STRTONUM(3C) Standard C Library Functions STRTONUM(3C)

strtonum, strtonumx, strtounum, strtounumx — reliably convert string value to an integer

Standard C Library (libc, -lc)

#include <stdlib.h>

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

long long
strtonumx(const char *nptr, long long minval, long long maxval, const char **errstr, int base);

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

unsigned long long
strtounumx(const char *nptr, unsigned long long minval, unsigned long long maxval, const char **errstr, int base);

The () and strtonumx() functions convert the string in nptr to a long long value, while the strtounum() and strtounumx() functions convert it to an unsigned long long value. These functions were designed to facilitate safe, robust programming and overcome the shortcomings of the atoi(3C) and strtol(3C) family of interfaces.

The string may begin with an arbitrary amount of whitespace (as determined by isspace(3C)) followed by a single optional ‘+’ or ‘-’ sign. For () and (), a ‘-’ sign is rejected with the error "too small"; unlike strtoull(3C), these functions never convert a negative string to its unsigned complement.

The remainder of the string is converted to a long long or unsigned long long value according to base 10 (for () and strtounum()) or the provided base (for strtonumx() and strtounumx()).

The value obtained is then checked against the provided minval and maxval bounds. If errstr is non-null, these functions store an error string in errstr indicating the failure.

For () and () the value of base is interpreted in the same way as described in strtoll(3C). In particular, if the value of base is 0, then the expected form of nptr is that of a decimal constant, octal constant or hexadecimal constant, any of which may be preceded by a ‘+’ or ‘-’ sign, although, as described above, a ‘-’ sign is rejected by strtounumx().

These functions return 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. errstr will be set to NULL on success; 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).

These functions will fail if:

The value to be returned falls outside of the specified range.
minval was larger than maxval.

The strtonum() and strtounum() functions will fail if:

The given string did not consist solely of digit characters.

The strtonumx() and strtounumx() functions will fail if:

The specified base was invalid, or the given string did not consist solely of characters which are valid in that base.

The strtounum() and strtounumx() functions will fail if:

The given string represented a negative number.

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, or a negative number was given to strtounum() or strtounumx().
"invalid"
The string did not consist solely of characters valid in the specified base (or base 10 for strtonum()).
"unparsable; invalid base specified"
The specified base was outside the permitted range.

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

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

strtonumx(), strtounum(), and strtounumx() are illumos extensions.

August 20, 2026 OmniOS