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


strtonum, strtonumx - 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);


The strtonum() and strtonumx() functions convert the string in nptr to a
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.

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

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

For strtonumx() 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.


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. 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

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).


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

ERANGE The value to be returned falls outside of the specified

EINVAL minval was larger than maxval.

The strtonum() function will fail if:

EINVAL The given string did not consist solely of digit

The strtonumx() function will fail if:

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

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 characters valid in the
specified base (or base 10 for strtonum()).
"unparsable; invalid base specified"
The specified base was outside the permitted range.





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() is an illumos extension.

illumos August 19, 2023 illumos