NAME
strtod,
strtof,
strtold
—
convert ASCII string to double, float, or long
double
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <stdlib.h>
double
strtod(
const char
* restrict nptr,
char **
restrict endptr);
float
strtof(
const char
* restrict nptr,
char **
restrict endptr);
long double
strtold(
const
char * restrict nptr,
char
** restrict endptr);
DESCRIPTION
The
strtod() function converts the initial portion of the
string pointed to by
nptr to
double
representation.
The
strtof() function converts the initial portion of the
string pointed to by
nptr to
float
representation.
The
strtold() function converts the initial portion of the
string pointed to by
nptr to
long
double representation.
The expected form of the string is an optional plus (‘+’) or minus
sign (‘-’) followed by one of the following:
- a sequence of digits optionally containing a decimal-point
character, optionally followed by an exponent. An exponent consists of an
‘E’ or ‘e’, followed by an optional plus or minus
sign, followed by a sequence of digits.
- one of
INF
or
INFINITY
, ignoring case.
- one of
NAN
or
NAN(n-char-sequence-opt)
, ignoring case. This
implementation currently does not interpret such a sequence.
Leading white-space characters in the string (as defined by the
isspace(3) function) are
skipped.
RETURN VALUES
The
strtod(),
strtof(), and
strtold() functions return the converted value, if any.
A character sequence
INF
or
INFINITY
is converted to infinity, if supported, else
to the largest finite floating-point number representable on the machine
(i.e., VAX).
A character sequence
NAN
or
NAN(n-char-sequence-opt)
is converted to a quiet NaN,
if supported, else remains unrecognized (i.e., VAX).
If
endptr is not
NULL
, a pointer
to the character after the last character used in the conversion is stored in
the location referenced by
endptr.
If no conversion is performed, zero is returned and the value of
nptr is stored in the location referenced by
endptr.
If the correct value is too large in magnitude to be represented
(‘overflow’), plus or minus
HUGE_VAL
,
HUGE_VALF
, or
HUGE_VALL
is
returned (according to the return type and sign of the value), and
ERANGE
is stored in
errno.
If the correct value is too small in magnitude to be represented normally with
full precision (‘underflow’), the closest subnormal value, or
zero, is returned, and
ERANGE
is stored in
errno.
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
errno to zero before calling
strtod(),
strtof(), or
strtold(); in the case of no
underflow or overflow, these functions preserve
errno.
To check for syntax errors, callers must
also check whether
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.
char *end;
double d;
...
errno = 0;
d = strtod(s, &end);
if (end == s)
errx(EXIT_FAILURE, "invalid syntax");
if (end[0] != '\0')
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 */
ERRORS
-
-
- [
ERANGE
]
- The conversion resulted in floating-point underflow or
overflow.
SEE ALSO
atof(3),
atoi(3),
atol(3),
math(3),
strtol(3),
strtoul(3)
STANDARDS
The
strtod() function conforms to
ANSI
X3.159-1989 (“ANSI C89”). The
strtof() and
strtold() functions conform
to
ISO/IEC 9899:1999 (“ISO C99”).
HISTORY
The
strtof() and
strtold() functions
appeared in
NetBSD 4.0.