1STRTOD(3P) POSIX Programmer's Manual STRTOD(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 strtod, strtof, strtold — convert a string to a double-precision number
13
15 #include <stdlib.h>
16 double strtod(const char *restrict nptr, char **restrict endptr);
18 float strtof(const char *restrict nptr, char **restrict endptr);
19 long double strtold(const char *restrict nptr, char **restrict endptr);
20
22 The functionality described on this reference page is aligned with the
23 ISO C standard. Any conflict between the requirements described here
24 and the ISO C standard is unintentional. This volume of POSIX.1‐2017
25 defers to the ISO C standard.
26 These functions shall convert the initial portion of the string pointed
28 to by nptr to double, float, and long double representation, respec‐
29 tively. First, they decompose the input string into three parts:
30 1. An initial, possibly empty, sequence of white-space characters (as
32 specified by isspace())
33 2. A subject sequence interpreted as a floating-point constant or rep‐
35 resenting infinity or NaN
36 3. A final string of one or more unrecognized characters, including
38 the terminating NUL character of the input string
39 Then they shall attempt to convert the subject sequence to a floating-
41 point number, and return the result.
42 The expected form of the subject sequence is an optional '+' or '-'
44 sign, then one of the following:
45 * A non-empty sequence of decimal digits optionally containing a
47 radix character; then an optional exponent part consisting of the
48 character 'e' or the character 'E', optionally followed by a '+' or
49 '-' character, and then followed by one or more decimal digits
50 * A 0x or 0X, then a non-empty sequence of hexadecimal digits option‐
52 ally containing a radix character; then an optional binary exponent
53 part consisting of the character 'p' or the character 'P', option‐
54 ally followed by a '+' or '-' character, and then followed by one
55 or more decimal digits
56 * One of INF or INFINITY, ignoring case
58 * One of NAN or NAN(n-char-sequenceopt), ignoring case in the NAN
60 part, where:
61 n-char-sequence:
64 digit
65 nondigit
66 n-char-sequence digit
67 n-char-sequence nondigit
68 The subject sequence is defined as the longest initial subsequence of
70 the input string, starting with the first non-white-space character,
71 that is of the expected form. The subject sequence contains no charac‐
72 ters if the input string is not of the expected form.
73 If the subject sequence has the expected form for a floating-point num‐
75 ber, the sequence of characters starting with the first digit or the
76 decimal-point character (whichever occurs first) shall be interpreted
77 as a floating constant of the C language, except that the radix charac‐
78 ter shall be used in place of a period, and that if neither an exponent
79 part nor a radix character appears in a decimal floating-point number,
80 or if a binary exponent part does not appear in a hexadecimal floating-
81 point number, an exponent part of the appropriate type with value zero
82 is assumed to follow the last digit in the string. If the subject
83 sequence begins with a <hyphen-minus>, the sequence shall be inter‐
84 preted as negated. A character sequence INF or INFINITY shall be inter‐
85 preted as an infinity, if representable in the return type, else as if
86 it were a floating constant that is too large for the range of the
87 return type. A character sequence NAN or NAN(n-char-sequenceopt) shall
88 be interpreted as a quiet NaN, if supported in the return type, else as
89 if it were a subject sequence part that does not have the expected
90 form; the meaning of the n-char sequences is implementation-defined. A
91 pointer to the final string is stored in the object pointed to by
92 endptr, provided that endptr is not a null pointer.
93 If the subject sequence has the hexadecimal form and FLT_RADIX is a
95 power of 2, the value resulting from the conversion is correctly
96 rounded.
97 The radix character is defined in the current locale (category
99 LC_NUMERIC). In the POSIX locale, or in a locale where the radix char‐
100 acter is not defined, the radix character shall default to a <period>
101 ('.').
102 In other than the C or POSIX locale, additional locale-specific subject
104 sequence forms may be accepted.
105 If the subject sequence is empty or does not have the expected form, no
107 conversion shall be performed; the value of nptr is stored in the
108 object pointed to by endptr, provided that endptr is not a null
109 pointer.
