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Re: snprintf return value

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Il 10/12/2018 11:24, Reinhardt Behm ha scritto:
> AT Monday 10 December 2018 18:20, pozz wrote:
> 
>> I usually have a static buffer and I don't want to use a dynamic buffer.
>>
>> char buf[BUFLEN];
>>
>> I have to construct a long string, and it is quite easy to call snprintf
>> multiple times:
>>
>>     int n = 0;
>>     n += snprintf(&buf[n], BUFLEN - n, ...);
>>     n += snprintf(&buf[n], BUFLEN - n, ...);
>>     n += snprintf(&buf[n], BUFLEN - n, ...);
>>     n += snprintf(&buf[n], BUFLEN - n, ...);
>>
>> I was convinced the return value was the real number of characters
>> written in buf, not counting final '\0'. In this case, the above code
>> works.
>>
>> I just noticed the return value of snprintf() is the number of
>> characters of the generated string as if the buffer was long enough. So
>> the above code doesn't work.
>>
>> The return value of snprintf() is useful if you want to dynamically
>> reallocate the buffer so it can store all the string. In my case I would
>> prefer to truncate the string at the end of the static buffer. The above
>> code should be redesigned as:
>>
>>     int n = 0;
>>     n += snprintf(&buf[n], BUFLEN - n, ...);
>>     if (n >= BUFLEN - n) return;
>>     n += snprintf(&buf[n], BUFLEN - n, ...);
>>     if (n >= BUFLEN - n) return;
>>     n += snprintf(&buf[n], BUFLEN - n, ...);
>>     if (n >= BUFLEN - n) return;
>>     n += snprintf(&buf[n], BUFLEN - n, ...);
>>     if (n >= BUFLEN - n) return;
>>
>> The code starts being not readable. I'm thinking to create a new
>> snprintf2():
>>
>>     int
>>     snprintf2(char *str, size_t size, const char *format, ...) {
>>         va_list args;
>>
>>         va_start(args, format);
>>         int n = vsnprintf(str, size, format, args);
>>         va_end(args);
>>         return (n >= size) ? (size - 1) : n;
>>     }
> 
> Keep in mind that snprintf can even return negative values as error codes.

Could you provide a test case for a negative return value from snprintf()?

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snprintf return value pozz <pozzugno@gmail.com> - 2018-12-10 11:20 +0100
  Re: snprintf return value Reinhardt Behm <rbehm@hushmail.com> - 2018-12-10 18:24 +0800
    Re: snprintf return value pozz <pozzugno@gmail.com> - 2018-12-10 11:32 +0100
      Re: snprintf return value Philipp Klaus Krause <pkk@spth.de> - 2019-01-01 12:59 +0100
  Re: snprintf return value David Brown <david.brown@hesbynett.no> - 2018-12-10 13:15 +0100
  Re: snprintf return value Clifford Heath <no.spam@please.net> - 2018-12-11 08:49 +1100
    Re: snprintf return value Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2018-12-11 11:41 -0500
      Re: snprintf return value Clifford Heath <no.spam@please.net> - 2018-12-12 10:34 +1100
        Re: snprintf return value David Brown <david.brown@hesbynett.no> - 2018-12-12 09:09 +0100
          Re: snprintf return value Clifford Heath <no.spam@please.net> - 2018-12-12 21:38 +1100
        Re: snprintf return value Les Cargill <lcargill99@comcast.com> - 2018-12-30 11:23 -0600
  Re: snprintf return value Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2018-12-11 00:47 +0100
    Re: snprintf return value Clifford Heath <no.spam@please.net> - 2018-12-11 13:31 +1100

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