1STRTOD(P) POSIX Programmer's Manual STRTOD(P)
2
6 strtod, strtof, strtold - convert a string to a double-precision number
7
9 #include <stdlib.h>
10 double strtod(const char *restrict nptr, char **restrict endptr);
12 float strtof(const char *restrict nptr, char **restrict endptr);
13 long double strtold(const char *restrict nptr, char **restrict endptr);
14
17 These functions shall convert the initial portion of the string pointed
18 to by nptr to double, float, and long double representation, respec‐
19 tively. First, they decompose the input string into three parts:
20 1. An initial, possibly empty, sequence of white-space characters (as
22 specified by isspace())
23 2. A subject sequence interpreted as a floating-point constant or rep‐
25 resenting infinity or NaN
26 3. A final string of one or more unrecognized characters, including
28 the terminating null byte of the input string
29 Then they shall attempt to convert the subject sequence to a floating-
31 point number, and return the result.
32 The expected form of the subject sequence is an optional plus or minus
34 sign, then one of the following:
35 * A non-empty sequence of decimal digits optionally containing a radix
37 character, then an optional exponent part
38 * A 0x or 0X, then a non-empty sequence of hexadecimal digits option‐
40 ally containing a radix character, then an optional binary exponent
41 part
42 * One of INF or INFINITY, ignoring case
44 * One of NAN or NAN(n-char-sequence_opt), ignoring case in the NAN
46 part, where:
47 n-char-sequence:
50 digit
51 nondigit
52 n-char-sequence digit
53 n-char-sequence nondigit
54 The subject sequence is defined as the longest initial subsequence of
56 the input string, starting with the first non-white-space character,
57 that is of the expected form. The subject sequence contains no charac‐
58 ters if the input string is not of the expected form.
59 If the subject sequence has the expected form for a floating-point num‐
61 ber, the sequence of characters starting with the first digit or the
62 decimal-point character (whichever occurs first) shall be interpreted
63 as a floating constant of the C language, except that the radix charac‐
64 ter shall be used in place of a period, and that if neither an exponent
65 part nor a radix character appears in a decimal floating-point number,
66 or if a binary exponent part does not appear in a hexadecimal floating-
67 point number, an exponent part of the appropriate type with value zero
68 is assumed to follow the last digit in the string. If the subject
69 sequence begins with a minus sign, the sequence shall be interpreted as
70 negated. A character sequence INF or INFINITY shall be interpreted as
71 an infinity, if representable in the return type, else as if it were a
72 floating constant that is too large for the range of the return type. A
73 character sequence NAN or NAN(n-char-sequence_opt) shall be interpreted
74 as a quiet NaN, if supported in the return type, else as if it were a
75 subject sequence part that does not have the expected form; the meaning
76 of the n-char sequences is implementation-defined. A pointer to the
77 final string is stored in the object pointed to by endptr, provided
78 that endptr is not a null pointer.
79 If the subject sequence has the hexadecimal form and FLT_RADIX is a
81 power of 2, the value resulting from the conversion is correctly
82 rounded.
83 The radix character is defined in the program's locale (category
85 LC_NUMERIC ). In the POSIX locale, or in a locale where the radix char‐
86 acter is not defined, the radix character shall default to a period (
87 '.' ).
88 In other than the C or POSIX locales, other implementation-defined
90 subject sequences may be accepted.
91 If the subject sequence is empty or does not have the expected form, no
93 conversion shall be performed; the value of str is stored in the object
94 pointed to by endptr, provided that endptr is not a null pointer.
95 The strtod() function shall not change the setting of errno if success‐
97 ful.
98 Since 0 is returned on error and is also a valid return on success, an
100 application wishing to check for error situations should set errno to
101 0, then call strtod(), strtof(), or strtold(), then check errno.
102
104 Upon successful completion, these functions shall return the converted
105 value. If no conversion could be performed, 0 shall be returned, and
106 errno may be set to [EINVAL].
107 If the correct value is outside the range of representable values,
109 ±HUGE_VAL, ±HUGE_VALF, or ±HUGE_VALL shall be returned (according to
110 the sign of the value), and errno shall be set to [ERANGE].
111 If the correct value would cause an underflow, a value whose magnitude
113 is no greater than the smallest normalized positive number in the
114 return type shall be returned and errno set to [ERANGE].
115
117 These functions shall fail if:
118 ERANGE The value to be returned would cause overflow or underflow.
120 These functions may fail if:
123 EINVAL No conversion could be performed.
125 The following sections are informative.
128
130 None.
131
133 If the subject sequence has the hexadecimal form and FLT_RADIX is not a
134 power of 2, and the result is not exactly representable, the result
135 should be one of the two numbers in the appropriate internal format
136 that are adjacent to the hexadecimal floating source value, with the
137 extra stipulation that the error should have a correct sign for the
138 current rounding direction.
139 If the subject sequence has the decimal form and at most DECIMAL_DIG
141 (defined in <float.h>) significant digits, the result should be cor‐
142 rectly rounded. If the subject sequence D has the decimal form and more
143 than DECIMAL_DIG significant digits, consider the two bounding, adja‐
144 cent decimal strings L and U, both having DECIMAL_DIG significant dig‐
145 its, such that the values of L, D, and U satisfy L <= D <= U. The
146 result should be one of the (equal or adjacent) values that would be
147 obtained by correctly rounding L and U according to the current round‐
148 ing direction, with the extra stipulation that the error with respect
149 to D should have a correct sign for the current rounding direction.
150 The changes to strtod() introduced by the ISO/IEC 9899:1999 standard
152 can alter the behavior of well-formed applications complying with the
153 ISO/IEC 9899:1990 standard and thus earlier versions of the base docu‐
154 ments. One such example would be:
155 int
158 what_kind_of_number (char *s)
159 {
160 char *endp;
161 double d;
162 long l;
163 d = strtod(s, &endp);
166 if (s != endp && *endp == `\0')
167 printf("It's a float with value %g\n", d);
168 else
169 {
170 l = strtol(s, &endp, 0);
171 if (s != endp && *endp == `\0')
172 printf("It's an integer with value %ld\n", 1);
173 else
174 return 1;
175 }
176 return 0;
177 }
178 If the function is called with:
180 what_kind_of_number ("0x10")
183 an ISO/IEC 9899:1990 standard-compliant library will result in the
185 function printing:
186 It's an integer with value 16
189 With the ISO/IEC 9899:1999 standard, the result is:
191 It's a float with value 16
194 The change in behavior is due to the inclusion of floating-point num‐
196 bers in hexadecimal notation without requiring that either a decimal
197 point or the binary exponent be present.
198
200 None.
201
203 None.
204
206 isspace() , localeconv() , scanf() , setlocale() , strtol() , the Base
207 Definitions volume of IEEE Std 1003.1-2001, Chapter 7, Locale,
208 <float.h>, <stdlib.h>
209
211 Portions of this text are reprinted and reproduced in electronic form
212 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
213 -- Portable Operating System Interface (POSIX), The Open Group Base
214 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
215 Electrical and Electronics Engineers, Inc and The Open Group. In the
216 event of any discrepancy between this version and the original IEEE and
217 The Open Group Standard, the original IEEE and The Open Group Standard
218 is the referee document. The original Standard can be obtained online
219 at http://www.opengroup.org/unix/online.html .
220IEEE/The Open Group 2003 STRTOD(P)