1 .\" Copyright (c) 1999 Andries Brouwer (aeb@cwi.nl)
3 .\" Earlier versions of this page influenced the present text.
4 .\" It was derived from a Berkeley page with version
5 .\" @(#)printf.3 6.14 (Berkeley) 7/30/91
6 .\" converted for Linux by faith@cs.unc.edu, updated by
7 .\" Helmut.Geyer@iwr.uni-heidelberg.de, agulbra@troll.no and Bruno Haible.
9 .\" %%%LICENSE_START(GPLv2+_DOC_FULL)
10 .\" This is free documentation; you can redistribute it and/or
11 .\" modify it under the terms of the GNU General Public License as
12 .\" published by the Free Software Foundation; either version 2 of
13 .\" the License, or (at your option) any later version.
15 .\" The GNU General Public License's references to "object code"
16 .\" and "executables" are to be interpreted as the output of any
17 .\" document formatting or typesetting system, including
18 .\" intermediate and printed output.
20 .\" This manual is distributed in the hope that it will be useful,
21 .\" but WITHOUT ANY WARRANTY; without even the implied warranty of
22 .\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
23 .\" GNU General Public License for more details.
25 .\" You should have received a copy of the GNU General Public
26 .\" License along with this manual; if not, see
27 .\" <http://www.gnu.org/licenses/>.
30 .\" 1999-11-25 aeb - Rewritten, using SUSv2 and C99.
31 .\" 2000-07-26 jsm28@hermes.cam.ac.uk - three small fixes
32 .\" 2000-10-16 jsm28@hermes.cam.ac.uk - more fixes
34 .TH PRINTF 3 2013-09-04 "GNU" "Linux Programmer's Manual"
36 printf, fprintf, sprintf, snprintf, vprintf, vfprintf, vsprintf,
37 vsnprintf \- formatted output conversion
41 .BI "int printf(const char *" format ", ...);"
43 .BI "int fprintf(FILE *" stream ", const char *" format ", ...);"
45 .BI "int sprintf(char *" str ", const char *" format ", ...);"
47 .BI "int snprintf(char *" str ", size_t " size ", const char *" format ", ...);"
49 .B #include <stdarg.h>
51 .BI "int vprintf(const char *" format ", va_list " ap );
53 .BI "int vfprintf(FILE *" stream ", const char *" format ", va_list " ap );
55 .BI "int vsprintf(char *" str ", const char *" format ", va_list " ap );
57 .BI "int vsnprintf(char *" str ", size_t " size ", const char *" format \
61 Feature Test Macro Requirements for glibc (see
62 .BR feature_test_macros (7)):
69 _BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500 || _ISOC99_SOURCE ||
70 _POSIX_C_SOURCE\ >=\ 200112L;
79 family produce output according to a
88 the standard output stream;
92 write output to the given output
99 write to the character string
108 bytes (including the terminating null byte (\(aq\e0\(aq)) to
116 are equivalent to the functions
121 respectively, except that they are called with a
123 instead of a variable number of arguments.
124 These functions do not call the
127 Because they invoke the
131 is undefined after the call.
135 These eight functions write the output under the control of a
137 string that specifies how subsequent arguments (or arguments accessed via
138 the variable-length argument facilities of
140 are converted for output.
142 C99 and POSIX.1-2001 specify that the results are undefined if a call to
148 would cause copying to take place between objects that overlap
149 (e.g., if the target string array and one of the supplied input arguments
150 refer to the same buffer).
153 Upon successful return, these functions return the number of characters
154 printed (excluding the null byte used to end output to strings).
160 do not write more than
162 bytes (including the terminating null byte (\(aq\e0\(aq)).
163 If the output was truncated due to this limit then the return value
164 is the number of characters (excluding the terminating null byte)
165 which would have been written to the final string if enough space
167 Thus, a return value of
169 or more means that the output was truncated.
170 (See also below under NOTES.)
172 If an output error is encountered, a negative value is returned.
173 .SS Format of the format string
174 The format string is a character string, beginning and ending
175 in its initial shift state, if any.
