1FPRINTF(3P) POSIX Programmer's Manual FPRINTF(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 dprintf, fprintf, printf, snprintf, sprintf — print formatted output
13
15 #include <stdio.h>
16 int dprintf(int fildes, const char *restrict format, ...);
18 int fprintf(FILE *restrict stream, const char *restrict format, ...);
19 int printf(const char *restrict format, ...);
20 int snprintf(char *restrict s, size_t n,
21 const char *restrict format, ...);
22 int sprintf(char *restrict s, const char *restrict format, ...);
23
25 Excluding dprintf(): The functionality described on this reference page
26 is aligned with the ISO C standard. Any conflict between the require‐
27 ments described here and the ISO C standard is unintentional. This vol‐
28 ume of POSIX.1‐2017 defers to the ISO C standard.
29 The fprintf() function shall place output on the named output stream.
31 The printf() function shall place output on the standard output stream
32 stdout. The sprintf() function shall place output followed by the null
33 byte, '\0', in consecutive bytes starting at *s; it is the user's
34 responsibility to ensure that enough space is available.
35 The dprintf() function shall be equivalent to the fprintf() function,
37 except that dprintf() shall write output to the file associated with
38 the file descriptor specified by the fildes argument rather than place
39 output on a stream.
40 The snprintf() function shall be equivalent to sprintf(), with the
42 addition of the n argument which states the size of the buffer referred
43 to by s. If n is zero, nothing shall be written and s may be a null
44 pointer. Otherwise, output bytes beyond the n‐1st shall be discarded
45 instead of being written to the array, and a null byte is written at
46 the end of the bytes actually written into the array.
47 If copying takes place between objects that overlap as a result of a
49 call to sprintf() or snprintf(), the results are undefined.
50 Each of these functions converts, formats, and prints its arguments
52 under control of the format. The format is a character string, begin‐
53 ning and ending in its initial shift state, if any. The format is com‐
54 posed of zero or more directives: ordinary characters, which are simply
55 copied to the output stream, and conversion specifications, each of
56 which shall result in the fetching of zero or more arguments. The
57 results are undefined if there are insufficient arguments for the for‐
58 mat. If the format is exhausted while arguments remain, the excess
59 arguments shall be evaluated but are otherwise ignored.
60 Conversions can be applied to the nth argument after the format in the
62 argument list, rather than to the next unused argument. In this case,
63 the conversion specifier character % (see below) is replaced by the
64 sequence "%n$", where n is a decimal integer in the range
65 [1,{NL_ARGMAX}], giving the position of the argument in the argument
66 list. This feature provides for the definition of format strings that
67 select arguments in an order appropriate to specific languages (see the
68 EXAMPLES section).
69 The format can contain either numbered argument conversion specifica‐
71 tions (that is, "%n$" and "*m$"), or unnumbered argument conversion
72 specifications (that is, % and *), but not both. The only exception to
73 this is that %% can be mixed with the "%n$" form. The results of mixing
74 numbered and unnumbered argument specifications in a format string are
75 undefined. When numbered argument specifications are used, specifying
76 the Nth argument requires that all the leading arguments, from the
77 first to the (N-1)th, are specified in the format string.
78 In format strings containing the "%n$" form of conversion specifica‐
80 tion, numbered arguments in the argument list can be referenced from
81 the format string as many times as required.
82 In format strings containing the % form of conversion specification,
84 each conversion specification uses the first unused argument in the
85 argument list.
86 All forms of the fprintf() functions allow for the insertion of a lan‐
88 guage-dependent radix character in the output string. The radix charac‐
89 ter is defined in the current locale (category LC_NUMERIC). In the
90 POSIX locale, or in a locale where the radix character is not defined,
91 the radix character shall default to a <period> ('.').
92 Each conversion specification is introduced by the '%' character or by
94 the character sequence "%n$", after which the following appear in
95 sequence:
96 * Zero or more flags (in any order), which modify the meaning of the
98 conversion specification.
