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.\"     from: @(#)strtod.3	8.1 (Berkeley) 6/4/93
.\"
.Dd November 4, 2016
.Dt STRTOD 3
.Os
.Sh NAME
.Nm strtod ,
.Nm strtof ,
.Nm strtold
.Nd convert
.Tn ASCII
string to double, float, or long double
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdlib.h
.Ft double
.Fn strtod "const char * restrict nptr" "char ** restrict endptr"
.Ft float
.Fn strtof "const char * restrict nptr" "char ** restrict endptr"
.Ft long double
.Fn strtold "const char * restrict nptr" "char ** restrict endptr"
.Sh DESCRIPTION
The
.Fn strtod
function converts the initial portion of the string
pointed to by
.Fa nptr
to
.Em double
representation.
.Pp
The
.Fn strtof
function converts the initial portion of the string
pointed to by
.Fa nptr
to
.Em float
representation.
.Pp
The
.Fn strtold
function converts the initial portion of the string
pointed to by
.Fa nptr
to
.Em long double
representation.
.Pp
The expected form of the string is an optional plus
.Pq Sq +
or minus sign
.Pq Sq \-
followed by one of the following:
.Bl -dash
.It
a sequence of digits optionally containing
a decimal-point character, optionally followed by an exponent.
An exponent consists of an
.Sq E
or
.Sq e ,
followed by an optional plus
or minus sign, followed by a sequence of digits.
.It
one of
.Li INF
or
.Li INFINITY ,
ignoring case.
.It
one of
.Li NAN
or
.Li NAN(n-char-sequence-opt) ,
ignoring case.
This implementation currently does not interpret such a sequence.
.El
.Pp
Leading white-space characters in the string (as defined by the
.Xr isspace 3
function) are skipped.
.Sh RETURN VALUES
The
.Fn strtod ,
.Fn strtof ,
and
.Fn strtold
functions return the converted value, if any.
.Pp
A character sequence
.Li INF
or
.Li INFINITY
is converted to \*(If,
if supported, else to the largest finite floating-point number representable
on the machine (i.e.,
.Tn VAX ) .
.Pp
A character sequence
.Li NAN
or
.Li NAN(n-char-sequence-opt)
is converted to a quiet \*(Na, if supported, else remains unrecognized (i.e.,
.Tn VAX ) .
.Pp
If
.Fa endptr
is not
.Dv NULL ,
a pointer to the character after the last character used
in the conversion is stored in the location referenced by
.Fa endptr .
.Pp
If no conversion is performed, zero is returned and the value of
.Fa nptr
is stored in the location referenced by
.Fa endptr .
.Pp
If the correct value is too large in magnitude to be represented
.Pq Sq overflow ,
plus or minus
.Dv HUGE_VAL ,
.Dv HUGE_VALF ,
or
.Dv HUGE_VALL
is returned (according to the return type and sign of the value), and
.Er ERANGE
is stored in
.Va errno .
.Pp
If the correct value is too small in magnitude to be represented
normally with full precision
.Pq Sq underflow ,
the closest subnormal value, or zero, is returned, and
.Er ERANGE
is stored in
.Va errno .
.Sh EXAMPLES
Since there is no out-of-band channel or sentinel value to indicate an
error, callers who wish to know whether there was overflow or underflow
must set
.Va errno
to zero before calling
.Fn strtod ,
.Fn strtof ,
or
.Fn strtold ;
in the case of no underflow or overflow, these functions preserve
.Va errno .
.Pp
To check for syntax errors, callers must
.Em also
check whether
.Fa endptr
was updated to reflect the true end of the string in order to determine
whether the full string was consumed or whether there were additional
erroneous characters in it.
.Bd -literal -offset abcd
char *end;
double d;

\&...

errno = 0;
d = strtod(s, &end);
if (end == s)
	errx(EXIT_FAILURE, "invalid syntax");
if (end[0] != '\e0')
	errx(EXIT_FAILURE, "trailing garbage");
if (errno) {
	assert(errno == ERANGE);
	assert(isinf(d) || d == 0 ||
	    fpclassify(d) == FP_SUBNORMAL);
	warnx("%s", isinf(d) ? "overflow" : "underflow");
}
/* d is the best floating-point approximation to the number in s */
.Ed
.Sh ERRORS
.Bl -tag -width Er
.It Bq Er ERANGE
The conversion resulted in floating-point underflow or overflow.
.El
.Sh SEE ALSO
.Xr atof 3 ,
.Xr atoi 3 ,
.Xr atol 3 ,
.Xr math 3 ,
.Xr strtol 3 ,
.Xr strtoul 3
.Sh STANDARDS
The
.Fn strtod
function
conforms to
.St -ansiC .
The
.Fn strtof
and
.Fn strtold
functions conform to
.St -isoC-99 .
.Sh HISTORY
The
.Fn strtof
and
.Fn strtold
functions appeared in
.Nx 4.0 .