Zephyr fopen() working in native posix application

[Dharshini-user]

Hi all,
I am Trying to build a Zephyr application as a native POSIX application, which has to run in WSL based Linux environment .My application involves making littlefs file mount,opening a file using fopen() ,writing hello into and closing the file.The application will use the text file thus created for some processing. The compilation is successful and the execution is not successful.
During execution lfs file mount is successful
case1: if path is /lfs/file_name.txt, then fopen fails stating that there is no such file or directory.
Case2: if path is just file_name.txt, then fopen opens a file in the Linux directory from where I run the Zephyr executable.
So I would like to know

1. Why fopen() fails to work with lfs mount?
   2.How to make fopen() work for lfs mounts?
   3.Suppose if I build the same application for a hardware and load it to the hardware board.Then in that case , my application will be able to use only lfs file mount structures right? So, based on the above working scenario,fopen cannot work with lfs file mounts and here we don't have a host file ( similar to Linux file system during Posix builds) system.How file access and operations could be made possible in this hardware case, using fopen?
   Thanks in advance.
   Best regards,
   Dharshini S

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[aescolar]

@Dharshini-user
One thing to be aware when building targeting native_sim (or native_posix) , is that if you forget to enable the Zephyr component that provides some function, but that function exists in the host C library, the call from the embedded code will eventually be linked against the host C library.
In your case, what seems to be happening is that you forgot to set CONFIG_POSIX_API=y in your application configuration,
and therefore are linking with your C library fopen(). And as a result when calling fopen you are opening a file in your host filesystem relative to where your zephyr.exe is executing.

If you forget to enable the needed components when building for a real HW target, the linking will instead just fail telling you fopen() does not exist.

If you really want to use fopen() from your embedded code you need to set CONFIG_POSIX_API in your project. But note that fopen is a shim on top of Zephyr's layer.
Maybe you wanted to use fs_open() ?https://docs.zephyrproject.org/latest/doxygen/html/group__file__system__api.html#ga9c90031ba3e5a10da8e00e81d53ef63b

[Dharshini-user]

@aescolar
Thankyou for your immediate response.
I am trying to use fopen() only. Though I set the option CONFIG_POSIX_API. I still get the same error message.
I couldn't find fopen() implementation(which internally calls the fs_open() ) in my Zephyr code. And I am using Zephyr version -3.6.99.
i)Has it got something to do with the version?
ii) I actually want my native POSIX application to create a file in the littlefs file mount, using fopen(). How could I do that, if CONFIG_POSIX_API is also throwing the same error?