99 * An optional minimum field width. If the converted value has fewer
101 bytes than the field width, it shall be padded with <space> charac‐
102 ters by default on the left; it shall be padded on the right if the
103 left-adjustment flag ('-'), described below, is given to the field
104 width. The field width takes the form of an <asterisk> ('*'),
105 described below, or a decimal integer.
106 * An optional precision that gives the minimum number of digits to
108 appear for the d, i, o, u, x, and X conversion specifiers; the num‐
109 ber of digits to appear after the radix character for the a, A, e,
110 E, f, and F conversion specifiers; the maximum number of signifi‐
111 cant digits for the g and G conversion specifiers; or the maximum
112 number of bytes to be printed from a string in the s and S conver‐
113 sion specifiers. The precision takes the form of a <period> ('.')
114 followed either by an <asterisk> ('*'), described below, or an
115 optional decimal digit string, where a null digit string is treated
116 as zero. If a precision appears with any other conversion speci‐
117 fier, the behavior is undefined.
118 * An optional length modifier that specifies the size of the argu‐
120 ment.
121 * A conversion specifier character that indicates the type of conver‐
123 sion to be applied.
124 A field width, or precision, or both, may be indicated by an <asterisk>
126 ('*'). In this case an argument of type int supplies the field width
127 or precision. Applications shall ensure that arguments specifying field
128 width, or precision, or both appear in that order before the argument,
129 if any, to be converted. A negative field width is taken as a '-' flag
130 followed by a positive field width. A negative precision is taken as if
131 the precision were omitted. In format strings containing the "%n$"
132 form of a conversion specification, a field width or precision may be
133 indicated by the sequence "*m$", where m is a decimal integer in the
134 range [1,{NL_ARGMAX}] giving the position in the argument list (after
135 the format argument) of an integer argument containing the field width
136 or precision, for example:
137 printf("%1$d:%2$.*3$d:%4$.*3$d\n", hour, min, precision, sec);
140 The flag characters and their meanings are:
142 ' (The <apostrophe>.) The integer portion of the result of a
144 decimal conversion (%i, %d, %u, %f, %F, %g, or %G) shall be
145 formatted with thousands' grouping characters. For other con‐
146 versions the behavior is undefined. The non-monetary grouping
147 character is used.
148 - The result of the conversion shall be left-justified within the
150 field. The conversion is right-justified if this flag is not
151 specified.
152 + The result of a signed conversion shall always begin with a
154 sign ('+' or '-'). The conversion shall begin with a sign only
155 when a negative value is converted if this flag is not speci‐
156 fied.
157 <space> If the first character of a signed conversion is not a sign or
159 if a signed conversion results in no characters, a <space>
160 shall be prefixed to the result. This means that if the <space>
161 and '+' flags both appear, the <space> flag shall be ignored.
162 # Specifies that the value is to be converted to an alternative
164 form. For o conversion, it shall increase the precision, if and
165 only if necessary, to force the first digit of the result to be
166 a zero (if the value and precision are both 0, a single 0 is
167 printed). For x or X conversion specifiers, a non-zero result
168 shall have 0x (or 0X) prefixed to it. For a, A, e, E, f, F, g,
169 and G conversion specifiers, the result shall always contain a
170 radix character, even if no digits follow the radix character.
171 Without this flag, a radix character appears in the result of
172 these conversions only if a digit follows it. For g and G con‐
173 version specifiers, trailing zeros shall not be removed from
174 the result as they normally are. For other conversion speci‐
175 fiers, the behavior is undefined.
176 0 For d, i, o, u, x, X, a, A, e, E, f, F, g, and G conversion
178 specifiers, leading zeros (following any indication of sign or
179 base) are used to pad to the field width rather than performing
180 space padding, except when converting an infinity or NaN. If
181 the '0' and '-' flags both appear, the '0' flag is ignored. For
182 d, i, o, u, x, and X conversion specifiers, if a precision is
183 specified, the '0' flag shall be ignored. If the '0' and
184 <apostrophe> flags both appear, the grouping characters are
185 inserted before zero padding. For other conversions, the behav‐
186 ior is undefined.
