strtod() -- General Function (libc)

Convert string to floating-point number
#include <stdlib.h>
double strtod(string, tailptr)
char *string; char **tailptr;

strtod()  converts  the  number  given  in  string  to  a  double-precision
floating-point number and returns its  value.  It is a more general version
of  the  function atof().  strtod()  also  stores a  pointer  to the  first
character  following the  number through tailptr,  provided tailptr  is not
NULL.

strtod() parses  the input string  into three portions:  beginning, subject
sequence, and tail.

The beginning  consists of zero  or more white-space  characters that begin
the string.

The  subject sequence  is the  portion  of the  input string  that strtod()
converts into  a floating-point  number.  It  consists of an  optional sign
character,  a nonempty  sequence of decimal  digits optionally  including a
decimal-point  character,  and  an  optional  exponent.   If  present,  the
exponent consists of  either `e' or `E' followed by  an optional sign and a
nonempty sequence  of decimal digits.   strtod() reads characters  until it
encounters either  a second decimal-point character  or exponent marker, or
any other non-numeral.

The  tail continues  from  the end  of  the subject  sequence  to the  null
character that ends the string.

strtod()  ignores the  beginning portion  of the  string.  It  converts the
subject  sequence  to  a double-precision  number.   Finally,  it sets  the
pointer pointed to by tailptr to  the address of the first character of the
string's tail.

strtod() returns  the double  generated from  the subject sequence.   If no
subject  sequence  could be  recognized,  it returns  zero  and stores  the
initial value  of string through tailptr. If the  number represented by the
subject  sequence is  too large  or too  small to fit  into a  double, then
strtod() sets  the global constant errno to ERANGE  and returns HUGE_VAL or
zero, respectively.  If, however,  the number given in the subject sequence
has  more digits  to the  right of  the decimal point  than can  be encoded
within an IEEE  double (which has a fraction of  53 bits), strtod trims the
excess digits before it converts the string.

Example

The following gives an example for strtod().

#include <stdlib.h>

main()
{
    static char st[] = " 123.4 567.8";
    char *head, *tail;

    for (head = st;; head = tail) {
        double amt = strtod(head, &tail);

         /* No token found is end of string */
        if (head == tail)
            break;
        printf("%f\n", amt);
    }
    exit(EXIT_SUCCESS);
}

See Also

atof(),
double,
errno,
libc,
limits.h,
stdlib.h,
strtol(),
strtoul()
ANSI Standard, §7.10.1.4

Notes

strtod() ignores  initial white space  in the string pointed  to by string;
white  space  is defined  as  being  all characters  so  recognized by  the
function isspace().

© 2012 by Stephen A. Ness