[] = "DEVICE";
  char *value; 
  value = frameManager.getDataFromFrame(frame, &key[0]);
  cout << "Retrieved value: " << value << endl;
  cout << "Retrieved value: " << frameManager.getDataFromFrame(frame, &key[0]) << endl;

  printf("%s",value);

  return 0;
}

and here the result:

• Retrieved value: y%R
• Retrieved value: ARM
• ARM

The first "cout" doesn't display the expected value. The second "cout" display the expected value and the printf too.

I don't understand what is the problem with the first "cout".

Thanks

Jocelyn

Answer 1

pValue points into local arrays, which get out of scope. That's undefined behavior. It might work, but your program might also crash, return wrong values (that's what you experience), corrupt your data or do any other arbitrary action.

Given that you're already using C++, consider using std::string as a result instead or point into the original frame (if possible).