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strtonum(3bsd) 3bsd strtonum(3bsd)


strtonumreliably convert string value to an integer


library “libbsd”


#include <limits.h>
#include <stdlib.h> (See libbsd(7) for include usage.)
long long
strtonum(const char *nptr, long long minval, long long maxval, const char **errstr);


The () function converts the string in nptr to a long long value.

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, () stores an error string in *errstr indicating the failure.


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 did not consist solely of digit characters; or minval was larger than maxval.
The given string was out of range.

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

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


atof(3), atoi(3), atol(3), atoll(3), sscanf(3), strtod(3), strtoi(3bsd), strtol(3), strtoll(3), strtou(3bsd), strtoul(3), strtoull(3)


strtonum() is an OpenBSD extension.


The strtonum() function first appeared in OpenBSD 3.6. strtonum() was redesigned in NetBSD 8.0 as strtoi(3bsd) and strtou(3bsd).


The strtonum() function was designed to facilitate safe, robust programming and overcome the shortcomings of the atoi(3) and strtol(3) family of interfaces, however there are problems with the strtonum() API:

  • will return 0 on failure; 0 might not be in range, so that necessitates an error check even if you want to avoid it
  • does not differentiate 'illegal' returns, so we can't tell the difference between partial and no conversions
  • returns english strings
  • can't set the base, or find where the conversion ended
  • hardcodes long long integer type
To overcome the shortcomings of strtonum() NetBSD provides strtou(3bsd) and strtoi(3bsd).
January 18, 2015 Linux 6.4.0-150600.23.7-default