I would like to know how to output the coordinates of each point(actually faces) of the inlet plane.
I tried to use the point_coordinates of n_points, but n_points --> Local number of points, this way can only output values for a small part of the entry plane.
What are the corresponding global variables (in the C function)?
There are no global variables for this, beyond the metadata (number of elements).
To output the global info, the easiest solution is to generate an additional postprocessing sub-mesh (or restart location) for this. Otherwise, to do this in a user-defined function, you need some specific parallel operations, since the data can be distributed across multiple processors. You can check the example in src/user_examples/cs_user_extra_operations-nusselt_calculation.c for something similar.
Actually, I want to divide the inlet plane into different regions and then assign different velocity fluctuations values for each area. So, my original idea was to divide the inlet plane according to the point coordinates. Since there is no such global variable, could I use some functions like cs_selector_get_b_face_list to get every element information of the plane?
Besides, I tried to output the point coordinates using the SEM method, I find that the total number of output points(n_points) was always 820, no matter how many entities I defined. It seems that the local number is a fixed value. Perhaps, I thought, is it possible to increase this fixed value?
Yes, you can use cs_selector_get_b_face_list with additional geometric criteria (for example) to select sub-zones.
You can also define your own virtual blocks and check for each face center in the main selection to which sub-block it belongs (assuming a division in rectangular sections), with a bit of programming in user-defined functions.
Regarding the SEM point coordinates, I am not familiar enough with the method to provide an answer. I would need to check the code in more detail to explain this.