Question

Is it possible to change the value being pointed to by a FILE pointer inside a function in C by passing by reference?

Here is an example to try and illustrate what I’m trying to do, I can modify the integer but not the file pointer, any ideas?

int main (void)
{
    FILE* stream;
    int a = 1;
    configure_stream(rand() % 100, &a, &stream);
    return 0;
}

void configure_stream(int value, int* modifyMe, FILE* stream)
{
    *modifyMe = rand() % 100;
    if (value > 50)
    {
        *stream = stderr;
    }
    else
    {
        *stream = stdout;
    }
}

Answer 1

I think you want to declare your configure_stream as follows

void configure_stream(int value, int* modifyMe, FILE** stream)
{
    *modifyMe = rand() % 100;
    if (value > 50)
    {
        *stream = stderr;
    }
    else
    {
        *stream = stdout;
    }
}

and use it with

configure_stream(rand() % 100, &a, &stream)

This gets a pointer pointing to a FILE pointer. With the pointer to the pointer you can modify the FILE pointer and without loosing it in the big jungle of memory