NAME

strtod, strtof, strtold — convert ASCII string to floating point

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

These conversion functions convert the initial portion of the string pointed to by nptr to double, float, and long double representation, respectively.

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

• a decimal significand, consisting of a sequence of decimal digits (optionally containing a decimal-point character) or
• a hexadecimal significand, consisting of a ``0X'' or ``0x'' followed by a sequence of hexadecimal digits (optionally containing a decimal-point character).

In both cases, the significand may be optionally followed by an exponent. An exponent consists of an ``E'' or ``e'' (for decimal constants) or a ``P'' or ``p'' (for hexadecimal constants), followed by an optional plus or minus sign, followed by a sequence of decimal digits. For decimal constants, the exponent indicates the power of 10 by which the significand should be scaled. For hexadecimal constants, the scaling is instead done by powers of 2.

If the portion of the string following the optional plus or minus sign begins with “INFINITY” or “INF”, ignoring case, it is interpreted as an infinity.

If the portion of the string following the optional plus or minus sign begins with “NAN”, ignoring case, it is interpreted as a quiet NaN. The syntax “NAN(s)”, where s is an alphanumeric string, produces the same value as the call nan("s"); (respectively, nanf("s"); and nanl("s");.)

In any of the above cases, leading white-space characters in the string (as defined by the isspace(3) function) are skipped. The decimal point character is defined in the program's locale (category LC_NUMERIC).

Extended locale versions of these functions are documented in strtod_l(3). See xlocale(3) for more information.

RETURN VALUES

The strtod(), strtof(), and strtold() functions return the converted value, if any, rounded to the nearest representable value of the corresponding type according to the rounding mode currently in effect. See fegetround(3) for more information on rounding modes. If flush-to-zero behavior is enabled in the current floating-point environment, the behavior is identical except that any subnormal values that would be returned will instead be returned as the correspondingly-signed zero.

If endptr is not NULL, a pointer to the character after the last character 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.

The input is converted by first rounding the value to the precision of the target type but allowing an unbounded exponent range. If the resulting exponent is too large for the target type, overflow is deemed to have occurred. Overflow is signalled by returning HUGE_VAL, HUGE_VALF, or HUGE_VALL (according to the sign of the input and type of the return value), and storing ERANGE in errno.

If the input string is not an explicit representation of zero and the correctly-rounded result is a subnormal or zero value, then ERANGE is stored in errno to indicate underflow has occurred.

ERRORS

[ERANGE]

Overflow or underflow occurred.

SEE ALSO

atof(3), atoi(3), atol(3), nan(3), strtod_l(3), strtol(3), strtoul(3), wcstod(3)

STANDARDS

The strtod() function conforms to ISO/IEC 9899:1999 (“ISO C99”).

Copyright (c) 1998 by Lucent Technologies
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