[chbot] Weirdness in parameter interpretation in C lib function

[Simeon Pilgrim]

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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