187 The length modifiers and their meanings are:
189 hh Specifies that a following d, i, o, u, x, or X conversion spec‐
191 ifier applies to a signed char or unsigned char argument (the
192 argument will have been promoted according to the integer pro‐
193 motions, but its value shall be converted to signed char or
194 unsigned char before printing); or that a following n conver‐
195 sion specifier applies to a pointer to a signed char argument.
196 h Specifies that a following d, i, o, u, x, or X conversion spec‐
198 ifier applies to a short or unsigned short argument (the argu‐
199 ment will have been promoted according to the integer promo‐
200 tions, but its value shall be converted to short or unsigned
201 short before printing); or that a following n conversion speci‐
202 fier applies to a pointer to a short argument.
203 l (ell) Specifies that a following d, i, o, u, x, or X conversion spec‐
205 ifier applies to a long or unsigned long argument; that a fol‐
206 lowing n conversion specifier applies to a pointer to a long
207 argument; that a following c conversion specifier applies to a
208 wint_t argument; that a following s conversion specifier
209 applies to a pointer to a wchar_t argument; or has no effect on
210 a following a, A, e, E, f, F, g, or G conversion specifier.
211 ll (ell-ell)
213 Specifies that a following d, i, o, u, x, or X conversion spec‐
214 ifier applies to a long long or unsigned long long argument; or
215 that a following n conversion specifier applies to a pointer to
216 a long long argument.
217 j Specifies that a following d, i, o, u, x, or X conversion spec‐
219 ifier applies to an intmax_t or uintmax_t argument; or that a
220 following n conversion specifier applies to a pointer to an
221 intmax_t argument.
222 z Specifies that a following d, i, o, u, x, or X conversion spec‐
224 ifier applies to a size_t or the corresponding signed integer
225 type argument; or that a following n conversion specifier
226 applies to a pointer to a signed integer type corresponding to
227 a size_t argument.
228 t Specifies that a following d, i, o, u, x, or X conversion spec‐
230 ifier applies to a ptrdiff_t or the corresponding unsigned type
231 argument; or that a following n conversion specifier applies to
232 a pointer to a ptrdiff_t argument.
233 L Specifies that a following a, A, e, E, f, F, g, or G conversion
235 specifier applies to a long double argument.
236 If a length modifier appears with any conversion specifier other than
238 as specified above, the behavior is undefined.
239 The conversion specifiers and their meanings are:
241 d, i The int argument shall be converted to a signed decimal in the
243 style "[-]dddd". The precision specifies the minimum number of
244 digits to appear; if the value being converted can be repre‐
245 sented in fewer digits, it shall be expanded with leading
246 zeros. The default precision is 1. The result of converting
247 zero with an explicit precision of zero shall be no characters.
248 o The unsigned argument shall be converted to unsigned octal for‐
250 mat in the style "dddd". The precision specifies the minimum
251 number of digits to appear; if the value being converted can be
252 represented in fewer digits, it shall be expanded with leading
253 zeros. The default precision is 1. The result of converting
254 zero with an explicit precision of zero shall be no characters.
255 u The unsigned argument shall be converted to unsigned decimal
257 format in the style "dddd". The precision specifies the minimum
258 number of digits to appear; if the value being converted can be
259 represented in fewer digits, it shall be expanded with leading
260 zeros. The default precision is 1. The result of converting
261 zero with an explicit precision of zero shall be no characters.
262 x The unsigned argument shall be converted to unsigned hexadeci‐
264 mal format in the style "dddd"; the letters "abcdef" are used.
265 The precision specifies the minimum number of digits to appear;
266 if the value being converted can be represented in fewer dig‐
267 its, it shall be expanded with leading zeros. The default pre‐
268 cision is 1. The result of converting zero with an explicit
269 precision of zero shall be no characters.
