CODEWITHSUNDEEP
cpointers
jazzdawg
jazzdawg

Reputation: 564

Changing the value a file pointer points to inside a C function

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;
    }
}

Upvotes: 0

Views: 1704

Answers (3)

cnicutar
cnicutar

Reputation: 182764

Try

void configure_stream(int value, int* modifyMe, FILE **stream)
                                                      ^
    *stream = stderr;

What you were trying wasn't correct:

void configure_stream(int value, int* modifyMe, FILE* stream)
    *stream  = stderr; /* Assign FILE * to FILE. */

EDIT

You should call it: configure_stream(..., &stream);

Upvotes: 2

Wizz
Wizz

Reputation: 1267

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

Upvotes: 2

Nikki Locke
Nikki Locke

Reputation: 2951

You are passing configure_stream a pointer to a FILE pointer. So the parameter should be declared as FILE **

Upvotes: 1

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