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strtou.3bsd - Man Page

convert a string value to an uintmax_t integer


library “libbsd”


#include <inttypes.h> (See libbsd(7) for include usage.)
strtou(const char * restrict nptr, char ** restrict endptr, int base, uintmax_t lo, uintmax_t hi, int *rstatus);


The strtou() function converts the string in nptr to an uintmax_t value. The strtou() function uses internally strtoumax(3) 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)) 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.


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);



The string did not contain any characters that were converted.


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


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


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), atoi(3), atol(3), atoll(3), strtod(3), strtoi(3bsd), strtoimax(3), strtol(3), strtoll(3), strtoul(3), strtoull(3), strtoumax(3)


The strtou() function is a NetBSD extension.


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


Ignores the current locale.


January 20, 2024