The body of the loop clearly deserves {} around it

!do_dealloc

return ()

Nit: No need for this 'blank line' between the copyright notices.

Might be remove list of suffixes from all sizes, just typeset it as a meta-symbol.
Then note somewhere that that size is a number with following accepted suffixes.

I hope you see what I mean.

Any reason to not include do_dealloc into next line?

Where is this enforced?

Isn't it useful to be able to omit the length and create a hole from the specified offset until EOF?

I thought it could be easily misused to accidentally turn a file into a completely sparse when you forgot to type the preceding dash for instance.

Around line 145, but I reordered it to be in the same line below this comment block.

Why? A signal most likely comes from user trying to stop the operation. Instead, the utility would try to restart.

In fact I do not think that EINTR is possible, because you do not install any signal handlers, but still I believe that this is simply wrong to do.

The ';' should be on separate line.

I do not understand this at all.

I think it would be cleaner if you added 'do_truncate' control var, set to 1 by default. This 'else' is quite confusing, I must admit.

At a quick glance, it looks like SPACECTL_DEALLOC is only defined if __BSD_VISIBLE, hence throwing:

/Users/runner/work/freebsd-src/freebsd-src/usr.bin/truncate/truncate.c:206:22: error: use of undeclared identifier 'SPACECTL_DEALLOC'
                        r = fspacectl(fd, SPACECTL_DEALLOC, &sr, 0, &sr);