176 The format string is composed of zero or more directives: ordinary
179 which are copied unchanged to the output stream;
180 and conversion specifications, each of which results in fetching zero or
181 more subsequent arguments.
182 Each conversion specification is introduced by
186 .IR "conversion specifier" .
187 In between there may be (in this order) zero or more
194 .IR "length modifier" .
196 The arguments must correspond properly (after type promotion) with the
197 conversion specifier.
198 By default, the arguments are used in the order
199 given, where each \(aq*\(aq and each conversion specifier asks for the next
200 argument (and it is an error if insufficiently many arguments are given).
201 One can also specify explicitly which argument is taken,
202 at each place where an argument is required, by writing "%m$" instead
203 of \(aq%\(aq and "*m$" instead of \(aq*\(aq,
204 where the decimal integer m denotes
205 the position in the argument list of the desired argument, indexed starting
211 printf("%*d", width, num);
219 printf("%2$*1$d", width, num);
224 The second style allows repeated references to the
226 The C99 standard does not include the style using \(aq$\(aq,
227 which comes from the Single UNIX Specification.
229 \(aq$\(aq is used, it must be used throughout for all conversions taking an
230 argument and all width and precision arguments, but it may be mixed
231 with "%%" formats which do not consume an argument.
233 gaps in the numbers of arguments specified using \(aq$\(aq; for example, if
234 arguments 1 and 3 are specified, argument 2 must also be specified
235 somewhere in the format string.
237 For some numeric conversions a radix character ("decimal point") or
238 thousands' grouping character is used.
239 The actual character used
244 uses \(aq.\(aq as radix character, and does not have a grouping character.
249 printf("%\(aq.2f", 1234567.89);
253 results in "1234567.89" in the POSIX locale, in "1234567,89" in the
254 nl_NL locale, and in "1.234.567,89" in the da_DK locale.
255 .SS The flag characters
256 The character % is followed by zero or more of the following flags:
259 The value should be converted to an "alternate form".
262 conversions, the first character of the output string is made zero
263 (by prefixing a 0 if it was not zero already).
268 conversions, a nonzero result has the string "0x" (or "0X" for
270 conversions) prepended to it.
281 conversions, the result will always contain a decimal point, even if no
282 digits follow it (normally, a decimal point appears in the results of those
283 conversions only if a digit follows).
288 conversions, trailing zeros are not removed from the result as they would
290 For other conversions, the result is undefined.
293 The value should be zero padded.
310 conversions, the converted value is padded on the left with zeros rather
316 flags both appear, the
319 If a precision is given with a numeric conversion
330 For other conversions, the behavior is undefined.
333 The converted value is to be left adjusted on the field boundary.
334 (The default is right justification.)
337 conversions, the converted value is padded on the right with blanks, rather
338 than on the left with blanks or zeros.
346 (a space) A blank should be left before a positive number
347 (or empty string) produced by a signed conversion.
350 A sign (+ or \-) should always be placed before a number produced by a signed
352 By default a sign is used only for negative numbers.
355 overrides a space if both are used.
357 The five flag characters above are defined in the C standard.
358 The SUSv2 specifies one further flag character.
361 For decimal conversion
369 the output is to be grouped with thousands' grouping characters
370 if the locale information indicates any.
371 Note that many versions of
373 cannot parse this option and will issue a warning.
375 include \fI%\(aqF\fP.
377 glibc 2.2 adds one further flag character.
380 For decimal integer conversion
384 the output uses the locale's alternative output digits, if any.
385 For example, since glibc 2.2.3 this will give Arabic-Indic digits
386 in the Persian ("fa_IR") locale.
387 .\" outdigits keyword in locale file
389 An optional decimal digit string (with nonzero first digit) specifying
390 a minimum field width.
391 If the converted value has fewer characters
392 than the field width, it will be padded with spaces on the left
393 (or right, if the left-adjustment flag has been given).
394 Instead of a decimal digit string one may write "*" or "*m$"
395 (for some decimal integer \fIm\fP) to specify that the field width
396 is given in the next argument, or in the \fIm\fP-th argument, respectively,
397 which must be of type
399 A negative field width is taken as a \(aq\-\(aq flag followed by a
400 positive field width.
