Linux manual pages Section 3  

Name

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

Synopsis

#include <stdlib.h>
double strtod( const char *nptr,
  char **endptr);
 
float strtof( const char *nptr,
  char **endptr);
 
long double strtold( const char *nptr,
  char **endptr);
 
[Note] Note
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
strtof(), strtold():
_XOPEN_SOURCE >= 600 || _ISOC99_SOURCE;
or cc -std=c99

DESCRIPTION

The strtod(), strtof(), and strtold() 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 (initial portion of the) string is optional leading white space as recognized by isspace(3), an optional plus ('+') or minus sign ('−') and then either (i) a decimal number, or (ii) a hexadecimal number, or (iii) an infinity, or (iv) a NAN (not-a-number).

A decimal number consists of a nonempty sequence of decimal digits possibly containing a radix character (decimal point, locale-dependent, usually '.'), optionally followed by a decimal exponent. A decimal exponent consists of an 'E' or 'e', followed by an optional plus or minus sign, followed by a nonempty sequence of decimal digits, and indicates multiplication by a power of 10.

A hexadecimal number consists of a "0x" or "0X" followed by a nonempty sequence of hexadecimal digits possibly containing a radix character, optionally followed by a binary exponent. A binary exponent consists of a 'P' or 'p', followed by an optional plus or minus sign, followed by a nonempty sequence of decimal digits, and indicates multiplication by a power of 2. At least one of radix character and binary exponent must be present.

An “infinity” is either "INF" or "INFINITY", disregarding case.

A NAN is "NAN" (disregarding case) optionally followed by '(', a sequence of characters, followed by ')'. The character string specifies in an implementation-dependent way the type of NAN.

RETURN VALUE

These functions return the converted value, if any.

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 would cause overflow, plus or minus HUGE_VAL (HUGE_VALF, HUGE_VALL) is returned (according to the sign of the value), and ERANGE is stored in errno. If the correct value would cause underflow, zero is returned and ERANGE is stored in errno.

ERRORS

ERANGE

Overflow or underflow occurred.

CONFORMING TO

C89 describes strtod(), C99 describes the other two functions.

NOTES

Since 0 can legitimately be returned on both success and failure, the calling program should set errno to 0 before the call, and then determine if an error occurred by checking whether errno has a nonzero value after the call.

EXAMPLE

See the example on the strtol(3) manual page; the use of the functions described in this manual page is similar.

SEE ALSO

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

COLOPHON

This page is part of release 3.24 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.