strtod() - strtof() - strtold() — Convert Character String to Double, Float, and Long Double

Format

#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);

Language Level

ANSI

Threadsafe

Yes

Locale Sensitive

The behavior of these functions might be affected by the LC_CTYPE and LC_NUMERIC categories of the current locale. For more information, see Understanding CCSIDs and Locales.

Description

The strtod(), strtof(), and strtold() functions convert a character string to a double, float, or long double value. The parameter nptr points to a sequence of characters that can be interpreted as a numeric binary floating-point value. These functions stop reading the string at the first character that is not recognized as part of a number. This character can be the null character at the end of the string.

The strtod(), strtof(), and strtold() functions expect nptr to point to a string with the following form:

The first character that does not fit this form stops the scan. In addition, a sequence of INFINITY or NAN (ignoring case) is allowed.

If an exponent is specified with the hexadecimal digit form, the exponent is interpreted as a binary (base 2) exponent. If an exponent is specified with the decimal digit form, the exponent is interpreted as a decimal (base 10) exponent.

Return Value

The strtod(), strtof(), and strtold() functions return the value of the floating-point number, except when the representation causes an underflow or overflow. For an overflow, strtof() returns HUGE_VALF or -HUGE_VALF; strtod() and strtold() return HUGE_VAL or -HUGE_VAL. For an underflow, all functions return 0.

In both cases, errno is set to ERANGE. If the string pointed to by nptr does not have the expected form, no conversion is performed and the value of nptr is stored in the object pointed to by endptr, provided that endptr is not a NULL pointer.

The strtod(), strtof(), and strtold() functions do not fail if a character other than a digit follows an E or e that is read as an exponent. For example, 100elf is converted to the floating-point value 100.0.

A character sequence of INFINITY (ignoring case) yields a value of INFINITY. A character value of NAN yields a Quiet Not-A-Number (NAN) value.

Example

This example converts the strings to double, float, and long double values. It prints the converted values and the substring that stopped the conversion.

#include <stdlib.h>
#include <stdio.h>
 
int main(void)
{
   char *string, *stopstring;
   double x;
   float f;
   long double ld;

   string = "3.1415926This stopped it";
   f = strtof(string, &stopstring);
   printf("string = \"%s\"\n", string);
   printf("   strtof = %f\n", f);
   printf("   Stopped scan at \"%s\"\n\n", stopstring);
   string = "100ergs";
   f = strtof(string, &stopstring);
   printf("string = \"%s\"\n", string);
   printf("   strtof = %f\n", f);
   printf("   Stopped scan at \"%s\"\n\n", stopstring);

   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 = "100ergs";
   x = strtod(string, &stopstring);
   printf("string = \"%s\"\n", string);
   printf("   strtod = %f\n", x);
   printf("   Stopped scan at \"%s\"\n\n", stopstring);

   string = "3.1415926This stopped it";
   ld = strtold(string, &stopstring);
   printf("string = \"%s\"\n", string);
   printf("   strtold = %lf\n", ld);
   printf("   Stopped scan at \"%s\"\n\n", stopstring);
   string = "100ergs";
   ld = strtold(string, &stopstring);
   printf("string = \"%s\"\n", string);
   printf("   strtold = %lf\n", ld);
   printf("   Stopped scan at \"%s\"\n\n", stopstring);
}

/***************** Output should be similar to: *****************
string = "3.1415926This stopped it"
   strtof = 3.141593
   Stopped scan at "This stopped it"

string = "100ergs"
   strtof = 100.000000
   Stopped scan at "ergs"

string = "3.1415926This stopped it"
   strtod = 3.141593
   Stopped scan at "This stopped it"
 
string = "100ergs"
   strtod = 100.000000
   Stopped scan at "ergs"

string = "3.1415926This stopped it"
   strtold = 3.141593
   Stopped scan at "This stopped it"
 
string = "100ergs"
   strtold = 100.000000
   Stopped scan at "ergs"

*/