401 In no case does a nonexistent or small field width cause truncation of a
402 field; if the result of a conversion is wider than the field width, the
403 field is expanded to contain the conversion result.
405 An optional precision, in the form of a period (\(aq.\(aq) followed by an
406 optional decimal digit string.
407 Instead of a decimal digit string one may write "*" or "*m$"
408 (for some decimal integer m) to specify that the precision
409 is given in the next argument, or in the m-th argument, respectively,
410 which must be of type
412 If the precision is given as just \(aq.\(aq, the precision is taken to
414 A negative precision is taken as if the precision were omitted.
415 This gives the minimum number of digits to appear for
423 conversions, the number of digits to appear after the radix character for
431 conversions, the maximum number of significant digits for
435 conversions, or the maximum number of characters to be printed from a
441 .SS The length modifier
442 Here, "integer conversion" stands for
453 A following integer conversion corresponds to a
457 argument, or a following
459 conversion corresponds to a pointer to a
464 A following integer conversion corresponds to a
467 .I unsigned short int
468 argument, or a following
470 conversion corresponds to a pointer to a
475 (ell) A following integer conversion corresponds to a
479 argument, or a following
481 conversion corresponds to a pointer to a
483 argument, or a following
485 conversion corresponds to a
487 argument, or a following
489 conversion corresponds to a pointer to
495 A following integer conversion corresponds to a
498 .I unsigned long long int
499 argument, or a following
501 conversion corresponds to a pointer to a
516 conversion corresponds to a
519 (C99 allows %LF, but SUSv2 does not.)
522 ("quad". 4.4BSD and Linux libc5 only.
524 This is a synonym for
528 A following integer conversion corresponds to an
535 A following integer conversion corresponds to a
546 A following integer conversion corresponds to a
550 The SUSv2 knows about only the length modifiers
578 .SS The conversion specifier
579 A character that specifies the type of conversion to be applied.
580 The conversion specifiers and their meanings are:
585 argument is converted to signed decimal notation.
586 The precision, if any, gives the minimum number of digits
587 that must appear; if the converted value requires fewer digits, it is
588 padded on the left with zeros.
589 The default precision is 1.
590 When 0 is printed with an explicit precision 0, the output is empty.
592 .BR o ", " u ", " x ", " X
595 argument is converted to unsigned octal
599 or unsigned hexadecimal
608 conversions; the letters
613 The precision, if any, gives the minimum number of digits
614 that must appear; if the converted value requires fewer digits, it is
615 padded on the left with zeros.
616 The default precision is 1.
617 When 0 is printed with an explicit precision 0, the output is empty.
622 argument is rounded and converted in the style
623 .RB [\-]d \&. ddd e \(+-dd
624 where there is one digit before the decimal-point character and the number
625 of digits after it is equal to the precision; if the precision is missing,
626 it is taken as 6; if the precision is zero, no decimal-point character
630 conversion uses the letter
634 to introduce the exponent.
635 The exponent always contains at least two
636 digits; if the value is zero, the exponent is 00.
641 argument is rounded and converted to decimal notation in the style
643 where the number of digits after the decimal-point character is equal to
644 the precision specification.
645 If the precision is missing, it is taken as
646 6; if the precision is explicitly zero, no decimal-point character appears.
647 If a decimal point appears, at least one digit appears before it.
649 (The SUSv2 does not know about
651 and says that character string representations for infinity and NaN
652 may be made available.
653 The C99 standard specifies "[\-]inf" or "[\-]infinity"
654 for infinity, and a string starting with "nan" for NaN, in the case of
656 conversion, and "[\-]INF" or "[\-]INFINITY" or "NAN*" in the case of
663 argument is converted in style
674 The precision specifies the number of significant digits.
675 If the precision is missing, 6 digits are given; if the precision is zero,
679 is used if the exponent from its conversion is less than \-4 or greater
680 than or equal to the precision.
681 Trailing zeros are removed from the
682 fractional part of the result; a decimal point appears only if it is
683 followed by at least one digit.
