Bytevector literals are marked as ‘SCM_F_BYTEVECTOR_IMMUTABLE’, but VM
instructions that access bytevectors do not check that flag, which can
lead to segfaults:
--8<---------------cut here---------------start------------->8---
$ cat t.scm
(use-modules (rnrs bytevectors))
(define bv #vu8(1 2 3))
(bytevector-u8-set! bv 0 1)
$ guild compile -O2 t.scm
wrote `/home/ludo/.cache/guile/ccache/3.0-LE-8-4.6/data/src/guile-3.0/t.scm.go'
$ guile t.scm
Segmentation fault
--8<---------------cut here---------------end--------------->8---
Conversely, the subrs from bytevector.c do check the flag:
--8<---------------cut here---------------start------------->8---
$ guild compile -O0 t.scm
wrote `/home/ludo/.cache/guile/ccache/3.0-LE-8-4.6/data/src/guile-3.0/t.scm.go'
$ guile t.scm
Backtrace:
In ice-9/boot-9.scm:
1752:10 6 (with-exception-handler _ _ #:unwind? _ # _)
In unknown file:
5 (apply-smob/0 #<thunk 7f2396b0e2e0>)
In ice-9/boot-9.scm:
724:2 4 (call-with-prompt ("prompt") #<procedure 7f2396b20c60 …> …)
In ice-9/eval.scm:
619:8 3 (_ #(#(#<directory (guile-user) 7f2396b14c80>)))
In ice-9/boot-9.scm:
2836:4 2 (save-module-excursion #<procedure 7f2396b06300 at ice-…>)
4388:12 1 (_)
In unknown file:
0 (bytevector-u8-set! #vu8(1 2 3) 0 1)
ERROR: In procedure bytevector-u8-set!:
In procedure bytevector-u8-set!: Wrong type argument in position 1 (expecting
mutable bytevector): #vu8(1 2 3)
$ guile --version
guile (GNU Guile) 3.0.8
Copyright (C) 2021 Free Software Foundation, Inc.
License LGPLv3+: GNU LGPL 3 or later <http://gnu.org/licenses/lgpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
--8<---------------cut here---------------end--------------->8---
I suppose the immutability test would have to be done not at the level
of individual VM instructions but at a higher-level in generated code,
such that the compiler could lift the test outside loops that access
bytevectors, similar to what it does with ‘heap-object?’.
Ludo’.
