Get number of characters read by sscanf?

Question

I'm parsing a string (a char*) and I'm using sscanf to parse numbers from the string into doubles, like so:

// char* expression;
double value = 0;
sscanf(expression, "%lf", &value);

This works great, but I would then like to continue parsing the string through conventional means. I need to know how many characters have been parsed by sscanf so that I may resume my manual parsing from the new offset.

Obviously, the easiest way would be to somehow calculate the number of characters that sscanf parses, but if there's no simple way to do that, I am open to alternative double parsing options. However, I'm currently using sscanf because it's fast, simple, and readable. Either way, I just need a way to evaluate the double and continue parsing after it.

score 54 · Accepted Answer · edited Feb 13 '15 at 05:01

You can use the format specifier %n and provide an additional int * argument to sscanf():

int pos;
sscanf(expression, "%lf%n", &value, &pos);

Description for format specifier n from the C99 standard:

No input is consumed. The corresponding argument shall be a pointer to signed integer into which is to be written the number of characters read from the input stream so far by this call to the fscanf function. Execution of a %n directive does not increment the assignment count returned at the completion of execution of the fscanf function. No argument is converted, but one is consumed. If the conversion specification includes an assignment suppressing character or a field width, the behavior is undefined.

Always check the return value of sscanf() to ensure that assignments were made, and subsequent code does not mistakenly process variables whose values were unchanged:

/* Number of assignments made is returned,
   which in this case must be 1. */
if (1 == sscanf(expression, "%lf%n", &value, &pos))
{
    /* Use 'value' and 'pos'. */
}

I can't seem to make it work for the [Microsoft implementation](https://msdn.microsoft.com/en-us/library/t6z7bya3.aspx?f=255&MSPPError=-2147217396) as such: `sscanf_s(p, "%s %d\n%n", toString, (int)_countof(toString), &itemCount, &pos);` The `pos` doesn't seem to change at all. — c00000fd, Nov 05 '17 at 08:00

score 4 · Answer 2 · answered Nov 21 '12 at 22:35

int i, j, k;
char s[20];

if (sscanf(somevar, "%d %19s %d%n", &i, s, &j, &k) != 3)
    ...something went wrong...

The variable k contains the character count up to the point where the end of the integer stored in j was scanned.

Note that the %n is not counted in the successful conversions. You can use %n several times in the format string if you need to.

Get number of characters read by sscanf?

2 Answers2

Linked