686 (C99; not in SUSv2) For
690 argument is converted to hexadecimal notation (using the letters abcdef)
692 .RB [\-] 0x h \&. hhhh p \(+-;
695 conversion the prefix
697 the letters ABCDEF, and the exponent separator
700 There is one hexadecimal digit before the decimal point,
701 and the number of digits after it is equal to the precision.
702 The default precision suffices for an exact representation of the value
703 if an exact representation in base 2 exists
704 and otherwise is sufficiently large to distinguish values of type
706 The digit before the decimal point is unspecified for nonnormalized
707 numbers, and nonzero but otherwise unspecified for normalized numbers.
712 modifier is present, the
714 argument is converted to an
715 .IR "unsigned char" ,
716 and the resulting character is written.
719 modifier is present, the
721 (wide character) argument is converted to a multibyte sequence by a call
724 function, with a conversion state starting in the initial state, and the
725 resulting multibyte string is written.
730 modifier is present: The
732 argument is expected to be a pointer to an array of character type (pointer
734 Characters from the array are written up to (but not
735 including) a terminating null byte (\(aq\\0\(aq);
736 if a precision is specified, no more than the number specified
738 If a precision is given, no null byte need be present;
739 if the precision is not specified, or is greater than the size of the
740 array, the array must contain a terminating null byte.
744 modifier is present: The
745 .I "const wchar_t\ *"
746 argument is expected to be a pointer to an array of wide characters.
747 Wide characters from the array are converted to multibyte characters
748 (each by a call to the
750 function, with a conversion state starting in the initial state before
751 the first wide character), up to and including a terminating null
753 The resulting multibyte characters are written up to
754 (but not including) the terminating null byte.
756 specified, no more bytes than the number specified are written, but
757 no partial multibyte characters are written.
758 Note that the precision
759 determines the number of
761 written, not the number of
764 .IR "screen positions" .
765 The array must contain a terminating null wide character, unless a
766 precision is given and it is so small that the number of bytes written
767 exceeds it before the end of the array is reached.
770 (Not in C99, but in SUSv2.)
776 (Not in C99, but in SUSv2.)
784 pointer argument is printed in hexadecimal (as if by
790 The number of characters written so far is stored into the integer
793 (or variant) pointer argument.
794 No argument is converted.
799 .IR strerror(errno) .
800 No argument is required.
803 A \(aq%\(aq is written.
804 No argument is converted.
805 The complete conversion
806 specification is \(aq%%\(aq.
816 functions conform to C89 and C99.
821 functions conform to C99.
823 Concerning the return value of
825 SUSv2 and C99 contradict each other: when
829 then SUSv2 stipulates an unspecified return value less than 1,
832 to be NULL in this case, and gives the return value (as always)
833 as the number of characters that would have been written in case
834 the output string has been large enough.
836 Linux libc4 knows about the five C standard flags.
837 It knows about the length modifiers \fBh\fP, \fBl\fP, \fBL\fP,
839 \fBc\fP, \fBd\fP, \fBe\fP, \fBE\fP, \fBf\fP, \fBF\fP,
840 \fBg\fP, \fBG\fP, \fBi\fP, \fBn\fP, \fBo\fP, \fBp\fP,
841 \fBs\fP, \fBu\fP, \fBx\fP, and \fBX\fP,
842 where \fBF\fP is a synonym for \fBf\fP.
843 Additionally, it accepts \fBD\fP, \fBO\fP, and \fBU\fP as synonyms
844 for \fBld\fP, \fBlo\fP, and \fBlu\fP.
845 (This is bad, and caused serious bugs later, when
846 support for \fB%D\fP disappeared.)
847 No locale-dependent radix character,
848 no thousands' separator, no NaN or infinity, no "%m$" and "*m$".
850 Linux libc5 knows about the five C standard flags and the \(aq flag,
851 locale, "%m$" and "*m$".
852 It knows about the length modifiers \fBh\fP, \fBl\fP, \fBL\fP,
853 \fBZ\fP, and \fBq\fP, but accepts \fBL\fP and \fBq\fP
854 both for \fIlong double\fP and for \fIlong long int\fP (this is a bug).