270 X Equivalent to the x conversion specifier, except that letters
272 "ABCDEF" are used instead of "abcdef".
273 f, F The double argument shall be converted to decimal notation in
275 the style "[-]ddd.ddd", where the number of digits after the
276 radix character is equal to the precision specification. If the
277 precision is missing, it shall be taken as 6; if the precision
278 is explicitly zero and no '#' flag is present, no radix charac‐
279 ter shall appear. If a radix character appears, at least one
280 digit appears before it. The low-order digit shall be rounded
281 in an implementation-defined manner.
282 A double argument representing an infinity shall be converted
284 in one of the styles "[-]inf" or "[-]infinity"; which style is
285 implementation-defined. A double argument representing a NaN
286 shall be converted in one of the styles "[-]nan(n-char-
287 sequence)" or "[-]nan"; which style, and the meaning of any n-
288 char-sequence, is implementation-defined. The F conversion
289 specifier produces "INF", "INFINITY", or "NAN" instead of
290 "inf", "infinity", or "nan", respectively.
291 e, E The double argument shall be converted in the style
293 "[-]d.ddde±dd", where there is one digit before the radix char‐
294 acter (which is non-zero if the argument is non-zero) and the
295 number of digits after it is equal to the precision; if the
296 precision is missing, it shall be taken as 6; if the precision
297 is zero and no '#' flag is present, no radix character shall
298 appear. The low-order digit shall be rounded in an implementa‐
299 tion-defined manner. The E conversion specifier shall produce a
300 number with 'E' instead of 'e' introducing the exponent. The
301 exponent shall always contain at least two digits. If the value
302 is zero, the exponent shall be zero.
303 A double argument representing an infinity or NaN shall be con‐
305 verted in the style of an f or F conversion specifier.
306 g, G The double argument representing a floating-point number shall
308 be converted in the style f or e (or in the style F or E in the
309 case of a G conversion specifier), depending on the value con‐
310 verted and the precision. Let P equal the precision if non-
311 zero, 6 if the precision is omitted, or 1 if the precision is
312 zero. Then, if a conversion with style E would have an exponent
313 of X:
314 -- If P>X≥-4, the conversion shall be with style f (or F) and
316 precision P-(X+1).
317 -- Otherwise, the conversion shall be with style e (or E) and
319 precision P-1.
320 Finally, unless the '#' flag is used, any trailing zeros shall
322 be removed from the fractional portion of the result and the
323 decimal-point character shall be removed if there is no frac‐
324 tional portion remaining.
325 A double argument representing an infinity or NaN shall be con‐
327 verted in the style of an f or F conversion specifier.
328 a, A A double argument representing a floating-point number shall be
330 converted in the style "[-]0xh.hhhhp±d", where there is one
331 hexadecimal digit (which shall be non-zero if the argument is a
332 normalized floating-point number and is otherwise unspecified)
333 before the decimal-point character and the number of hexadeci‐
334 mal digits after it is equal to the precision; if the precision
335 is missing and FLT_RADIX is a power of 2, then the precision
336 shall be sufficient for an exact representation of the value;
337 if the precision is missing and FLT_RADIX is not a power of 2,
338 then the precision shall be sufficient to distinguish values of
339 type double, except that trailing zeros may be omitted; if the
340 precision is zero and the '#' flag is not specified, no deci‐
341 mal-point character shall appear. The letters "abcdef" shall be
342 used for a conversion and the letters "ABCDEF" for A conver‐
343 sion. The A conversion specifier produces a number with 'X' and
344 'P' instead of 'x' and 'p'. The exponent shall always contain
345 at least one digit, and only as many more digits as necessary
346 to represent the decimal exponent of 2. If the value is zero,
347 the exponent shall be zero.
348 A double argument representing an infinity or NaN shall be con‐
350 verted in the style of an f or F conversion specifier.
351 c The int argument shall be converted to an unsigned char, and
353 the resulting byte shall be written.
