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12 .TH STRTOD 3C "Nov 1, 2003"
13 .SH NAME
14 strtod, strtof, strtold, atof \- convert string to floating-point number
15 .SH SYNOPSIS
16 .LP
17 .nf
18 #include <stdlib.h>
20 \fBdouble\fR \fBstrtod\fR(\fBconst char *restrict\fR \fInptr\fR, \fBchar **restrict\fR \fIendptr\fR);
21 .fi
23 .LP
24 .nf
25 \fBfloat\fR \fBstrtof\fR(\fBconst char *restrict\fR \fInptr\fR, \fBchar **restrict\fR \fIendptr\fR);
26 .fi
28 .LP
29 .nf
30 \fBlong double\fR \fBstrtold\fR(\fBconst char *restrict\fR \fInptr\fR, \fBchar **restrict\fR \fIendptr\fR);
31 .fi
33 .LP
34 .nf
35 \fBdouble\fR \fBatof\fR(\fBconst char *\fR\fIstr\fR);
36 .fi
38 .SH DESCRIPTION
39 .sp
40 .LP
41 The \fBstrtod()\fR, \fBstrtof()\fR, and \fBstrtold()\fR functions convert the
42 initial portion of the string pointed to by \fInptr\fR to \fBdouble\fR,
43 \fBfloat\fR, and \fBlong double\fR representation, respectively. First they
44 decompose the input string into three parts:
45 .RS +4
46 .TP
47 1.
48 An initial, possibly empty, sequence of white-space characters (as specified
49 by \fBisspace\fR(3C))
50 .RE
51 .RS +4
52 .TP
53 2.
54 A subject sequence interpreted as a floating-point constant or representing
55 infinity or NaN
56 .RE
57 .RS +4
58 .TP
59 3.
60 A final string of one or more unrecognized characters, including the
61 terminating null byte of the input string.
62 .RE
63 .sp
64 .LP
65 Then they attempt to convert the subject sequence to a floating-point number,
66 and return the result.
67 .sp
68 .LP
69 The expected form of the subject sequence is an optional plus or minus sign,
70 then one of the following:
71 .RS +4
72 .TP
73 .ie t \(bu
74 .el o
75 A non-empty sequence of digits optionally containing a radix character, then an
76 optional exponent part
77 .RE
78 .RS +4
79 .TP
80 .ie t \(bu
81 .el o
82 A 0x or 0X, then a non-empty sequence of hexadecimal digits optionally
83 containing a radix character, then an optional binary exponent part
84 .RE
85 .RS +4
86 .TP
87 .ie t \(bu
88 .el o
89 One of INF or INFINITY, ignoring case
90 .RE
91 .RS +4
92 .TP
93 .ie t \(bu
94 .el o
95 One of NAN or NAN(\fIn-char-sequence\fR(\fIopt\fR)), ignoring case in the NAN
96 part, where:
97 .sp
98 .in +2
99 .nf
100 n-char-sequence:
101 digit
102 nondigit
103 n-char-sequence digit
104 n-char-sequence nondigit
105 .fi
106 .in -2
108 .RE
109 .sp
110 .LP
111 In default mode for \fBstrtod()\fR, only decimal, INF/INFINITY, and
112 NAN/NAN(\fIn-char-sequence\fR) forms are recognized. In C99/SUSv3 mode,
113 hexadecimal strings are also recognized.
114 .sp
115 .LP
116 In default mode for \fBstrtod()\fR, the \fIn-char-sequence\fR in the
117 NAN(\fIn-char-equence\fR) form can contain any character except ')' (right
118 parenthesis) or '\e0' (null). In C99/SUSv3 mode, the \fIn-char-sequence\fR can
119 contain only upper and lower case letters, digits, and '_' (underscore).
120 .sp
121 .LP
122 The \fBstrtof()\fR and \fBstrtold()\fR functions always function in
123 C99/SUSv3-conformant mode.
124 .sp
125 .LP
126 The subject sequence is defined as the longest initial subsequence of the input
127 string, starting with the first non-white-space character, that is of the
128 expected form. The subject sequence contains no characters if the input string
129 is not of the expected form.
130 .sp
131 .LP
132 If the subject sequence has the expected form for a floating-point number, the
133 sequence of characters starting with the first digit or the decimal-point
134 character (whichever occurs first) is interpreted as a floating constant of the
135 C language, except that the radix character is used in place of a period, and
136 that if neither an exponent part nor a radix character appears in a decimal
137 floating-point number, or if a binary exponent part does not appear in a
138 hexadecimal floating-point number, an exponent part of the appropriate type
139 with value zero is assumed to follow the last digit in the string. If the
140 subject sequence begins with a minus sign, the sequence is interpreted as
141 negated. A character sequence INF or INFINITY is interpreted as an infinity. A
142 character sequence NAN or NAN(\fIn-char-sequence\fR(\fIopt\fR)) is interpreted
143 as a quiet NaN. A pointer to the final string is stored in the object pointed
144 to by \fIendptr\fR, provided that \fIendptr\fR is not a null pointer.
