Thread: change file mode

I made a function that receives a file pointer. That file pointer was set in write mode from main. How can i change it inside the function to read mode now? If i use fopen or freopen i have to give filename but don't we already have that information once the function receives the FILE pointer as argument? I don't want to create a new parameter just for this.

Code:

void ListStuff(FILE *f)
{
//How do i turn f into read mode here?

}

int main()
{
    
    
    char fileName[50];
    printf("file name? ");
    scanf("%s", fileName);
    char auxFile[50];
    sprintf(auxFile, "%s.txt", fileName);
    FILE *f;
    f=fopen(auxFile, "w");
    if(f==NULL)
    {
        printf("Error\n");
        return 0;
    }
    ListStuff(f);