354 If an l (ell) qualifier is present, the wint_t argument shall
356 be converted as if by an ls conversion specification with no
357 precision and an argument that points to a two-element array of
358 type wchar_t, the first element of which contains the wint_t
359 argument to the ls conversion specification and the second ele‐
360 ment contains a null wide character.
361 s The argument shall be a pointer to an array of char. Bytes
363 from the array shall be written up to (but not including) any
364 terminating null byte. If the precision is specified, no more
365 than that many bytes shall be written. If the precision is not
366 specified or is greater than the size of the array, the appli‐
367 cation shall ensure that the array contains a null byte.
368 If an l (ell) qualifier is present, the argument shall be a
370 pointer to an array of type wchar_t. Wide characters from the
371 array shall be converted to characters (each as if by a call to
372 the wcrtomb() function, with the conversion state described by
373 an mbstate_t object initialized to zero before the first wide
374 character is converted) up to and including a terminating null
375 wide character. The resulting characters shall be written up to
376 (but not including) the terminating null character (byte). If
377 no precision is specified, the application shall ensure that
378 the array contains a null wide character. If a precision is
379 specified, no more than that many characters (bytes) shall be
380 written (including shift sequences, if any), and the array
381 shall contain a null wide character if, to equal the character
382 sequence length given by the precision, the function would need
383 to access a wide character one past the end of the array. In no
384 case shall a partial character be written.
385 p The argument shall be a pointer to void. The value of the
387 pointer is converted to a sequence of printable characters, in
388 an implementation-defined manner.
389 n The argument shall be a pointer to an integer into which is
391 written the number of bytes written to the output so far by
392 this call to one of the fprintf() functions. No argument is
393 converted.
394 C Equivalent to lc.
396 S Equivalent to ls.
398 % Print a '%' character; no argument is converted. The complete
400 conversion specification shall be %%.
401 If a conversion specification does not match one of the above forms,
403 the behavior is undefined. If any argument is not the correct type for
404 the corresponding conversion specification, the behavior is undefined.
405 In no case shall a nonexistent or small field width cause truncation of
407 a field; if the result of a conversion is wider than the field width,
408 the field shall be expanded to contain the conversion result. Charac‐
409 ters generated by fprintf() and printf() are printed as if fputc() had
410 been called.
411 For the a and A conversion specifiers, if FLT_RADIX is a power of 2,
413 the value shall be correctly rounded to a hexadecimal floating number
414 with the given precision.
415 For a and A conversions, if FLT_RADIX is not a power of 2 and the
417 result is not exactly representable in the given precision, the result
418 should be one of the two adjacent numbers in hexadecimal floating style
419 with the given precision, with the extra stipulation that the error
420 should have a correct sign for the current rounding direction.
421 For the e, E, f, F, g, and G conversion specifiers, if the number of
423 significant decimal digits is at most DECIMAL_DIG, then the result
424 should be correctly rounded. If the number of significant decimal dig‐
425 its is more than DECIMAL_DIG but the source value is exactly repre‐
426 sentable with DECIMAL_DIG digits, then the result should be an exact
427 representation with trailing zeros. Otherwise, the source value is
428 bounded by two adjacent decimal strings L < U, both having DECIMAL_DIG
429 significant digits; the value of the resultant decimal string D should
430 satisfy L <= D <= U, with the extra stipulation that the error should
431 have a correct sign for the current rounding direction.
432 The last data modification and last file status change timestamps of
434 the file shall be marked for update:
435 1. Between the call to a successful execution of fprintf() or printf()
437 and the next successful completion of a call to fflush() or
438 fclose() on the same stream or a call to exit() or abort()
439 2. Upon successful completion of a call to dprintf()
441
443 Upon successful completion, the dprintf(), fprintf(), and printf()
444 functions shall return the number of bytes transmitted.
445 Upon successful completion, the sprintf() function shall return the
447 number of bytes written to s, excluding the terminating null byte.
