NAME
strtod
, strtof
,
strtold
—
convert ASCII string to double, float,
or long double
LIBRARY
library “libc”
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
Thestrtod
()
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
orINFINITY
, ignoring case. - one of
NAN
orNAN(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
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.