[chbot] Weirdness in parameter interpretation in C lib function

[Simeon Pilgrim]

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
> >>
> >> _______________________________________________
> >> Chchrobotics mailing list Chchrobotics at lists.ourshack.com
> >> https://lists.ourshack.com/mailman/listinfo/chchrobotics
> >> Mail Archives: http://lists.ourshack.com/pipermail/chchrobotics/
> >> Meetings usually 3rd Monday each month. See http://kiwibots.org for
> venue, directions and dates.
> >> When replying, please edit your Subject line to reflect new subjects.
> >
> > _______________________________________________
> > Chchrobotics mailing list Chchrobotics at lists.ourshack.com
> > https://lists.ourshack.com/mailman/listinfo/chchrobotics
> > Mail Archives: http://lists.ourshack.com/pipermail/chchrobotics/
> > Meetings usually 3rd Monday each month. See http://kiwibots.org for
> venue, directions and dates.
> > When replying, please edit your Subject line to reflect new subjects.
>
> _______________________________________________
> Chchrobotics mailing list Chchrobotics at lists.ourshack.com
> https://lists.ourshack.com/mailman/listinfo/chchrobotics
> Mail Archives: http://lists.ourshack.com/pipermail/chchrobotics/
> Meetings usually 3rd Monday each month. See http://kiwibots.org for
> venue, directions and dates.
> When replying, please edit your Subject line to reflect new subjects.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.ourshack.com/pipermail/chchrobotics/attachments/20240730/19ec1a00/attachment-0001.html>