Override global function like fopen/ofstream

Question

I want to know what file is opened and in what mode and for this I want to override the global functions like fopen/ofstream and other file handling function

Required:

    FILE* fopen(filename, mode){ //Our own fopen
    printf("%s-%s",filename, mode)// print filename na dmode
    return fopen(filename, mode); // actual fopen
    }

    void main(){
    FILE * fp = fopen("name.txt", "w");
    fprintf(fp, "%s", "hello World");
    fclose(fp);
    }

Output should be: name.txt-w

One solution is to create a wrapper with some other name for fopen but as fopen is called from various places I don't want to use different name

Is there a way to implement above scenario?

please pick the language you are actually using. At least one of the tags is offtopic — 463035818_is_not_an_ai, Jul 26 '21 at 11:23
What is calling `fopen` - your own code or some library function? — Paul Sanders, Jul 26 '21 at 11:25
This is called “interposing.” Searching for “interposing” or “interposition” will yield further information. it is operating-system specific, so there is not a general answer; you need to say which operating system(s) you want to do this for. — Eric Postpischil, Jul 26 '21 at 11:51
You could make a wrapper macro to keep the name: `#define fopen(fname, mode) (printf("%s-%s", (fname), (mode)), fopen((fname), (mode)))`. Should work as an expression as well. — mediocrevegetable1, Jul 26 '21 at 12:39
@mediocrevegetable1 Returning a pointer like fopen won't be possible from a macro — Sumit Trehan, Jul 26 '21 at 12:49
@SumitTrehan Just tested (with C++ but should be same for C) and it seems to be returning the correct type thanks to the use of the comma operator: https://godbolt.org/z/ohY8jao6a — mediocrevegetable1, Jul 26 '21 at 12:54
Works in C too: https://onlinegdb.com/BNTQurouU Run the code and then check the generated file. — mediocrevegetable1, Jul 26 '21 at 13:03
@mediocrevegetable1 Ok I was not using the comma operator correctly. — Sumit Trehan, Jul 26 '21 at 14:34

Override global function like fopen/ofstream

0 Answers0