110 These functions shall not change the setting of errno if successful.
112 Since 0 is returned on error and is also a valid return on success, an
114 application wishing to check for error situations should set errno to
115 0, then call strtod(), strtof(), or strtold(), then check errno.
116
118 Upon successful completion, these functions shall return the converted
119 value. If no conversion could be performed, 0 shall be returned, and
120 errno may be set to [EINVAL].
121 If the correct value is outside the range of representable values,
123 ±HUGE_VAL, ±HUGE_VALF, or ±HUGE_VALL shall be returned (according to
124 the sign of the value), and errno shall be set to [ERANGE].
125 If the correct value would cause an underflow, a value whose magnitude
127 is no greater than the smallest normalized positive number in the
128 return type shall be returned and errno set to [ERANGE].
129
131 These functions shall fail if:
132 ERANGE The value to be returned would cause overflow or underflow.
134 These functions may fail if:
136 EINVAL No conversion could be performed.
138 The following sections are informative.
140
142 None.
143
145 If the subject sequence has the hexadecimal form and FLT_RADIX is not a
146 power of 2, and the result is not exactly representable, the result
147 should be one of the two numbers in the appropriate internal format
148 that are adjacent to the hexadecimal floating source value, with the
149 extra stipulation that the error should have a correct sign for the
150 current rounding direction.
151 If the subject sequence has the decimal form and at most DECIMAL_DIG
153 (defined in <float.h>) significant digits, the result should be cor‐
154 rectly rounded. If the subject sequence D has the decimal form and more
155 than DECIMAL_DIG significant digits, consider the two bounding, adja‐
156 cent decimal strings L and U, both having DECIMAL_DIG significant dig‐
157 its, such that the values of L, D, and U satisfy L <= D <= U. The
158 result should be one of the (equal or adjacent) values that would be
159 obtained by correctly rounding L and U according to the current round‐
160 ing direction, with the extra stipulation that the error with respect
161 to D should have a correct sign for the current rounding direction.
162 The changes to strtod() introduced by the ISO/IEC 9899:1999 standard
164 can alter the behavior of well-formed applications complying with the
165 ISO/IEC 9899:1990 standard and thus earlier versions of this standard.
166 One such example would be:
167 int
170 what_kind_of_number (char *s)
171 {
172 char *endp;
173 double d;
174 long l;
175 d = strtod(s, &endp);
177 if (s != endp && *endp == `\0')
178 printf("It's a float with value %g\n", d);
179 else
180 {
181 l = strtol(s, &endp, 0);
182 if (s != endp && *endp == `\0')
183 printf("It's an integer with value %ld\n", 1);
184 else
185 return 1;
186 }
187 return 0;
188 }
189 If the function is called with:
191 what_kind_of_number ("0x10")
194 an ISO/IEC 9899:1990 standard-compliant library will result in the
196 function printing:
197 It's an integer with value 16
200 With the ISO/IEC 9899:1999 standard, the result is:
202 It's a float with value 16
205 The change in behavior is due to the inclusion of floating-point num‐
207 bers in hexadecimal notation without requiring that either a decimal
208 point or the binary exponent be present.
209
211 None.
212
214 None.
215
217 fscanf(), isspace(), localeconv(), setlocale(), strtol()
218 The Base Definitions volume of POSIX.1‐2017, Chapter 7, Locale,
220 <float.h>, <stdlib.h>
221
223 Portions of this text are reprinted and reproduced in electronic form
224 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
225 table Operating System Interface (POSIX), The Open Group Base Specifi‐
226 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
227 Electrical and Electronics Engineers, Inc and The Open Group. In the
228 event of any discrepancy between this version and the original IEEE and
229 The Open Group Standard, the original IEEE and The Open Group Standard
230 is the referee document. The original Standard can be obtained online
231 at http://www.opengroup.org/unix/online.html .
232 Any typographical or formatting errors that appear in this page are
234 most likely to have been introduced during the conversion of the source
235 files to man page format. To report such errors, see https://www.ker‐
236 nel.org/doc/man-pages/reporting_bugs.html .
237IEEE/The Open Group 2017 STRTOD(3P)