strtonum.3

.\" $OpenBSD: strtonum.3,v 1.19 2022/09/11 06:38:11 jmc Exp $
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.\" Copyright (c) 2004 Ted Unangst
.\"
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.Dd $Mdocdate: September 11 2022 $
.Dt STRTONUM 3
.Os
.Sh NAME
.Nm strtonum
.Nd reliably convert string value to an integer
.Sh SYNOPSIS
.In stdlib.h
.Ft long long
.Fo strtonum
.Fa "const char *nptr"
.Fa "long long minval"
.Fa "long long maxval"
.Fa "const char **errstr"
.Fc
.Sh DESCRIPTION
The
.Fn strtonum
function converts the string in
.Fa nptr
to a
.Vt long long
value.
The
.Fn strtonum
function was designed to facilitate safe, robust programming
and overcome the shortcomings of the
.Xr atoi 3
and
.Xr strtol 3
family of interfaces.
.Pp
The string may begin with an arbitrary amount of whitespace
(as determined by
.Xr isspace 3 )
followed by a single optional
.Ql +
or
.Ql -
sign.
.Pp
The remainder of the string is converted to a
.Vt long long
value according to base 10.
.Pp
The value obtained is then checked against the provided
.Fa minval
and
.Fa maxval
bounds.
If
.Fa errstr
is non-null,
.Fn strtonum
stores an error string in
.Fa *errstr
indicating the failure.
.Sh RETURN VALUES
The
.Fn strtonum
function returns the result of the conversion,
unless the value would exceed the provided bounds or is invalid.
On error, 0 is returned,
.Va errno
is set, and
.Fa errstr
will point to an error message.
.Fa *errstr
will be set to
.Dv NULL
on success;
this fact can be used to differentiate
a successful return of 0 from an error.
.Sh EXAMPLES
Using
.Fn strtonum
correctly is meant to be simpler than the alternative functions.
.Bd -literal -offset indent
int iterations;
const char *errstr;

iterations = strtonum(optarg, 1, 64, &errstr);
if (errstr != NULL)
	errx(1, "number of iterations is %s: %s", errstr, optarg);
.Ed
.Pp
The above example will guarantee that the value of iterations is between
1 and 64 (inclusive).
.Sh ERRORS
.Bl -tag -width Er
.It Bq Er ERANGE
The given string was out of range.
.It Bq Er EINVAL
The given string did not consist solely of digit characters.
.It Bq Er EINVAL
.Ar minval
was larger than
.Ar maxval .
.El
.Pp
If an error occurs,
.Fa errstr
will be set to one of the following strings:
.Pp
.Bl -tag -width "too largeXX" -compact
.It Qq too large
The result was larger than the provided maximum value.
.It Qq too small
The result was smaller than the provided minimum value.
.It Qq invalid
The string did not consist solely of digit characters.
.El
.Sh SEE ALSO
.Xr atof 3 ,
.Xr atoi 3 ,
.Xr atol 3 ,
.Xr atoll 3 ,
.Xr sscanf 3 ,
.Xr strtod 3 ,
.Xr strtol 3 ,
.Xr strtoul 3
.Sh STANDARDS
.Fn strtonum
is an
.Ox
extension.
The existing alternatives, such as
.Xr atoi 3
and
.Xr strtol 3 ,
are either impossible or difficult to use safely.
.Sh HISTORY
The
.Fn strtonum
function first appeared in
.Ox 3.6 .