Standard C Library Functions strtod(3C)
NAME
strtod, strtof, strtold, atof - convert string to floating-
point number
SYNOPSIS
#include
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
characters (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 charac-
ters, including 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 contain-
ing a radix character, then an optional exponent
part
o A 0x or 0X, then a non-empty sequence of hexade-
cimal digits optionally containing a radix charac-
ter, then an optional binary exponent part
o One of INF or INFINITY, ignoring case
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Standard C Library Functions strtod(3C)
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, hexadecimal strings are also recognized.
In default mode for strtod(), the n-char-sequence in the
NAN(n-char-equence) form can contain any character except
')' (right parenthesis) or '\0' (null). In C99/SUSv3 mode,
the n-char-sequence can contain only upper and lower case
letters, digits, and '' (underscore).
The strtof() and strtold() functions always function in
C99/SUSv3-conformant 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 characters if the input string
is not of the expected form.
If the subject sequence has the expected form for a
floating-point number, the sequence of characters starting
with the first digit or the decimal-point character (which-
ever 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 binary exponent part does not appear in a
hexadecimal floating-point number, an exponent part of the
appropriate type with value zero is assumed 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.
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If the subject sequence has either the decimal or hexade-
cimal 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 LCNUMERIC). In the POSIX locale, or in a locale
where the radix character is not defined, the radix charac-
ter 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 successful.
The atof(str) function call is equivalent to strtod(nptr,
(char **)NUL).
RETURN VALUES
Upon successful completion, these functions return the con-
verted value. If no conversion could be performed, 0 is
returned.
If the correct value is outside the range of representable
values, ]HUGEVAL, ]HUGEVALF, or ]HUGEVAL is returned
(according to the sign of the value), a floating point over-
flow 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 floating point underflow exception is raised,
and errno is set to ERANGE.
ERORS
These functions will fail if:
ERANGE The value to be returned would cause overflow or
underflow
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Standard C Library Functions strtod(3C)
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 situa-
tions 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 ear-
lier versions of IE Std 1003.1-200x. One such example
would be:
int
whatkindofnumber (char *s)
{
char *endp;
double d;
long l;
d = strtod(s, &endp);
if (s != endp && *endp == ` ')
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:
whatkindofnumber ("0x10")
an ISO/IEC 9899: 1990 standard-compliant library will result
in the function printing:
It's an integer with value 16
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Standard C Library Functions strtod(3C)
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 numbers in hexadecimal notation without requiring that
either a decimal point or the binary exponent be present.
ATRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
ATRIBUTE TYPE ATRIBUTE VALUE
CSI Enabled
Interface Stability Standard
MT-Level MT-Safe with exceptions
SEE ALSO
isspace(3C), localeconv(3C), scanf(3C), setlocale(3C),
strtol(3C), attributes(5), standards(5)
NOTES
The strtod() and atof() functions can be used safely in mul-
tithreaded applications, as long as setlocale(3C) is not
called to change the locale.
The DESCRIPTION and RETURN VALUES sections above are very
similar to the wording used by the Single UNIX Specification
version 2 (SUSv2) and the 1989 C Standard to describe the
behavior of the strtod() function. Since some users have
reported that they find the description confusing, the fol-
lowing notes might be helpful.
1. The strtod() function does not modify the string
pointed to by str and does not malloc() space to
hold the decomposed portions of the input string.
2. If endptr is not (char **)NUL, strtod() will set
the pointer pointed to by endptr to the first byte
of the "final string of unrecognized characters".
(If all input characters were processed, the
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Standard C Library Functions strtod(3C)
pointer pointed to by endptr will be set to point
to the null character at the end of the input
string.)
3. If strtod() returns 0.0, one of the following
occurred:
a. The "subject sequence" was not an empty string,
but evaluated to 0.0. (In this case, errno
will be left unchanged.)
b. The "subject sequence" was an empty string . In
this case, errno will be left unchanged. (The
Single UNIX Specification version 2 allows
errno to be set to EINVAL or to be left
unchanged. The C Standard does not specify any
specific behavior in this case.)
c. The "subject sequence" specified a numeric
value whose conversion resulted in a floating
point underflow. In this case, an underflow
exception is raised and errno is set to ERANGE.
Note that the standards do not require that implementa-
tions distinguish between these three cases. An appli-
cation can determine case (b) by making sure that there
are no leading white-space characters in the string
pointed to by str and giving strtod() an endptr that is
not (char **)NUL. If endptr points to the first char-
acter of str when strtod() returns, you have detected
case (b). Case (c) can be detected by examining the
underflow flag or by looking for a non-zero digit before
the exponent part of the "subject sequence". Note, how-
ever, that the decimal-point character is locale-
dependent.
4. If strtod() returns ]HUGEVAL or -HUGEVAL, one of
the following occurred:
a. If ]HUGEVAL is returned and errno is set to
ERANGE, a floating point overflow occurred
while processing a positive value, causing a
floating point overflow exception to be raised.
b. If -HUGEVAL is returned and errno is set to
ERANGE, a floating point overflow occurred
while processing a negative value, causing a
floating point overflow exception to be raised.
c. If strtod() does not set errno to ERANGE, the
value specified by the "subject string" con-
verted to ]HUGEVAL or -HUGEVAL, respectively.
Note that if errno is set to ERANGE when strtod() is
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Standard C Library Functions strtod(3C)
called, case (c) can be distinguished from cases (a) and
(b) by examining either ERANGE or the overflow flag.
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