145 .sp
146 .LP
147 If the subject sequence has either the decimal or hexadecimal form, the value
148 resulting from the conversion is rounded correctly according to the prevailing
149 floating point rounding direction mode. The conversion also raises floating
150 point inexact, underflow, or overflow exceptions as appropriate.
151 .sp
152 .LP
153 The radix character is defined in the program's locale (category
154 \fBLC_NUMERIC\fR). In the POSIX locale, or in a locale where the radix
155 character is not defined, the radix character defaults to a period ('.').
156 .sp
157 .LP
158 If the subject sequence is empty or does not have the expected form, no
159 conversion is performed; the value of \fInptr\fR is stored in the object
160 pointed to by \fIendptr\fR, provided that \fIendptr\fR is not a null pointer.
161 .sp
162 .LP
163 The \fBstrtod()\fR function does not change the setting of \fBerrno\fR if
164 successful.
165 .sp
166 .LP
167 The \fBatof(\fR\fIstr\fR\fB)\fR function call is equivalent to
168 \fBstrtod(\fR\fInptr\fR\fB, (char **)NULL)\fR.
169 .SH RETURN VALUES
170 .sp
171 .LP
172 Upon successful completion, these functions return the converted value. If no
173 conversion could be performed, \fB0\fR is returned.
174 .sp
175 .LP
176 If the correct value is outside the range of representable values,
177 \fB\(+-HUGE_VAL\fR, \fB\(+-HUGE_VALF\fR, or \fB\(+-HUGE_VALL\fR is returned
178 (according to the sign of the value), a floating point overflow exception is
179 raised, and \fBerrno\fR is set to \fBERANGE\fR.
180 .sp
181 .LP
182 If the correct value would cause an underflow, the correctly rounded result
183 (which may be normal, subnormal, or zero) is returned, a floating point
184 underflow exception is raised, and \fBerrno\fR is set to \fBERANGE\fR.
185 .SH ERRORS
186 .sp
187 .LP
188 These functions will fail if:
189 .sp
190 .ne 2
191 .na
192 \fB\fBERANGE\fR\fR
193 .ad
194 .RS 10n
195 The value to be returned would cause overflow or underflow
196 .RE
198 .sp
199 .LP
200 These functions may fail if:
201 .sp
202 .ne 2
203 .na
204 \fB\fBEINVAL\fR\fR
205 .ad
206 .RS 10n
207 No conversion could be performed.
208 .RE
210 .SH USAGE
211 .sp
212 .LP
213 Since 0 is returned on error and is also a valid return on success, an
214 application wishing to check for error situations should set \fBerrno\fR to 0,
215 then call \fBstrtod()\fR, \fBstrtof()\fR, or \fBstrtold()\fR, then check
216 \fBerrno\fR.
217 .sp
218 .LP
219 The changes to \fBstrtod()\fR introduced by the ISO/IEC 9899: 1999 standard can
220 alter the behavior of well-formed applications complying with the ISO/IEC 9899:
221 1990 standard and thus earlier versions of IEEE Std 1003.1-200x. One such
222 example would be:
223 .sp
224 .in +2
225 .nf
226 int
227 what_kind_of_number (char *s)
228 {
229 char *endp;
230 double d;
231 long l;
232 d = strtod(s, &endp);
233 if (s != endp && *endp == `\0')
234 printf("It's a float with value %g\en", d);
235 else
236 {
237 l = strtol(s, &endp, 0);
238 if (s != endp && *endp == `\e0')
239 printf("It's an integer with value %ld\en", 1);
240 else
241 return 1;
242 }
243 return 0;
244 }
245 .fi
246 .in -2
248 .sp
249 .LP
250 If the function is called with:
251 .sp
252 .in +2
253 .nf
254 what_kind_of_number ("0x10")
255 .fi
256 .in -2
258 .sp
259 .LP
260 an ISO/IEC 9899: 1990 standard-compliant library will result in the function
261 printing:
262 .sp
263 .in +2
264 .nf
265 It's an integer with value 16
266 .fi
267 .in -2
269 .sp
270 .LP
271 With the ISO/IEC 9899: 1999 standard, the result is:
272 .sp
273 .in +2
274 .nf
275 It's a float with value 16
276 .fi
277 .in -2
279 .sp
280 .LP
281 The change in behavior is due to the inclusion of floating-point numbers in
282 hexadecimal notation without requiring that either a decimal point or the
283 binary exponent be present.
