Reference: `scanf` Function

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Basic usage

To read one integer from standard input, use

int variable;
scanf("%d", &variable);  // pass an address!

Similar to printf, a format specifier %d tells scanf that it needs to read a number, which will be placed in the specified location in memory pointed to by &variable.

Wondering what & means? Go read the page about pointers first, then come back here.

Reading multiple variables

int x, y, z;
scanf("%d%d%d", &x, &y, &z);

This usage will assume that there are spaces or linebreaks between values, e.g. 1 2 42. If there's something else, like slashes, you could let scanf know that it must skip delimiters. Let's read a date:

int year, month, day;
scanf("%d/%d/%d", &year, &month, &day);

This will accept inputs like 2025/5/11.

`scanf` returns number of values read

Sometimes you don't know if your input has finished, or not. It is important to know that scanf returns number of values successfully read. In the above example with the dates, if everything is fine, it will return 3. If not, it might return 0 if no values were read (maybe there was something but not a number), 1 if only one value was there, and so on.

The best way to use scanf is to always compare it with the expected number of parameters you read.

if (scanf("%d/%d/%d", &year, &month, &day) != 3) {
	printf("invalid input!\n");
}

Reading a string

scanf can read words with %s. It will stop reading when it finds a space or a line break character. Note that a string is an array of characters, and an array can be "decayed" to a pointer to its 0-th element, so you don't need to take an address when reading into a character array:

char str[100];
scanf("%s", str);  // overflow is possible!

To prevent a possible overlow (reading beyond the boundary of your array), you can limit the number of characters that scanf will read. Remember to leave one extra character to terminate your string with a zero!

char str[100];
scanf("%99s", str); // will not read more than 99 characters

If you want to read the whole line, not stopping at the first space character, use fgets instead:

char str[100];
fgets(str, 100, stdin); // be careful: the last character might not be 0

Skip a value

Sometimes you want scanf to read a value but you don't want to store it in a variable. Use %*d (or %*s, etc.) to skip one value:

int a, b;
scanf("%d %*d %d", &a, &b); // reads 3 numbers, but stores only two

Other formats

Here are format specifiers that you might need to use:

%d: signed integers
%u: unsigned integers
%f: float variables
%lf: double variables
%s: strings
%lld: long long int variables
%llu: unsigned long long int variables

Playground

If you want to test how scanf works, do it here!

#include <stdio.h>

int main() {
	int year, month, day;
	
	if (scanf("%d/%d/%d", &year, &month, &day) != 3) {
		printf("invalid input!\n");
		return 1; /* we return a non-zero exit code if there's an error */	
	}

	printf("we read year: %d, month: %d, day: %d\n", year, month, day);
	return 0;
}

Reference: scanf Function