popen Segfault

Sample Excerpt

....

FILE *fp;

....

while( true ){

    if(fp == nullptr) pclose(fp);    //  <---seg. fault happens here. (see below)
    // or
    //if(fp == 0)       pclose(fp);
    // or
    //if(fp == NULL) pclose(fp);

    ...
    fp = popen(...);
    ...
    pclose(fp);
 
    //-------------------------------------------------------------------------------
    //  NOTE: The above segfault will evaporate if you manually nullify fp.
    //-------------------------------------------------------------------------------
    fp = nullptr;      //  <---- This will fix the above segfault.
    //-------------------------------------------------------------------------------
    ...
}