284 .SH ATTRIBUTES
285 .sp
286 .LP
287 See \fBattributes\fR(5) for descriptions of the following attributes:
288 .sp
290 .sp
291 .TS
292 box;
293 c | c
294 l | l .
295 ATTRIBUTE TYPE ATTRIBUTE VALUE
296 _
297 CSI Enabled
298 _
299 Interface Stability Standard
300 _
301 MT-Level MT-Safe with exceptions
302 .TE
304 .SH SEE ALSO
305 .sp
306 .LP
307 \fBisspace\fR(3C), \fBlocaleconv\fR(3C), \fBscanf\fR(3C), \fBsetlocale\fR(3C),
308 \fBstrtol\fR(3C), \fBattributes\fR(5), \fBstandards\fR(5)
309 .SH NOTES
310 .sp
311 .LP
312 The \fBstrtod()\fR and \fBatof()\fR functions can be used safely in
313 multithreaded applications, as long as \fBsetlocale\fR(3C) is not called to
314 change the locale.
315 .sp
316 .LP
317 The DESCRIPTION and RETURN VALUES sections above are very similar to the
318 wording used by the Single UNIX Specification version 2 (SUSv2) and the 1989 C
319 Standard to describe the behavior of the \fBstrtod()\fR function. Since some
320 users have reported that they find the description confusing, the following
321 notes might be helpful.
322 .RS +4
323 .TP
324 1.
325 The \fBstrtod()\fR function does not modify the string pointed to by
326 \fIstr\fR and does not \fBmalloc()\fR space to hold the decomposed portions of
327 the input string.
328 .RE
329 .RS +4
330 .TP
331 2.
332 If \fIendptr\fR is not \fB(char **)NULL\fR, \fBstrtod()\fR will set the
333 pointer pointed to by \fIendptr\fR to the first byte of the "final string of
334 unrecognized characters". (If all input characters were processed, the pointer
335 pointed to by \fIendptr\fR will be set to point to the null character at the
336 end of the input string.)
337 .RE
338 .RS +4
339 .TP
340 3.
341 If \fBstrtod()\fR returns 0.0, one of the following occurred:
342 .RS +4
343 .TP
344 a.
345 The "subject sequence" was not an empty string, but evaluated to 0.0. (In
346 this case, \fBerrno\fR will be left unchanged.)
347 .RE
348 .RS +4
349 .TP
350 b.
351 The "subject sequence" was an empty string . In this case, \fBerrno\fR will
352 be left unchanged. (The Single UNIX Specification version 2 allows \fBerrno\fR
353 to be set to \fBEINVAL\fR or to be left unchanged. The C Standard does not
354 specify any specific behavior in this case.)
355 .RE
356 .RS +4
357 .TP
358 c.
359 The "subject sequence" specified a numeric value whose conversion resulted
360 in a floating point underflow. In this case, an underflow exception is raised
361 and \fBerrno\fR is set to \fBERANGE\fR.
362 .RE
363 Note that the standards do not require that implementations distinguish between
364 these three cases. An application can determine case (b) by making sure that
365 there are no leading white-space characters in the string pointed to by
366 \fIstr\fR and giving \fBstrtod()\fR an \fIendptr\fR that is not \fB(char
367 **)NULL\fR. If \fIendptr\fR points to the first character of \fIstr\fR when
368 \fBstrtod()\fR returns, you have detected case (b). Case (c) can be detected
369 by examining the underflow flag or by looking for a non-zero digit before the
370 exponent part of the "subject sequence". Note, however, that the decimal-point
371 character is locale-dependent.
372 .RE
373 .RS +4
374 .TP
375 4.
376 If \fBstrtod()\fR returns \fB+HUGE_VAL\fR or \fB\(miHUGE_VAL\fR, one of the
377 following occurred:
378 .RS +4
379 .TP
380 a.
381 If \fB+HUGE_VAL\fR is returned and \fBerrno\fR is set to \fBERANGE\fR, a
382 floating point overflow occurred while processing a positive value, causing a
383 floating point overflow exception to be raised.
384 .RE
385 .RS +4
386 .TP
387 b.
388 If \fB\(miHUGE_VAL\fR is returned and \fBerrno\fR is set to \fBERANGE\fR, a
389 floating point overflow occurred while processing a negative value, causing a
390 floating point overflow exception to be raised.
391 .RE
392 .RS +4
393 .TP
394 c.
395 If \fBstrtod()\fR does not set \fBerrno\fR to \fBERANGE\fR, the value
396 specified by the "subject string" converted to \fB+HUGE_VAL\fR or
397 \fB\(miHUGE_VAL\fR, respectively.
398 .RE
399 Note that if \fBerrno\fR is set to \fBERANGE\fR when \fBstrtod()\fR is called,
400 case (c) can be distinguished from cases (a) and (b) by examining either
401 \fBERANGE\fR or the overflow flag.
402 .RE