448 Upon successful completion, the snprintf() function shall return the
450 number of bytes that would be written to s had n been sufficiently
451 large excluding the terminating null byte.
452 If an output error was encountered, these functions shall return a neg‐
454 ative value and set errno to indicate the error.
455 If the value of n is zero on a call to snprintf(), nothing shall be
457 written, the number of bytes that would have been written had n been
458 sufficiently large excluding the terminating null shall be returned,
459 and s may be a null pointer.
460
462 For the conditions under which dprintf(), fprintf(), and printf() fail
463 and may fail, refer to fputc() or fputwc().
464 In addition, all forms of fprintf() shall fail if:
466 EILSEQ A wide-character code that does not correspond to a valid char‐
468 acter has been detected.
469 EOVERFLOW
471 The value to be returned is greater than {INT_MAX}.
472 The dprintf() function may fail if:
474 EBADF The fildes argument is not a valid file descriptor.
476 The dprintf(), fprintf(), and printf() functions may fail if:
478 ENOMEM Insufficient storage space is available.
480 The snprintf() function shall fail if:
482 EOVERFLOW
484 The value of n is greater than {INT_MAX}.
485 The following sections are informative.
487
489 Printing Language-Independent Date and Time
490 The following statement can be used to print date and time using a lan‐
491 guage-independent format:
492 printf(format, weekday, month, day, hour, min);
495 For American usage, format could be a pointer to the following string:
497 "%s, %s %d, %d:%.2d\n"
500 This example would produce the following message:
502 Sunday, July 3, 10:02
505 For German usage, format could be a pointer to the following string:
507 "%1$s, %3$d. %2$s, %4$d:%5$.2d\n"
510 This definition of format would produce the following message:
512 Sonntag, 3. Juli, 10:02
515 Printing File Information
517 The following example prints information about the type, permissions,
518 and number of links of a specific file in a directory.
519 The first two calls to printf() use data decoded from a previous stat()
521 call. The user-defined strperm() function shall return a string similar
522 to the one at the beginning of the output for the following command:
523 ls -l
526 The next call to printf() outputs the owner's name if it is found using
528 getpwuid(); the getpwuid() function shall return a passwd structure
529 from which the name of the user is extracted. If the user name is not
530 found, the program instead prints out the numeric value of the user ID.
531 The next call prints out the group name if it is found using get‐
533 grgid(); getgrgid() is very similar to getpwuid() except that it shall
534 return group information based on the group number. Once again, if the
535 group is not found, the program prints the numeric value of the group
536 for the entry.
537 The final call to printf() prints the size of the file.
539 #include <stdio.h>
542 #include <sys/types.h>
543 #include <pwd.h>
544 #include <grp.h>
545 char *strperm (mode_t);
547 ...
548 struct stat statbuf;
549 struct passwd *pwd;
550 struct group *grp;
551 ...
552 printf("%10.10s", strperm (statbuf.st_mode));
553 printf("%4d", statbuf.st_nlink);
554 if ((pwd = getpwuid(statbuf.st_uid)) != NULL)
556 printf(" %-8.8s", pwd->pw_name);
557 else
558 printf(" %-8ld", (long) statbuf.st_uid);
559 if ((grp = getgrgid(statbuf.st_gid)) != NULL)
561 printf(" %-8.8s", grp->gr_name);
562 else
563 printf(" %-8ld", (long) statbuf.st_gid);
564 printf("%9jd", (intmax_t) statbuf.st_size);
566 ...
567 Printing a Localized Date String
569 The following example gets a localized date string. The nl_langinfo()
570 function shall return the localized date string, which specifies the
571 order and layout of the date. The strftime() function takes this infor‐
572 mation and, using the tm structure for values, places the date and time
573 information into datestring. The printf() function then outputs dat‐
574 estring and the name of the entry.
575 #include <stdio.h>
578 #include <time.h>
579 #include <langinfo.h>
580 ...
