strtod(3) - NetBSD Manual Pages

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STRTOD(3)               NetBSD Library Functions Manual              STRTOD(3)


NAME

     strtod, strtof, strtold -- convert ASCII string to double, float, or long
     double


LIBRARY

     Standard C Library (libc, -lc)


SYNOPSIS

     #include <stdlib.h>

     double
     strtod(const char * restrict nptr, char ** restrict endptr);

     float
     strtof(const char * restrict nptr, char ** restrict endptr);

     long double
     strtold(const char * restrict nptr, char ** restrict endptr);


DESCRIPTION

     The strtod() function converts the initial portion of the string pointed
     to by nptr to double representation.

     The strtof() function converts the initial portion of the string pointed
     to by nptr to float representation.

     The strtold() function converts the initial portion of the string pointed
     to by nptr to long double representation.

     The expected form of the string is an optional plus (`+') or minus sign
     (`-') followed by one of the following:

     -   a sequence of digits optionally containing a decimal-point character,
         optionally followed by an exponent.  An exponent consists of an `E'
         or `e', followed by an optional plus or minus sign, followed by a
         sequence of digits.

     -   one of INF or INFINITY, ignoring case.

     -   one of NAN or NAN(n-char-sequence-opt), ignoring case.  This imple-
         mentation currently does not interpret such a sequence.

     Leading white-space characters in the string (as defined by the
     isspace(3) function) are skipped.


RETURN VALUES

     The strtod(), strtof(), and strtold() functions return the converted
     value, if any.

     A character sequence INF or INFINITY is converted to infinity, if sup-
     ported, else to the largest finite floating-point number representable on
     the machine (i.e., VAX).

     A character sequence NAN or NAN(n-char-sequence-opt) is converted to a
     quiet NaN, if supported, else remains unrecognized (i.e., VAX).

     If endptr is not NULL, a pointer to the character after the last charac-
     ter used in the conversion is stored in the location referenced by
     endptr.

     If no conversion is performed, zero is returned and the value of nptr is
     stored in the location referenced by endptr.

     If the correct value is too large in magnitude to be represented
     (`overflow'), plus or minus HUGE_VAL, HUGE_VALF, or HUGE_VALL is returned
     (according to the return type and sign of the value), and ERANGE is
     stored in errno.

     If the correct value is too small in magnitude to be represented normally
     with full precision (`underflow'), the closest subnormal value, or zero,
     is returned, and ERANGE is stored in errno.


EXAMPLES

     Since there is no out-of-band channel or sentinel value to indicate an
     error, callers who wish to know whether there was overflow or underflow
     must set errno to zero before calling strtod(), strtof(), or strtold();
     in the case of no underflow or overflow, these functions preserve errno.

     To check for syntax errors, callers must also check whether endptr was
     updated to reflect the true end of the string in order to determine
     whether the full string was consumed or whether there were additional
     erroneous characters in it.

         char *end;
         double d;

         ...

         errno = 0;
         d = strtod(s, &end);
         if (end == s)
                 errx(EXIT_FAILURE, "invalid syntax");
         if (end[0] != '\0')
                 errx(EXIT_FAILURE, "trailing garbage");
         if (errno) {
                 assert(errno == ERANGE);
                 assert(isinf(d) || d == 0 ||
                     fpclassify(d) == FP_SUBNORMAL);
                 warnx("%s", isinf(d) ? "overflow" : "underflow");
         }
         /* d is the best floating-point approximation to the number in s */


ERRORS

     [ERANGE]           The conversion resulted in floating-point underflow or
                        overflow.


SEE ALSO

     atof(3), atoi(3), atol(3), math(3), strtol(3), strtoul(3)


STANDARDS

     The strtod() function conforms to ANSI X3.159-1989 (``ANSI C89'').  The
     strtof() and strtold() functions conform to ISO/IEC 9899:1999
     (``ISO C99'').


HISTORY

     The strtof() and strtold() functions appeared in NetBSD 4.0.

NetBSD 8.0                     November 4, 2016                     NetBSD 8.0