Use Files in 8M Nano

I am using the 8m nano development kit.
I want to use files, but the open function gives me the error:

warning: implicit declaration of function ‘open’; did you mean ‘popen’? [-Wimplicit-function-declaration]
43 | fd_Serial = open (MODEMDEVICE, O_RDWR | O_NOCTTY);
also with the types O_RDWR | O_NOCTTY.

In my main.c I have included
#include .

When I try to search the module with the find command, it returns the following error:
root @ ccimx8mn-dvk: / usr # find / -type f -name “types.h”
[1152.265429] EXT4-fs error (device mmcblk0p3): htree_dirblock_to_tree: 997: inode # 10378: comm find: Directory block failed checksum

I have solved the problem with the open function.
But the error when I do a find on the module, I would like to know what it is.