581 struct dirent *dp;
582 struct tm *tm;
583 char datestring[256];
584 ...
585 strftime(datestring, sizeof(datestring), nl_langinfo (D_T_FMT), tm);
586 printf(" %s %s\n", datestring, dp->d_name);
588 ...
589 Printing Error Information
591 The following example uses fprintf() to write error information to
592 standard error.
593 In the first group of calls, the program tries to open the password
595 lock file named LOCKFILE. If the file already exists, this is an
596 error, as indicated by the O_EXCL flag on the open() function. If the
597 call fails, the program assumes that someone else is updating the pass‐
598 word file, and the program exits.
599 The next group of calls saves a new password file as the current pass‐
601 word file by creating a link between LOCKFILE and the new password file
602 PASSWDFILE.
603 #include <sys/types.h>
606 #include <sys/stat.h>
607 #include <fcntl.h>
608 #include <stdio.h>
609 #include <stdlib.h>
610 #include <unistd.h>
611 #include <string.h>
612 #include <errno.h>
613 #define LOCKFILE "/etc/ptmp"
615 #define PASSWDFILE "/etc/passwd"
616 ...
617 int pfd;
618 ...
619 if ((pfd = open(LOCKFILE, O_WRONLY | O_CREAT | O_EXCL,
620 S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)) == -1)
621 {
622 fprintf(stderr, "Cannot open /etc/ptmp. Try again later.\n");
623 exit(1);
624 }
625 ...
626 if (link(LOCKFILE,PASSWDFILE) == -1) {
627 fprintf(stderr, "Link error: %s\n", strerror(errno));
628 exit(1);
629 }
630 ...
631 Printing Usage Information
633 The following example checks to make sure the program has the necessary
634 arguments, and uses fprintf() to print usage information if the
635 expected number of arguments is not present.
636 #include <stdio.h>
639 #include <stdlib.h>
640 ...
641 char *Options = "hdbtl";
642 ...
643 if (argc < 2) {
644 fprintf(stderr, "Usage: %s -%s <file\n", argv[0], Options); exit(1);
645 }
646 ...
647 Formatting a Decimal String
649 The following example prints a key and data pair on stdout. Note use
650 of the <asterisk> ('*') in the format string; this ensures the correct
651 number of decimal places for the element based on the number of ele‐
652 ments requested.
653 #include <stdio.h>
656 ...
657 long i;
658 char *keystr;
659 int elementlen, len;
660 ...
661 while (len < elementlen) {
662 ...
663 printf("%s Element%0*ld\n", keystr, elementlen, i);
664 ...
665 }
666 Creating a Pathname
668 The following example creates a pathname using information from a pre‐
669 vious getpwnam() function that returned the password database entry of
670 the user.
671 #include <stdint.h>
674 #include <stdio.h>
675 #include <stdlib.h>
676 #include <string.h>
677 #include <sys/types.h>
678 #include <unistd.h>
679 ...
680 char *pathname;
681 struct passwd *pw;
682 size_t len;
683 ...
684 // digits required for pid_t is number of bits times
685 // log2(10) = approx 10/33
686 len = strlen(pw->pw_dir) + 1 + 1+(sizeof(pid_t)*80+32)/33 +
687 sizeof ".out";
688 pathname = malloc(len);
689 if (pathname != NULL)
690 {
691 snprintf(pathname, len, "%s/%jd.out", pw->pw_dir,
692 (intmax_t)getpid());
693 ...
694 }
695 Reporting an Event
697 The following example loops until an event has timed out. The pause()
698 function waits forever unless it receives a signal. The fprintf()
699 statement should never occur due to the possible return values of
700 pause().
701 #include <stdio.h>
704 #include <unistd.h>
705 #include <string.h>
706 #include <errno.h>
707 ...
708 while (!event_complete) {
709 ...
710 if (pause() != -1 || errno != EINTR)
711 fprintf(stderr, "pause: unknown error: %s\n", strerror(errno));
712 }
713 ...
