Are there any practical applications for the format %n in printf/scanf family?

Question

int x;
printf("hello %n World\n", &x);
printf("%d\n", x);

Answer 1

It's not so useful for printf(), but it can be very useful for sscanf(), especially if you're parsing a string in multiple iterations. fscanf() and scanf() automatically advance their internal pointers by the amount of input read, but sscanf() does not. For example:

char stringToParse[256];
...
char *curPosInString = stringToParse;  // start parsing at the beginning
int bytesRead;
while(needsParsing())
{
    sscanf(curPosInString, "(format string)%n", ..., &bytesRead);  // check the return value here
    curPosInString += bytesRead;  // Advance read pointer
    ...
}