In Raspberry Pi 4, I am trying to read a series of files(more than 1000 files) in the specific directory with the fopen function in the for loop, but fopen cannot read the file if it exceeds a certain number of iterations. How do I solve this?
When you open file using fopen the system uses a file descriptor to point to that file. And there are only so many of them available. A quick  google search says microsoft usually has 512 file descriptors. Also, the file is loaded to memory. So, loading a lot of files will use up your memory fast. You should close each file after you are done with them. This usually is not a problem when working with a few files. But in case like yours where thousands of files are necessary, they should be closed as soon as possible after using them.
Using fopen in Raspberry pi4