The pretty printer treats precision in %s directives as a request to print exactly as many characters from the string argument when what precision normally (in C) means is the maximum number of characters to read from the string. It doesn't mean to read past the terminating NUL.
The attached patch fixes that. Tested on x86_64-linux. Martin
PR c++/81586 - valgrind error in output_buffer_append_r with -Wall gcc/ChangeLog: PR c++/81586 * pretty-print.c (pp_format): Correct the handling of %s precision. diff --git a/gcc/pretty-print.c b/gcc/pretty-print.c index 570dec7..a79191b 100644 --- a/gcc/pretty-print.c +++ b/gcc/pretty-print.c @@ -667,7 +667,17 @@ pp_format (pretty_printer *pp, text_info *text) } s = va_arg (*text->args_ptr, const char *); - pp_append_text (pp, s, s + n); + + /* Negative precision is treated as if it were omitted. */ + if (n < 0) + n = INT_MAX; + + /* Append the lesser of precision and strlen (s) characters. */ + size_t len = strlen (s); + if ((unsigned) n < len) + len = n; + + pp_append_text (pp, s, s + len); } break;
