How does this C code (from lua library, Torch) even compile/work? -

see https://github.com/torch/nn/blob/master/generic/tanh.c

for example,

 static int nn_(tanh_updateoutput)(lua_state *l) {    thtensor *input = luat_checkudata(l, 2, torch_tensor);    thtensor *output = luat_getfieldcheckudata(l, 1, "output", torch_tensor);     thtensor_(resizeas)(output, input);     if (input->ndimension == 1 || !thtensor_(iscontiguous)(input) || !thtensor_(iscontiguous)(output))    {     th_tensor_apply2(real, output, real, input,   \      *output_data = tanh(*input_data););     }   else   {    real* ptr_output = thtensor_(data)(output);    real* ptr_input  = thtensor_(data)(input);    long i;  #pragma omp parallel private(i) for(i = 0; < thtensor_(nelement)(input); i++)   ptr_output[i] = tanh(ptr_input[i]); } return 1; } 

first, don't know how interpret first line:

 static int nn_(tanh_updateoutput)(lua_state *l) 

what arguments here? tanh_updateoutput refer to? "nn_" have special meaning?

second, "th_tensor_apply2" , "thtensor_(...)" both used don't see defined? there no other includes in file?

nn_ macro. can find definition searching repository "#define nn_"; it's in init.c:

#define nn_(name) th_concat_3(nn_, real, name) 

you can keep following chain of macro definitions, , you'll end token pasting thing makes nn_(tanh_updateoutput) expand name of function.

(it's weird generic/tanh.c doesn't have includes; generic/tanh.c must included other file. that's unusual .c files.)