strtod, strtof, strtold, atof - convert string to floating-point number
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);
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,
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 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:
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-
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-con‐
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
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.
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
The radix character is defined in the program's locale (category
LC_NUMERIC). In the POSIX locale, or in a locale where the radix char‐
acter 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‐
The atof(str) function call is equivalent to strtod(nptr, (char
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.
These functions will fail if:
The value to be returned would cause overflow or underflow
These functions may fail if:
No conversion could be performed.
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
1003.1-200x. One such example would be:
what_kind_of_number (char *s)
d = strtod(s, &endp);
if (s != endp && *endp == ` ')
printf("It's a float with value %g\n", d);
l = strtol(s, &endp, 0);
if (s != endp && *endp == `\0')
printf("It's an integer with value %ld\n", 1);
If the function is called with:
an ISO/IEC 9899: 1990 standard-compliant library will result in the
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.
See attributes(5) for descriptions of the following attributes:
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
│CSI │ Enabled │
│Interface Stability │ Standard │
│MT-Level │ MT-Safe with exceptions │
SEE ALSOisspace(3C), localeconv(3C), scanf(3C), setlocale(3C), strtol(3C),
The strtod() and atof() functions can be used safely in multithreaded
applications, as long as setlocale(3C) is not called to change the
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 confus‐
ing, the following 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 **)NULL, strtod() will set the
pointer pointed to by endptr to the first byte of the "final
string of unrecognized characters". (If all input charac‐
ters were processed, the pointer pointed to by endptr will
be set to point to the null character at the end of the
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
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 implementations distin‐
guish between these three cases. An application 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 **)NULL. If endptr points to the first character 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, however, that the decimal-point character is locale-depen‐
4. If strtod() returns +HUGE_VAL or −HUGE_VAL, one of the fol‐
a. If +HUGE_VAL is returned and errno is set to ERANGE, a
floating point overflow occurred while processing a pos‐
itive value, causing a floating point overflow exception
to be raised.
b. If −HUGE_VAL is returned and errno is set to ERANGE, a
floating point overflow occurred while processing a neg‐
ative 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" converted to +HUGE_VAL
or −HUGE_VAL, respectively.
Note that if errno is set to ERANGE when strtod() is called, case
(c) can be distinguished from cases (a) and (b) by examining either
ERANGE or the overflow flag.
Nov 1, 2003 STRTOD(3C)