714 Printing Monetary Information
716 The following example uses strfmon() to convert a number and store it
717 as a formatted monetary string named convbuf. If the first number is
718 printed, the program prints the format and the description; otherwise,
719 it just prints the number.
720 #include <monetary.h>
723 #include <stdio.h>
724 ...
725 struct tblfmt {
726 char *format;
727 char *description;
728 };
729 struct tblfmt table[] = {
731 { "%n", "default formatting" },
732 { "%11n", "right align within an 11 character field" },
733 { "%#5n", "aligned columns for values up to 99999" },
734 { "%=*#5n", "specify a fill character" },
735 { "%=0#5n", "fill characters do not use grouping" },
736 { "%^#5n", "disable the grouping separator" },
737 { "%^#5.0n", "round off to whole units" },
738 { "%^#5.4n", "increase the precision" },
739 { "%(#5n", "use an alternative pos/neg style" },
740 { "%!(#5n", "disable the currency symbol" },
741 };
742 ...
743 float input[3];
744 int i, j;
745 char convbuf[100];
746 ...
747 strfmon(convbuf, sizeof(convbuf), table[i].format, input[j]);
748 if (j == 0) {
750 printf("%s%s%s\n", table[i].format,
751 convbuf, table[i].description);
752 }
753 else {
754 printf("%s\n", convbuf);
755 }
756 ...
757 Printing Wide Characters
759 The following example prints a series of wide characters. Suppose that
760 "L`@`" expands to three bytes:
761 wchar_t wz [3] = L"@@"; // Zero-terminated
764 wchar_t wn [3] = L"@@@"; // Unterminated
765 fprintf (stdout,"%ls", wz); // Outputs 6 bytes
767 fprintf (stdout,"%ls", wn); // Undefined because wn has no terminator
768 fprintf (stdout,"%4ls", wz); // Outputs 3 bytes
769 fprintf (stdout,"%4ls", wn); // Outputs 3 bytes; no terminator needed
770 fprintf (stdout,"%9ls", wz); // Outputs 6 bytes
771 fprintf (stdout,"%9ls", wn); // Outputs 9 bytes; no terminator needed
772 fprintf (stdout,"%10ls", wz); // Outputs 6 bytes
773 fprintf (stdout,"%10ls", wn); // Undefined because wn has no terminator
774 In the last line of the example, after processing three characters,
776 nine bytes have been output. The fourth character must then be examined
777 to determine whether it converts to one byte or more. If it converts to
778 more than one byte, the output is only nine bytes. Since there is no
779 fourth character in the array, the behavior is undefined.
780
782 If the application calling fprintf() has any objects of type wint_t or
783 wchar_t, it must also include the <wchar.h> header to have these
784 objects defined.
785
787 If an implementation detects that there are insufficient arguments for
788 the format, it is recommended that the function should fail and report
789 an [EINVAL] error.
790
792 None.
793
795 Section 2.5, Standard I/O Streams, fputc(), fscanf(), setlocale(),
796 strfmon(), wcrtomb()
797 The Base Definitions volume of POSIX.1‐2017, Chapter 7, Locale, <int‐
799 types.h>, <stdio.h>, <wchar.h>
800
802 Portions of this text are reprinted and reproduced in electronic form
803 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
804 table Operating System Interface (POSIX), The Open Group Base Specifi‐
805 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
806 Electrical and Electronics Engineers, Inc and The Open Group. In the
807 event of any discrepancy between this version and the original IEEE and
808 The Open Group Standard, the original IEEE and The Open Group Standard
809 is the referee document. The original Standard can be obtained online
810 at http://www.opengroup.org/unix/online.html .
811 Any typographical or formatting errors that appear in this page are
813 most likely to have been introduced during the conversion of the source
814 files to man page format. To report such errors, see https://www.ker‐
815 nel.org/doc/man-pages/reporting_bugs.html .
816IEEE/The Open Group 2017 FPRINTF(3P)