bug#35062: [PATCH v3 2/3] constify a bit of xterm.c

[Paul Eggert]

I'd rather not use 'const' on locals. Let me tell you a story as to why.

I once worked with someone who insisted on using the 'register' keyword on every 
C local variable that was never taken the address of, on the grounds that this 
meant the reader could easily see that the variable could never be modified 
indirectly via a pointer and that this made the code easier to read because you 
didn't need to worry about aliasing.

I disagreed then, and still disagree. Saying 'register' nearly all the time 
clutters up the code, and the cost is not worth the benefit in C. It's pretty 
easy for a human reader to determine whether a local variable is taken the 
address of somewher in its function. (If it's hard, then write an Elisp function 
that will tell you. :-) In hindsight, perhaps C should have been designed so 
that 'register' was the default for local variables, and that one needed a 
special word to say "watch out! this variable might have its address taken!"; 
but the ship has sailed.

'const' is like 'register' in this respect. Putting 'const' nearly everywhere 
clutters C code. It's pretty easy for a human reader to determine whether a 
local variable is modified (if it's hard, write an Elisp function :-). In 
hindsight, perhaps C should have been designed so that 'const' was the default 
for local variables; but the ship has sailed there too.