Manual Pages  — STRTONUM

strtonum – reliably convert string value to an integer

SYNOPSIS DESCRIPTION RETURN VALUES EXAMPLES ERRORS SEE ALSO STANDARDS HISTORY

#include <stdlib.h>

long long
strtonum(const char *nptr, long long minval, long long maxval, const char **errstr);

The strtonum() function returns the result of the conversion, unless the value would exceed the provided bounds or is invalid. On error, 0 is returned, errno is set, and errstr will point to an error message. On success, *errstr will be set to NULL; this fact can be used to differentiate a successful return of 0 from an error.

Using strtonum() correctly is meant to be simpler than the alternative functions.

int iterations;
const char *errstr;

iterations = strtonum(optarg, 1, 64, &errstr);
if (errstr != NULL)
        errx(1, "number of iterations is %s: %s", errstr, optarg);

The above example will guarantee that the value of iterations is between 1 and 64 (inclusive).

[ERANGE]
	The given string was out of range.
[EINVAL]
	The given string did not consist solely of digit characters.
[EINVAL]
	The supplied minval was larger than maxval.

If an error occurs, errstr will be set to one of the following strings:

too large	The result was larger than the provided maximum value.
too small	The result was smaller than the provided minimum value.
invalid	The string did not consist solely of digit characters.

The strtonum() function first appeared in OpenBSD 3.6 .