Re: RFC: Followup to PR91593

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Re: RFC: Followup to PR91593

jerry DeLisle-3
Copying gcc list for additional thoughts on a possible bogus warning.

On 9/29/19 9:02 AM, Jerry DeLisle wrote:
> Hi all,
>
--- snip ---

> diff --git a/libgfortran/io/write.c b/libgfortran/io/write.c
> index 4ef35561fdd..fc046efbe34 100644
> --- a/libgfortran/io/write.c
> +++ b/libgfortran/io/write.c
> @@ -1031,7 +1031,7 @@ btoa_big (const char *s, char *buffer, int len,
> GFC_UINTEGER_LARGEST *n)
>     else
>       {
>         const char *p = s + len - 1;
> -      for (i = 0; i < len; i++)
> +      for (i = 0; i < len - 1; i++)
>       {
>         char c = *p;
>

--- snip ---

The first attempt to fix (above) is completely off.  I have tried various
combinations of code changes and I am beginning to think the warning is bogus:

In function ‘btoa_big’,
     inlined from ‘write_b’ at ../../../trunk/libgfortran/io/write.c:1217:11:
../../../trunk/libgfortran/io/write.c:1052:6: warning: writing 1 byte into a
region of size 0 [-Wstringop-overflow=]
  1052 |   *q = '\0';
       |   ~~~^~~~~~

Using gdb I have watched the pointer address stored in q and the setting of the
string of bytes doing the binary to ascii conversion. I have also checked the
length of the buffer being used and its is what I would expect with length of 129.

However, the warning only goes away if I add an additional 8 bytes to the buffer
(suspicious).

So doing the following eliminates the warning:

diff --git a/libgfortran/io/write.c b/libgfortran/io/write.c
index 4ef35561fdd..fd0e46851e4 100644
--- a/libgfortran/io/write.c
+++ b/libgfortran/io/write.c
@@ -1204,7 +1204,7 @@ void
  write_b (st_parameter_dt *dtp, const fnode *f, const char *source, int len)
  {
    const char *p;
-  char itoa_buf[GFC_BTOA_BUF_SIZE];
+  char itoa_buf[GFC_BTOA_BUF_SIZE + 8];
    GFC_UINTEGER_LARGEST n = 0;

    if (len > (int) sizeof (GFC_UINTEGER_LARGEST))

Any suggestions? I am certainly not seeing it.

Regards,

Jerry