---
name: "strtou"
section: "3bsd"
platform: "linux"
package: "libbsd"
version: "0.12.2-2"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/strtou"
---

# strtou(3bsd)

convert a string value to an uintmax\_t integer

## NAME

**strtou** – convert a string value to an uintmax\_t integer

## LIBRARY

library “libbsd”

## SYNOPSIS

**#include \<inttypes.h\>**

(See [libbsd(7)](/libbsd/7.md) for include usage.)

*uintmax\_t*\

**strtou**(*const char \* restrict nptr*, *char \*\* restrict endptr*, *int base*, *uintmax\_t lo*, *uintmax\_t hi*, *int \*rstatus*);

## DESCRIPTION

The **strtou**() function converts the string in *nptr* to an *uintmax\_t* value. The **strtou**() function uses internally [strtoumax(3)](/strtoumax/3.md) and ensures that the result is always in the range \[ *lo*, *..* *hi* \]. In addition it always places 0 on success or a conversion status in the *rstatus* argument, avoiding the errno gymnastics the other functions require. The *rstatus* argument can be NULL if conversion status is to be ignored.

The string may begin with an arbitrary amount of white space (as determined by [isspace(3)](/isspace/3.md)) followed by a single optional ‘+’ or ‘-’ sign. If *base* is zero or 16, the string may then include a ‘0x’ or ‘0X’ prefix, and the number will be read in base 16; otherwise, a zero *base* is taken as 10 (decimal) unless the next character is ‘0’, in which case it is taken as 8 (octal).

The remainder of the string is converted to an *uintmax\_t* value in the obvious manner, stopping at the end of the string or at the first character which is not a valid digit in the given base. (In bases above 10, the letter ‘A’ in either upper or lower case represents 10, ‘B’ represents 11, and so forth, with ‘Z’ representing 35.)

If *endptr* is non-nil, **strtou**() stores the address of the first invalid character in *\*endptr*. If there were no digits at all, however, **strtou**() stores the original value of *nptr* in *\*endptr*. (Thus, if *\*nptr* is not ‘\\0’ but *\*\*endptr* is ‘\\0’ on return, the entire string was valid.)

## RETURN VALUES

The **strtou**() function always returns the closest value in the range specified by the *lo* and *hi* arguments.

The *errno* value is guaranteed to be left unchanged.

Errors are stored as the conversion status in the *rstatus* argument.

## EXAMPLES

The following example will always return a number in \[1..99\] range no matter what the input is, and warn if the conversion failed.

```
int e;
uintmax_t lval = strtou(buf, NULL, 0, 1, 99, &e);
if (e)
	warnc(e, "conversion of `%s' to a number failed, using %ju",
	    buf, lval);
```

## ERRORS

- \[ECANCELED\]

  The string did not contain any characters that were converted.

- \[EINVAL\]

  The *base* is not between 2 and 36 and does not contain the special value 0.

- \[ENOTSUP\]

  The string contained non-numeric characters that did not get converted. In this case, *endptr* points to the first unconverted character.

- \[ERANGE\]

  The given string was out of range; the value converted has been clamped; or the range given was invalid, i.e. *lo* \> *hi*.

The range check is more important than the unconverted characters check, and it is performed first. If a program needs to know if there were unconverted characters when an out of range number has been provided, it needs to supply and test *endptr.*

## SEE ALSO

[atof(3)](/atof/3.md), [atoi(3)](/atoi/3.md), [atol(3)](/atol/3.md), [atoll(3)](/atoll/3.md), [strtod(3)](/strtod/3.md), [strtoi(3bsd)](/strtoi/3bsd.md), [strtoimax(3)](/strtoimax/3.md), [strtol(3)](/strtol/3.md), [strtoll(3)](/strtoll/3.md), [strtoul(3)](/strtoul/3.md), [strtoull(3)](/strtoull/3.md), [strtoumax(3)](/strtoumax/3.md)

## STANDARDS

The **strtou**() function is a NetBSD extension.

## HISTORY

The **strtou**() function first appeared in NetBSD 7.0. OpenBSD introduced the [strtonum(3bsd)](/strtonum/3bsd.md) function for the same purpose, but the interface makes it impossible to properly differentiate illegal returns.

## BUGS

Ignores the current locale.