Thanks for reporting your issue. Please make sure these boxes are checked before submitting your issue - thank you!

- [x] I looked for a similar issue and couldn't find any.
- [x] I tried with the latest version of GPAC. Installers available at https://gpac.io/downloads/gpac-nightly-builds/
- [x] I give enough information for contributors to reproduce my issue (meaningful title, github labels, platform and compiler, command-line ...). I can share files anonymously with this dropbox: https://www.mediafire.com/filedrop/filedrop_hosted.php?drop=eec9e058a9486fe4e99c33021481d9e1826ca9dbc242a6cfaab0fe95da5e5d95

Detailed guidelines: https://gpac.io/bug-reporting/

## OverView
I am trying to write m4a, m4v files like below   
![image](https://github.com/gpac/gpac/assets/80387186/d4e61a64-00c0-444f-ad68-a4d7ad2d0e84)   

How to write only moof and mdat using Gpac Module API?  

I am trying like below. but it failed to wrtie.  
```
file_ptr = gf_isom_open("output.mp4", GF_ISOM_OPEN_WRITE, NULL);
gf_isom_set_brand_info(file_ptr, GF_ISOM_BRAND_MP42, 0);
gf_isom_setup_track_fragment
gf_isom_start_fragment
gf_isom_fragment_add_sample
fwrite((void*)&file_ptr->moof, 1, sizeof(GF_MovieFragmentBox), wfp);
```
It is right to find it myself, but time is urgent so please understand that I am asking questions.
How to Write Only Moof and Mdat Boxes?
This is a GitHub Issue
repo:gpac
owner:gpac
Title : How to Write Only Moof and Mdat Boxes?
Issue date:

--- start body ---
Thanks for reporting your issue. Please make sure these boxes are checked before submitting your issue - thank you!

- [x] I looked for a similar issue and couldn't find any.
- [x] I tried with the latest version of GPAC. Installers available at https://gpac.io/downloads/gpac-nightly-builds/
- [x] I give enough information for contributors to reproduce my issue (meaningful title, github labels, platform and compiler, command-line ...). I can share files anonymously with this dropbox: https://www.mediafire.com/filedrop/filedrop_hosted.php?drop=eec9e058a9486fe4e99c33021481d9e1826ca9dbc242a6cfaab0fe95da5e5d95

Detailed guidelines: https://gpac.io/bug-reporting/

## OverView
I am trying to write m4a, m4v files like below   
![image](https://github.com/gpac/gpac/assets/80387186/d4e61a64-00c0-444f-ad68-a4d7ad2d0e84)   

How to write only moof and mdat using Gpac Module API?  

I am trying like below. but it failed to wrtie.  
```
file_ptr = gf_isom_open("output.mp4", GF_ISOM_OPEN_WRITE, NULL);
gf_isom_set_brand_info(file_ptr, GF_ISOM_BRAND_MP42, 0);
gf_isom_setup_track_fragment
gf_isom_start_fragment
gf_isom_fragment_add_sample
fwrite((void*)&file_ptr->moof, 1, sizeof(GF_MovieFragmentBox), wfp);
```
It is right to find it myself, but time is urgent so please understand that I am asking questions.
--- end body ---
