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strtod(3c)
strtod(3C) Standard C Library Functions strtod(3C)
NAME
strtod, strtof, strtold, atof - convert string to floating-point number
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);
double atof(const char *str);
DESCRIPTION
The strtod(), strtof(), and strtold() functions convert the initial
portion of the string pointed to by nptr to double, float, and long
double representation, respectively. First they decompose the input
string into three parts:
1. An initial, possibly empty, sequence of white-space charac‐
ters (as specified by isspace(3C))
2. A subject sequence interpreted as a floating-point constant
or representing infinity or NaN
3. A final string of one or more unrecognized characters, in‐
cluding the terminating null byte of the input string.
Then they attempt to convert the subject sequence to a floating-point
number, and return the result.
The expected form of the subject sequence is an optional plus or minus
sign, then one of the following:
o A non-empty sequence of digits optionally containing a radix
character, then an optional exponent part
o A 0x or 0X, then a non-empty sequence of hexadecimal digits
optionally containing a radix character, then an optional
binary exponent part
o One of INF or INFINITY, ignoring case
o One of NAN or NAN(n-char-sequence(opt)), ignoring case in
the NAN part, where:
n-char-sequence:
digit
nondigit
n-char-sequence digit
n-char-sequence nondigit
In default mode for strtod(), only decimal, INF/INFINITY, and
NAN/NAN(n-char-sequence) forms are recognized. In C99/SUSv3 mode, hexa‐
decimal strings are also recognized.
In default mode for strtod(), the n-char-sequence in the NAN(n-char-se‐
quence) form can contain any character except ')' (right parenthesis)
or '\0' (null). In C99/SUSv3 mode, the n-char-sequence can contain only
upper and lowercase letters, digits, and '_' (underscore).
The strtof() and strtold() functions always function in C99/SUSv3-con‐
formant mode.
The subject sequence is defined as the longest initial subsequence of
the input string, starting with the first non-white-space character,
that is of the expected form. The subject sequence contains no charac‐
ters if the input string is not of the expected form.
If the subject sequence has the expected form for a floating-point num‐
ber, the sequence of characters starting with the first digit or the
decimal-point character (whichever occurs first) is interpreted as a
floating constant of the C language, except that the radix character is
used in place of a period, and that if neither an exponent part nor a
radix character appears in a decimal floating-point number, or if a bi‐
nary exponent part does not appear in a hexadecimal floating-point num‐
ber, an exponent part of the appropriate type with value zero is as‐
sumed to follow the last digit in the string. If the subject sequence
begins with a minus sign, the sequence is interpreted as negated. A
character sequence INF or INFINITY is interpreted as an infinity. A
character sequence NAN or NAN(n-char-sequence(opt)) is interpreted as a
quiet NaN. A pointer to the final string is stored in the object
pointed to by endptr, provided that endptr is not a null pointer.
If the subject sequence has either the decimal or hexadecimal form, the
value resulting from the conversion is rounded correctly according to
the prevailing floating-point rounding direction mode. The conversion
also raises floating-point inexact, underflow, or overflow exceptions
as appropriate.
The radix character is defined in the program's locale (category LC_NU‐
MERIC). In the POSIX locale, or in a locale where the radix character
is not defined, the radix character defaults to a period ('.').
If the subject sequence is empty or does not have the expected form, no
conversion is performed; the value of nptr is stored in the object
pointed to by endptr, provided that endptr is not a null pointer.
The strtod() function does not change the setting of errno if success‐
ful.
The atof(str) function call is equivalent to strtod(nptr, (char
**)NULL).
RETURN VALUES
Upon successful completion, these functions return the converted value.
If no conversion could be performed, 0 is returned.
If the correct value is outside the range of representable values,
±HUGE_VAL, ±HUGE_VALF, or ±HUGE_VALL is returned (according to the sign
of the value), a floating-point overflow exception is raised, and errno
is set to ERANGE.
If the correct value would cause an underflow, the correctly rounded
result (which may be normal, subnormal, or zero) is returned, a float‐
ing-point underflow exception is raised, and errno is set to ERANGE.
ERRORS
These functions will fail if:
ERANGE The value to be returned would cause overflow or underflow
These functions may fail if:
EINVAL No conversion could be performed.
USAGE
Since 0 is returned on error and is also a valid return on success, an
application wishing to check for error situations should set errno to
0, then call strtod(), strtof(), or strtold(), then check errno.
The changes to strtod() introduced by the ISO/IEC 9899: 1999 standard
can alter the behavior of well-formed applications complying with the
ISO/IEC 9899: 1990 standard and thus earlier versions of IEEE Std
1003.1-2008. One such example would be:
int
what_kind_of_number (char *s)
{
char *endp;
double d;
long l;
d = strtod(s, &endp);
if (s != endp && *endp == '\0')
printf("It's a float with value %g\n", d);
else
{
l = strtol(s, &endp, 0);
if (s != endp && *endp == '\0')
printf("It's an integer with value %ld\n", 1);
else
return 1;
}
return 0;
}
If the function is called with:
what_kind_of_number ("0x10")
an ISO/IEC 9899: 1990 standard-compliant library will result in the
function printing:
It's an integer with value 16
With the ISO/IEC 9899: 1999 standard, the result is:
It's a float with value 16
The change in behavior is due to the inclusion of floating-point num‐
bers in hexadecimal notation without requiring that either a decimal
point or the binary exponent be present.
ATTRIBUTES
See attributes(7) for descriptions of the following attributes:
tab() box; cw(2.75i) |cw(2.75i) lw(2.75i) |lw(2.75i) ATTRIBUTE TYPEAT‐
TRIBUTE VALUE _ CSIEnabled _ Interface StabilityCommitted _ MT-LevelMT-
Safe _ StandardSee standards(7).
SEE ALSO
strtol(3C), isspace(3C), localeconv(3C), scanf(3C), setlocale(3C), at‐
tributes(7), standards(7)
Oracle Solaris 11.4 11 May 2021 strtod(3C)