Dev Notes

Software Development Resources by David Egan.

Integer Input in C


C, Linux
David Egan

Objective: get an integer input from stdin.

At first glance, scanf() looks like a reasonable way to collect integer input. The function allows formatted input to be collected (it’s name comes from the words “scan formatted”).

To scan integer input, first declare an integer and pass a pointer to this to scanf:

int input;
scanf("%d", &input);

However, because stdin is inherently unconstrained (users can type anything), scanf() may not be the best choice. You need to handle whitespace characters appropriately as well as clearing the input stream for subsequent input because scanf() doesn’t read newlines.

Note that fscanf() which scans a FILE stream and sscanf() which scans a string are rational and useful choices because they operate on structured data.

fgets: Read the Entire Line

It may be better to read the whole line using fgets(), and then process the input as required:

  • Use fgets() to read an entire line of stdin within a while loop.
  • Check the length of the input buffer - even though fgets() discards extra input, the user should be informed if input has exceeded available space and allowed to re-enter the data.
  • Convert input to an integer using strtol().
  • If input meets the length constraint and can be interpreted as an integer, the loop ends.

Example: Read Integer

// file: integer-input.h
#ifndef INTEGER_INPUT_H
#define INTEGER_INPUT_H

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>

#define MAX_DIGITS 12

/**
 * This function removes surplus characters from the input buffer.
 * Otherwise, if more than the permitted number of characters have been entered during the
 * call to fgets(), the surplus characters (after MAX_DIGITS chars) remain in the input buffer
 * and will be wrongly accepted as input on the next iteration of the loop.
 * */
static inline void ClearInputBuffer() 
{
	char c = 0;
	// Loop over input buffer and consume chars until buffer is empty
	while ((c = getchar()) != '\n' && c != EOF);
}

void getIntegerFromStdin(int *inputInteger)
{
	char *inputBuffer = malloc(sizeof(char) * MAX_DIGITS);
	memset(inputBuffer, 0, MAX_DIGITS);
	char *input = NULL;
	while (input == NULL) {
		// Note that fgets returns inputBuffer on success.
		// This becomes important when freeing - free either `input` or
		// `inputBuffer` to avoid an attempted double-free error.
		input = fgets(inputBuffer, MAX_DIGITS, stdin);
		
		// If fgets() receives less than MAX_DIGITS, the last char in the array is '\n'.
		// Therefore if the last char is not '\n', too many characters were entered.
		if (inputBuffer[strlen(inputBuffer) - 1] != '\n') {
			fprintf(stderr, "[ERROR]: Too many characters: max input is %d chars.\n", MAX_DIGITS);
			ClearInputBuffer();
			input = NULL;
			continue;
		}

		// Check that the input can be intepreted as an integer
		// Convert to integer using `strtol()`
		errno = 0;
		char *endptr = NULL;
		*inputInteger = strtol(input, &endptr, 10);
		
		// If an integer was not found, endptr remains set to input
		if (input == endptr) {
			// Remove trailing newline by adding NUL at the index of the
			// terminating '\n' character. See man strcspn - this function
			// gets the length of a prefix substring.
			input[strcspn(input, "\n")] = 0;
			printf("Invalid input: no integer found in %s.\n", input);
			input = NULL;
		}
		if (errno != 0) {
			fprintf(stderr, "[ERROR]: That doesn't look like an integer.\n");
			input = NULL;
		}
	}
	free(inputBuffer);
}

#endif

Example of usage:

// file: main.c
#include <stdio.h>
#include "integer-input.h"

int main()
{
	puts("Enter integer:");
	int x;
	getIntegerFromStdin(&x);
	printf("You entered: %d\n", x);
	return 0;
}

References

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