strtod(3) - Manual pages

NAME

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

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 implementation 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 supported, 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 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.

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 */