STRTOD(3C) Standard C Library Functions STRTOD(3C)


strtod, strtof, strtold, atof - convert string to floating-point number


#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);


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 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:

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-
sequence) 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 (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 as

The radix character is defined in the program's locale (category
LC_NUMERIC). 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 successful.

The atof(str) function call is equivalent to strtod(nptr, (char **)NULL).


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. HUGE_VAL, HUGE_VALF, and HUGE_VALL are described
in math.h(3HEAD).

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.


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)
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);
l = strtol(s, &endp, 0);
if (s != endp && *endp == '\0')
printf("It's an integer with value %ld\n", 1);
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 numbers
in hexadecimal notation without requiring that either a decimal point or
the binary exponent be present.


See attributes(7) for descriptions of the following attributes:

|CSI | Enabled |
|Interface Stability | Standard |
|MT-Level | MT-Safe with exceptions |


isspace(3C), localeconv(3C), scanf(3C), setlocale(3C), strtol(3C),
math.h(3HEAD), attributes(7), standards(7)


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 confusing, 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 characters were
processed, the 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

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
distinguish 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

4. If strtod() returns +HUGE_VAL or -HUGE_VAL, one of the
following occurred:

a. If +HUGE_VAL 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 -HUGE_VAL 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" 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.

illumos April 21, 2021 STRTOD(3C)