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strtof, strtod, strtold

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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 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.

Contents

[edit] Parameters

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

[edit] 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.

[edit] Example

#include <stdio.h>
#include <stdlib.h>
 
int main(void) 
{
    char *str = " 111.11 22.22 0X1.BC70A3D70A3D7P+6  1.18973e+4932";
    char *str_end;
 
    printf("%f\n", strtod(str,&str_end));
    printf("str_end --> [%c]\n", *str_end);
    printf("%f\n", strtod(str_end,&str_end));
    printf("%g\n", strtod(str_end,&str_end));
    printf("%f\n", strtod(str_end, NULL));
 
    return 0;
}

Output:

111.110001
str_end --> [ ]
22.219999
111.11
inf

[edit] See also

converts a byte string to a floating point value
(function) [edit]
C++ documentation for strtof, strtod, strtold