[chbot] Weirdness in parameter interpretation in C lib function

[Simeon Pilgrim]

```
char* test(uint8_t index)
{
#define DEST_BUF_SIZE 100

  if (index < NUMRESULTS) {
    static char buf[DEST_BUF_SIZE];

    snprintf(buf, DEST_BUF_SIZE, "SSID %s\n", scandata[index].ssid);
    return buf;
  }
  return "INVALID INDEX";
}
```

On Tue, 30 Jul 2024 at 11:05, Simeon Pilgrim <simeon.pilgrim at gmail.com>
wrote:

> you have provided a pointer (to memory), AND you have declared it 30 bytes
> long. This is what YOU HAVE done. The compiler is HELPING YOU by pointing
> out 37 > 30, which it is.
>
> If you want the compiler to help you correctly, tell the compiler the
> length of the data you have, which is 100.
>
>
>
>
>
> On Tue, 30 Jul 2024 at 10:58, Robin Gilks <gb7ipd at gmail.com> wrote:
>
>> But I've provided 100 bytes, not 30!!
>>
>> On Tue, Jul 30, 2024 at 10:40 AM Simeon Pilgrim
>> <simeon.pilgrim at gmail.com> wrote:
>> >
>> > the second parameter should be the size of the destination buffer, thus
>> the code should be:
>> >
>> > snprintf (buf, 100, "SSID %s\n",
>> >
>> > or
>> >
>> > snprintf (buf, sizeof(buf), "SSID %s\n",
>> >
>> > your format string is 7 tokens (6 + zero termination) + the 30 byte
>> string ssid, which is larger than 30 you have provided.
>> >
>> > The snprintf functions are "safe truncation" functions, not handy dandy
>> truncation utilities, thus the warning that you might have truncation.
>> >
>> > On Tue, 30 Jul 2024 at 10:27, Robin Gilks <gb7ipd at gmail.com> wrote:
>> >>
>> >> Since there are a few C programmers in the group I thought I'd ask
>> >> this (interesting?) question
>> >>
>> >> Here is a bit of sample code to illustrate:
>> >>
>> >>
>> ----------------------------------------------------------------------------
>> >> #include <stdio.h>
>> >> #include <stdint.h>
>> >>
>> >>
>> >> typedef struct
>> >> {
>> >> char ssid[30];
>> >> int8_t rssi;
>> >> } ScanResult;
>> >>
>> >> #define NUMRESULTS 50 // how many tracked simultaneously
>> >>
>> >> static ScanResult scandata[NUMRESULTS];
>> >>
>> >> void
>> >> main (void)
>> >> {
>> >> }
>> >>
>> >>
>> >> char * test (uint8_t index)
>> >> {
>> >> static char buf[100];
>> >>
>> >> snprintf (buf, sizeof (scandata[index].ssid), "SSID %s\n",
>> >> scandata[index].ssid);
>> >> return buf;
>> >>
>> >> }
>> >>
>> ----------------------------------------------------------------------------
>> >> Save as test.c; compile with gcc test.c
>> >>
>> >> The warning indicates that the destination buffer may be too small
>> >>
>> >> test.c: In function ‘test’:
>> >> test.c:25:56: warning: ‘%s’ directive output may be truncated writing
>> >> up to 29 bytes into a region of size 25 [-Wformat-truncation=]
>> >>    25 |    snprintf (buf, sizeof (scandata[index].ssid), "SSID %s\n",
>> >> scandata[index].ssid);
>> >>       |                                                        ^~
>> >> test.c:25:4: note: ‘snprintf’ output between 7 and 36 bytes into a
>> >> destination of size 30
>> >>    25 |    snprintf (buf, sizeof (scandata[index].ssid), "SSID %s\n",
>> >> scandata[index].ssid);
>> >>       |
>> >>
>> >> This appear to be treating the size parameter in the snprintf as being
>> >> the size of the output buffer 'buf' (which is 100 bytes long) but
>> >> surely it should be applying the restriction on the format string that
>> >> includes  scandata[index].ssid (which is 30 bytes long)
>> >>
>> >> Am I just having brain fade or is that just fundamentally wrong?
>> >>
>> >> --
>> >> Robin Gilks
>> >>
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