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PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface
may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface
may not be implemented on Linux.
NAME
strtod, strtof, strtold — convert a string to a double-precision 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);
DESCRIPTION
The functionality described on this reference page is aligned with the ISO C standard. Any conflict
between the requirements described here and the ISO C standard is unintentional. This volume of
POSIX.1‐2017 defers to the ISO C standard.
These functions shall 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())
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 NUL character of the
input string
Then they shall 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 '+' or '-' sign, then one of the following:
* A non-empty sequence of decimal digits optionally containing a radix character; then an optional
exponent part consisting of the character 'e' or the character 'E', optionally followed by a '+' or
'-' character, and then followed by one or more decimal digits
* A 0x or 0X, then a non-empty sequence of hexadecimal digits optionally containing a radix character;
then an optional binary exponent part consisting of the character 'p' or the character 'P',
optionally followed by a '+' or '-' character, and then followed by one or more decimal digits
* One of INF or INFINITY, ignoring case
* One of NAN or NAN(n-char-sequenceopt), ignoring case in the NAN part, where:
n-char-sequence:
digit
nondigit
n-char-sequence digit
n-char-sequence nondigit
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) shall be
interpreted as a floating constant of the C language, except that the radix character shall be 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 <hyphen-minus>, the sequence shall be interpreted as
negated. A character sequence INF or INFINITY shall be interpreted as an infinity, if representable in
the return type, else as if it were a floating constant that is too large for the range of the return
type. A character sequence NAN or NAN(n-char-sequenceopt) shall be interpreted as a quiet NaN, if
supported in the return type, else as if it were a subject sequence part that does not have the expected
form; the meaning of the n-char sequences is implementation-defined. 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 the hexadecimal form and FLT_RADIX is a power of 2, the value resulting from
the conversion is correctly rounded.
The radix character is defined in the current locale (category LC_NUMERIC). In the POSIX locale, or in a
locale where the radix character is not defined, the radix character shall default to a <period> ('.').
In other than the C or POSIX locale, additional locale-specific subject sequence forms may be accepted.
If the subject sequence is empty or does not have the expected form, no conversion shall be performed;
the value of nptr is stored in the object pointed to by endptr, provided that endptr is not a null
pointer.
These functions shall not change the setting of errno if successful.
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.
RETURN VALUE
Upon successful completion, these functions shall return the converted value. If no conversion could be
performed, 0 shall be returned, and errno may be set to [EINVAL].
If the correct value is outside the range of representable values, ±HUGE_VAL, ±HUGE_VALF, or ±HUGE_VALL
shall be returned (according to the sign of the value), and errno shall be set to [ERANGE].
If the correct value would cause an underflow, a value whose magnitude is no greater than the smallest
normalized positive number in the return type shall be returned and errno set to [ERANGE].
ERRORS
These functions shall fail if:
ERANGE The value to be returned would cause overflow or underflow.
These functions may fail if:
EINVAL No conversion could be performed.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
If the subject sequence has the hexadecimal form and FLT_RADIX is not a power of 2, and the result is not
exactly representable, the result should be one of the two numbers in the appropriate internal format
that are adjacent to the hexadecimal floating source value, with the extra stipulation that the error
should have a correct sign for the current rounding direction.
If the subject sequence has the decimal form and at most DECIMAL_DIG (defined in <float.h>) significant
digits, the result should be correctly rounded. If the subject sequence D has the decimal form and more
than DECIMAL_DIG significant digits, consider the two bounding, adjacent decimal strings L and U, both
having DECIMAL_DIG significant digits, such that the values of L, D, and U satisfy L <= D <= U. The
result should be one of the (equal or adjacent) values that would be obtained by correctly rounding L and
U according to the current rounding direction, with the extra stipulation that the error with respect to
D should have a correct sign for the current rounding direction.
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 this
standard. 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 numbers in hexadecimal notation without
requiring that either a decimal point or the binary exponent be present.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
fscanf(), isspace(), localeconv(), setlocale(), strtol()
The Base Definitions volume of POSIX.1‐2017, Chapter 7, Locale, <float.h>, <stdlib.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original
IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document.
The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page are most likely to have been introduced
during the conversion of the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .