strtonum man page

strtonum — reliably convert string value to an integer


library “libbsd”


#include <limits.h>
#include <bsd/stdlib.h>

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


The strtonum() function converts the string in nptr to a long long value. The strtonum() function was designed to facilitate safe, robust programming and overcome the shortcomings of the atoi(3) and strtol(3) family of interfaces.

The string may begin with an arbitrary amount of whitespace (as determined by isspace(3)) followed by a single optional ‘+’ or ‘-’ sign.

The remainder of the string is converted to a long long value according to base 10.

The value obtained is then checked against the provided minval and maxval bounds. If errstr is non-null, strtonum() stores an error string in *errstr indicating the failure.

Return Values

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) 
	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).



The given string was out of range.


The given string did not consist solely of digit characters.


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.


The string did not consist solely of digit characters.

See Also

atof(3), atoi(3), atol(3), atoll(3), sscanf(3), strtod(3), strtol(3), strtoul(3)


The strtonum() function is a BSD extension. The existing alternatives, such as atoi(3) and strtol(3), are either impossible or difficult to use safely.


The strtonum() function first appeared in OpenBSD 3.6.


April 29, 2004