855 It no longer recognizes \fBF\fP, \fBD\fP, \fBO\fP, and \fBU\fP,
856 but adds the conversion character
859 .IR strerror(errno) .
861 glibc 2.0 adds conversion characters \fBC\fP and \fBS\fP.
863 glibc 2.1 adds length modifiers \fBhh\fP, \fBj\fP, \fBt\fP, and \fBz\fP
864 and conversion characters \fBa\fP and \fBA\fP.
866 glibc 2.2 adds the conversion character \fBF\fP with C99 semantics,
867 and the flag character \fBI\fP.
869 Some programs imprudently rely on code such as the following
871 sprintf(buf, "%s some further text", buf);
875 However, the standards explicitly note that the results are undefined
876 if source and destination buffers overlap when calling
882 .\" http://sourceware.org/bugzilla/show_bug.cgi?id=7075
883 Depending on the version of
885 used, and the compiler options employed, calls such as the above will
887 produce the expected results.
889 The glibc implementation of the functions
893 conforms to the C99 standard, that is, behaves as described above,
894 since glibc version 2.1.
895 Until glibc 2.0.6 they would return \-1
896 when the output was truncated.
898 .\" UNIX V7 defines the three routines
902 .\" and has the flag \-, the width or precision *, the length modifier l,
903 .\" and the conversions doxfegcsu, and also D,O,U,X as synonyms for ld,lo,lu,lx.
904 .\" This is still true for 2.9.1BSD, but 2.10BSD has the flags
905 .\" #, + and <space> and no longer mentions D,O,U,X.
910 .\" and warns not to use D,O,U,X.
911 .\" 4.3BSD Reno has the flag 0, the length modifiers h and L,
912 .\" and the conversions n, p, E, G, X (with current meaning)
913 .\" and deprecates D,O,U.
914 .\" 4.4BSD introduces the functions
917 .\" .BR vsnprintf (),
918 .\" and the length modifier q.
919 .\" FreeBSD also has functions
922 .\" .BR vasprintf (),
923 .\" that allocate a buffer large enough for
925 .\" In glibc there are functions
929 .\" that print to a file descriptor instead of a stream.
935 assume an arbitrarily long string, callers must be careful not to overflow
936 the actual space; this is often impossible to assure.
938 of the strings produced is locale-dependent and difficult to predict.
948 Linux libc4.[45] does not have a
950 but provides a libbsd that contains an
954 that is, one that ignores the
959 with early libc4 leads to serious security problems.
963 often indicates a bug, since
965 may contain a % character.
968 comes from untrusted user input, it may contain \fB%n\fP, causing the
970 call to write to memory and creating a security hole.
972 .\" Some floating-point conversions under early libc4
973 .\" caused memory leaks.
977 to five decimal places:
983 fprintf(stdout, "pi = %.5f\en", 4 * atan(1.0));
987 To print a date and time in the form "Sunday, July 3, 10:02",
992 are pointers to strings:
997 fprintf(stdout, "%s, %s %d, %.2d:%.2d\en",
998 weekday, month, day, hour, min);
1002 Many countries use the day-month-year order.
1003 Hence, an internationalized version must be able to print
1004 the arguments in an order specified by the format:
1009 fprintf(stdout, format,
1010 weekday, month, day, hour, min);
1016 depends on locale, and may permute the arguments.
1021 "%1$s, %3$d. %2$s, %4$d:%5$.2d\en"
1025 one might obtain "Sonntag, 3. Juli, 10:02".
1027 To allocate a sufficiently large string and print into it
1028 (code correct for both glibc 2.0 and glibc 2.1):
1030 If truncation occurs in glibc versions prior to 2.0.6, this is treated as an
1031 error instead of being handled gracefully.
1039 make_message(const char *fmt, ...)
1042 int size = 100; /* Guess we need no more than 100 bytes */
1046 if ((p = malloc(size)) == NULL)
1051 /* Try to print in the allocated space */
1054 n = vsnprintf(p, size, fmt, ap);
1057 /* Check error code */
1064 /* If that worked, return the string */
1069 /* Else try again with more space */
1071 size = n + 1; /* Precisely what is needed */
1074 if ((np = realloc (p, size)) == NULL) {