strtof, strtod, strtold

float       strtof( const char *restrict str, char **restrict str_end );

double      strtod( const char          *str, char          **str_end );

double      strtod( const char *restrict str, char **restrict str_end );

long double strtold( const char *restrict str, char **restrict str_end );

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

Function discards any whitespace characters (as determined by 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 to base 10)

• 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 a null pointer, it is ignored.

Parameters

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 (errno is set to ERANGE) and HUGE_VAL, HUGE_VALF or HUGE_VALL is returned. If no conversion can be performed, ​0​ is returned.

Example

#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));
}

Parsing '111.11 -2.22 Nan nan(2) 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

• C17 standard (ISO/IEC 9899:2018):
• 7.22.1.3 The strtod, strtof, and strtold functions (p: 249-251)
• 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

See also