Section (3) strtod

Linux manual pages Section 3  


strtod, strtof, strtold — convert ASCII string to floating-point number


#include <stdlib.h>
double strtod( const char *nptr,
  char **endptr);
float strtof( const char *nptr,
  char **endptr);
long double strtold( const char *nptr,
  char **endptr);
[Note] Note
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
strtof(), strtold():


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.

The expected form of the (initial portion of the) string is optional leading white space as recognized by isspace(3), an optional plus (_zsingle_quotesz_+_zsingle_quotesz_) or minus sign (_zsingle_quotesz_−_zsingle_quotesz_) and then either (i) a decimal number, or (ii) a hexadecimal number, or (iii) an infinity, or (iv) a NAN (not-a-number).

A decimal number consists of a nonempty sequence of decimal digits possibly containing a radix character (decimal point, locale-dependent, usually _zsingle_quotesz_._zsingle_quotesz_), optionally followed by a decimal exponent. A decimal exponent consists of an _zsingle_quotesz_E_zsingle_quotesz_ or _zsingle_quotesz_e_zsingle_quotesz_, followed by an optional plus or minus sign, followed by a nonempty sequence of decimal digits, and indicates multiplication by a power of 10.

A hexadecimal number consists of a 0x or 0X followed by a nonempty sequence of hexadecimal digits possibly containing a radix character, optionally followed by a binary exponent. A binary exponent consists of a _zsingle_quotesz_P_zsingle_quotesz_ or _zsingle_quotesz_p_zsingle_quotesz_, followed by an optional plus or minus sign, followed by a nonempty sequence of decimal digits, and indicates multiplication by a power of 2. At least one of radix character and binary exponent must be present.

An infinity is either INF or INFINITY, disregarding case.

A NAN is NAN (disregarding case) optionally followed by a string, (n-char-sequence), where n-char-sequence specifies in an implementation-dependent way the type of NAN (see NOTES).


These functions return the converted value, if any.

If endptr is not NULL, a pointer to the character after the last character used in the conversion is stored in the location referenced by endptr.

If no conversion is performed, zero is returned and (unless endptr is null) the value of nptr is stored in the location referenced by endptr.

If the correct value would cause overflow, plus or minus HUGE_VAL (HUGE_VALF, HUGE_VALL) is returned (according to the sign of the value), and ERANGE is stored in errno. If the correct value would cause underflow, zero is returned and ERANGE is stored in errno.



Overflow or underflow occurred.


For an explanation of the terms used in this section, see attributes(7).

Interface Attribute Value
strtod(), strtof(), strtold() Thread safety MT-Safe locale


POSIX.1-2001, POSIX.1-2008, C99.

strtod() was also described in C89.


Since 0 can legitimately be returned on both success and failure, the calling program should set errno to 0 before the call, and then determine if an error occurred by checking whether errno has a nonzero value after the call.

In the glibc implementation, the n-char-sequence that optionally follows NAN is interpreted as an integer number (with an optional _zsingle_quotesz_0_zsingle_quotesz_ or _zsingle_quotesz_0x_zsingle_quotesz_ prefix to select base 8 or 16) that is to be placed in the mantissa component of the returned value.


See the example on the strtol(3) manual page; the use of the functions described in this manual page is similar.


atof(3), atoi(3), atol(3), nan(3), nanf(3), nanl(3), strfromd(3), strtol(3), strtoul(3)


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    (#)strtod.3 5.3 (Berkeley) 6/29/91

Modified Sun Aug 21 17:16:22 1994 by Rik Faith (
Modified Sat May 04 19:34:31 MET DST 1996 by Michael Haardt
Added strof, strtold, aeb, 2001-06-07