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strtod, _strtod_l, wcstod, _wcstod_l

Convert strings to a double-precision value.


double strtod(
   const char *strSource,
   char **endptr
double _strtod_l(
   const char *strSource,
   char **endptr,
   _locale_t locale
double wcstod(
   const wchar_t *strSource,
   wchar_t **endptr
double _wcstod_l(
   const wchar_t *strSource,
   wchar_t **endptr,
   _locale_t locale


Null-terminated string to convert.

Pointer to character that stops scan.

The locale to use.

Return value

strtod returns the value of the floating-point number, except when the representation would cause an overflow, in which case the function returns +/-HUGE_VAL. The sign of HUGE_VAL matches the sign of the value that can't be represented. strtod returns 0 if no conversion can be performed or an underflow occurs.

wcstod returns values analogously to strtod:

  • For both functions, errno is set to ERANGE if overflow or underflow occurs.
  • If there are invalid parameters, errno is set to EINVAL and the invalid parameter handler is invoked, as described in Parameter validation.

For more information on this and other return codes, see errno, _doserrno, _sys_errlist, and _sys_nerr.


Each function converts the input string strSource to a double. The strtod function converts strSource to a double-precision value. strtod stops reading the string strSource at the first character it can't recognize as part of a number. This character may be the terminating null character. wcstod is a wide-character version of strtod; its strSource argument is a wide-character string. These functions behave identically otherwise.

By default, this function's global state is scoped to the application. To change this behavior, see Global state in the CRT.

Generic-text routine mappings

TCHAR.H routine _UNICODE and _MBCS not defined _MBCS defined _UNICODE defined
_tcstod strtod strtod wcstod
_tcstod_l _strtod_l _strtod_l _wcstod_l

The LC_NUMERIC category setting of the current locale determines recognition of the radix point character in strSource. For more information, see setlocale. The functions without the _l suffix use the current locale; _strtod_l is identical to _strtod except the former uses the locale passed in instead. For more information, see Locale.

If endptr isn't NULL, a pointer to the character that stopped the scan is stored at the location pointed to by endptr. If no conversion can be performed (no valid digits were found or an invalid base was specified), the value of strSource is stored at the location pointed to by endptr.

strtod expects strSource to point to a string of one of the following forms:

[whitespace] [sign] {digits [radix digits] | radix digits} [{e | E} [sign] digits]
[whitespace] [sign] {0x | 0X} {hexdigits [radix hexdigits] | radix hexdigits} [{p | P} [sign] digits]
[whitespace] [sign] {INF | INFINITY}
[whitespace] [sign] NAN [sequence]

The optional leading whitespace may consist of space and tab characters, which are ignored.
sign is either plus (+) or minus (-).
digits are one or more decimal digits.
hexdigits are one or more hexadecimal digits.
radix is the radix point character, either a period (.) in the default "C" locale, or the locale-specific value if the current locale is different or when locale is specified.
A sequence is a sequence of alphanumeric or underscore characters.

In both decimal and hexadecimal number forms, if no digits appear before the radix point character, at least one must appear after the radix point character.

In the decimal form, the decimal digits can be followed by an exponent, which consists of an introductory letter (e or E) and an optionally signed integer.

In the hexadecimal form, the hexadecimal digits can be followed by an exponent, which consists of an introductory letter (p or P) and an optionally signed decimal integer that represents the exponent as a power of 2.

In either form, if there isn't an exponent part or a radix point character, a radix point character is assumed to follow the last digit in the string.

Case is ignored in both the INF and NAN forms. The first character that doesn't fit one of these forms stops the scan.

The UCRT versions of these functions don't support conversion of Fortran-style (d or D) exponent letters. This non-standard extension was supported by earlier versions of the CRT, and may be a breaking change for your code. The UCRT versions support hexadecimal strings and round-tripping of INF and NAN values, which weren't supported in earlier versions. This support can also cause breaking changes in your code. For example, the string "0x1a" would be interpreted by strtod as 0.0 in previous versions, but as 26.0 in the UCRT version.


Routine Required header
strtod, _strtod_l C: <stdlib.h> C++: <cstdlib> or <stdlib.h>
wcstod, _wcstod_l C: <stdlib.h> or <wchar.h> C++: <cstdlib>, <stdlib.h>, or <wchar.h>

For more compatibility information, see Compatibility.


// crt_strtod.c
// This program uses strtod to convert a
// string to a double-precision value; strtol to
// convert a string to long integer values; and strtoul
// to convert a string to unsigned long-integer values.

#include <stdlib.h>
#include <stdio.h>

int main(void)
    char *string, *stopstring;
    double x;
    long   l;
    int    base;
    unsigned long ul;

    string = "3.1415926This stopped it";
    x = strtod(string, &stopstring);
    printf("string = %s\n", string);
    printf("   strtod = %f\n", x);
    printf("   Stopped scan at: %s\n\n", stopstring);

    string = "-10110134932This stopped it";
    l = strtol(string, &stopstring, 10);
    printf("string = %s\n", string);
    printf("   strtol = %ld\n", l);
    printf("   Stopped scan at: %s\n\n", stopstring);

    string = "10110134932";
    printf("string = %s\n", string);

    // Convert string using base 2, 4, and 8:
    for (base = 2; base <= 8; base *= 2)
        // Convert the string:
        ul = strtoul(string, &stopstring, base);
        printf("   strtol = %ld (base %d)\n", ul, base);
        printf("   Stopped scan at: %s\n", stopstring);

    // NaN
    x = strtod("+nan", &stopstring);
    printf("\n%f\n", x);

    // INF
    x = strtod("-INF", &stopstring);
    printf("\n%f\n", x);

    // e - exponent
    x = strtod("1.18973e+49", &stopstring);
    printf("\n%f\n", x);

    // doesn't handle Fortran style
    x = strtod("1.18973d+49", &stopstring);
    printf("\n%f\n", x);
    printf("No Fortran style support. Stopped parsing at %s\n", stopstring);
string = 3.1415926This stopped it
   strtod = 3.141593
   Stopped scan at: This stopped it

string = -10110134932This stopped it
   strtol = -2147483648
   Stopped scan at: This stopped it

string = 10110134932
   strtol = 45 (base 2)
   Stopped scan at: 34932
   strtol = 4423 (base 4)
   Stopped scan at: 4932
   strtol = 2134108 (base 8)
   Stopped scan at: 932




No Fortran style support. Stopped parsing at d+49

See also

Data conversion
Math and floating-point support
Interpretation of multibyte-character sequences
String to numeric value functions
strtol, wcstol, _strtol_l, _wcstol_l
strtoul, _strtoul_l, wcstoul, _wcstoul_l
atof, _atof_l, _wtof, _wtof_l
_create_locale, _wcreate_locale