[chbot] Weirdness in parameter interpretation in C lib function

[Robin Gilks]

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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