What is the use of the %n format specifier in C?

c printf
josh picture josh · Aug 4, 2010 · Viewed 104.4k times · Source

What is the use of the %n format specifier in C? Could anyone explain with an example?

Answer

jamesdlin picture jamesdlin · Aug 4, 2010

Most of these answers explain what %n does (which is to print nothing and to write the number of characters printed thus far to an int variable), but so far no one has really given an example of what use it has. Here is one:

int n;
printf("%s: %nFoo\n", "hello", &n);
printf("%*sBar\n", n, "");

will print:

hello: Foo
       Bar

with Foo and Bar aligned. (It's trivial to do that without using %n for this particular example, and in general one always could break up that first printf call:

int n = printf("%s: ", "hello");
printf("Foo\n");
printf("%*sBar\n", n, "");

Whether the slightly added convenience is worth using something esoteric like %n (and possibly introducing errors) is open to debate.)

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