strtof, strtod, strtold

Defined in header `<stdlib.h>`
float strtof( const char restrict str, char *restrict str_end );		(since C99)
double strtod( const char str, char *str_end );		(until C99)
double strtod( const char restrict str, char *restrict str_end );		(since C99)
long double strtold( const char restrict str, char *restrict str_end );		(since C99)

Interprets a floating-point value in a byte string pointed to by str.

Function discards any whitespace characters (as determined by std::isspace()) until first non-whitespace character is found. Then it takes as many characters as possible to form a valid floating-point representation and converts them to a floating-point value. The valid floating-point value can be one of the following:

decimal floating-point expression. It consists of the following parts:

(optional) plus or minus sign
nonempty sequence of decimal digits optionally containing decimal-point character (as determined by the current C locale) (defines significand)
(optional) e or E followed with optional minus or plus sign and nonempty sequence of decimal digits (defines exponent)

binary floating-point expression. It consists of the following parts:

(optional) plus or minus sign
0x or 0X
nonempty sequence of hexadecimal digits optionally containing a decimal-point character (as determined by the current C locale) (defines significand)
(optional) p or P followed with optional minus or plus sign and nonempty sequence of decimal digits (defines exponent)

infinity expression. It consists of the following parts:

(optional) plus or minus sign
INF or INFINITY ignoring case

not-a-number expression. It consists of the following parts:

(optional) plus or minus sign
NAN or NAN(char_sequence) ignoring case of the NAN part. char_sequence can only contain alphanumeric characters. The result is a quiet NaN floating-point value.

any other expression that may be accepted by the currently installed C locale

The functions sets the pointer pointed to by str_end to point to the character past the last character interpreted. If str_end is NULL, it is ignored.

Parameters

str	-	pointer to the null-terminated byte string to be interpreted
str_end	-	pointer to a pointer to character.

Return value

Floating-point value corresponding to the contents of str on success. If the converted value falls out of range of corresponding return type, range error occurs and HUGE_VAL, HUGE_VALF or HUGE_VALL is returned. If no conversion can be performed, 0 is returned.

Example

Run this code

#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
 
int main(void)
{
    // parsing with error handling
    const char *p = "111.11 -2.22 Nan nan(2) inF 0X1.BC70A3D70A3D7P+6  1.18973e+4932zzz";
    printf("Parsing '%s':\n", p);
    char *end;
    for (double f = strtod(p, &end); p != end; f = strtod(p, &end))
    {
        printf("'%.*s' -> ", (int)(end-p), p);
        p = end;
        if (errno == ERANGE){
            printf("range error, got ");
            errno = 0;
        }
        printf("%f\n", f);
    }
 
    // parsing without error handling
    printf("\"  -0.0000000123junk\"  -->  %g\n", strtod("  -0.0000000123junk", NULL));
    printf("\"junk\"                 -->  %g\n", strtod("junk", NULL));
}

Possible output:

Parsing '111.11 -2.22 Nan inF 0X1.BC70A3D70A3D7P+6  1.18973e+4932zzz':
'111.11' -> 111.110000
' -2.22' -> -2.220000
' Nan' -> nan
' nan(2)' -> nan
' inF' -> inf
' 0X1.BC70A3D70A3D7P+6' -> 111.110000
'  1.18973e+4932' -> range error, got inf
"  -0.0000000123junk"  -->  -1.23e-08
"junk"                 -->  0

References

C11 standard (ISO/IEC 9899:2011):

7.22.1.3 The strtod, strtof, and strtold functions (p: 342-344)

C99 standard (ISO/IEC 9899:1999):

7.20.1.3 The strtod, strtof, and strtold functions (p: 308-310)

C89/C90 standard (ISO/IEC 9899:1990):

4.10.1.4 The strtod function

Language
headers
Type support
Dynamic memory management
Error handling
Program utilities
Variadic function support
Date and time utilities
Strings library
Algorithms
Numerics
Input/output support
Localization support
Thread support (C11)
Atomic operations (C11)
Technical Specifications

atof	converts a byte string to a floating-point value (function)
wcstofwcstodwcstold (C99)(C95)(C99)	converts a wide string to a floating-point value (function)
C++ documentation for strtof, strtod, strtold

strtof, strtod, strtold

Parameters

Return value

Example

References

See also