How to capture length of sscanf'd string?

I'm parsing a string that follows a predictable pattern:

1. 1 character
2. an integer (one or more digits)
3. 1 colon
4. a string, whose length came from #2

For example:

s5:stuff

I can see easily how to parse this with PCRE or the like, but I'd rather stick to plain string ops for the sake of speed.

I know I'll need to do it in 2 steps because I can't allocate the destination string until I know its length. My problem is gracefully getting the offset for the start of said string. Some code:

unsigned start = 0;
char type = serialized[start++]; // get the type tag
int len = 0;
char* dest = NULL;
char format[20];
//...
switch (type) {
  //...
  case 's':
    // Figure out the 开发者_开发问答length of the target string...
    sscanf(serialized + start, "%d", &len);
    // <code type='graceful'>
    // increment start by the STRING LENGTH of whatever %d was
    // </code>
    // Don't forget to skip over the colon...
    ++start;
    // Build a format string which accounts for length...
    sprintf(format, "%%%ds", len);
    // Finally, grab the target string...
    sscanf(serialized + start, format, string);
    break;
  //...
}

That code is roughly taken from what I have (which isn't complete because of the issue at hand) but it should get the point across. Maybe I'm taking the wrong approach entirely. What's the most graceful way to do this? The solution can either C or C++ (and I'd actually like to see the competing methods if there are enough responses).

You can use the %n conversion specifier, which doesn't consume any input - instead, it expects an int * parameter, and writes the number of characters consumed from the input into it:

int consumed;

sscanf(serialized + start, "%d%n", &len, &consumed);
start += consumed;

(But don't forget to check that sscanf() returned > 0!)

Use the %n format specifier to write the number of characters read so far to an integer argument.

Here's a C++ solution, it could be better, and is hard-coded specifically to deal with your example input, but shouldn't require much modification to get working.

std::stringstream ss;

char type;
unsigned length;
char dummy;
std::string value;

ss << "s5:Helloxxxxxxxxxxx";

ss >> type;
ss >> length;
ss >> dummy;
ss.width(length);
ss >> value;

std::cout << value << std::endl;

Disclaimer:

I'm a noob at C++.

You can probably just use atoi which will ignore the colon.

e.g. len = atoi(serialized + start);

The only thing with atoi is that if it returns zero it could mean either the conversion failed, or that the length was truly zero. So it's not always the most appropriate function.

if you replace you colon with a space scanf will stop on it and you can get the size malloc the size then run another scanf to get the rest of the string`

int main (int argc, const char * argv[]) {
char foo[20];
char *test;

scanf("%s",foo); //"hello world"
printf("foo = %s\n", foo);//prints hello
//get size
    test = malloc(sizeof(char)* 10);//replace 10 with your string size
    scanf("%s", test);
printf("test = %s\n", test);//prints world

return 0;
}

`

Seems like the format is overspecified... (using a variable length field to specify the length of a variable length field).

If you're using GCC, I'd suggest

if (sscanf(serialized,"%c%d:%as",&type,&len,&dest)<3) return -1;
/* use type, dest; ignore len */
free(dest);
return 0;