1PRINTF(3)                  Linux Programmer's Manual                 PRINTF(3)
2
3
4

NAME

6       printf,   fprintf,  sprintf,  snprintf,  vprintf,  vfprintf,  vsprintf,
7       vsnprintf - formatted output conversion
8

SYNOPSIS

10       #include <stdio.h>
11
12       int printf(const char *format, ...);
13       int fprintf(FILE *stream, const char *format, ...);
14       int sprintf(char *str, const char *format, ...);
15       int snprintf(char *str, size_t size, const char *format, ...);
16
17       #include <stdarg.h>
18
19       int vprintf(const char *format, va_list ap);
20       int vfprintf(FILE *stream, const char *format, va_list ap);
21       int vsprintf(char *str, const char *format, va_list ap);
22       int vsnprintf(char *str, size_t size, const char *format, va_list ap);
23
24   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
25
26       snprintf(), vsnprintf(): _BSD_SOURCE || _XOPEN_SOURCE >= 500 ||
27       _ISOC99_SOURCE; or cc -std=c99
28

DESCRIPTION

30       The functions in the printf() family produce output according to a for‐
31       mat as described below.  The functions  printf()  and  vprintf()  write
32       output  to stdout, the standard output stream; fprintf() and vfprintf()
33       write  output  to  the  given  output  stream;  sprintf(),  snprintf(),
34       vsprintf() and vsnprintf() write to the character string str.
35
36       The  functions  snprintf()  and  vsnprintf()  write  at most size bytes
37       (including the trailing null byte ('\0')) to str.
38
39       The functions vprintf(), vfprintf(), vsprintf(), vsnprintf() are equiv‐
40       alent  to  the  functions  printf(),  fprintf(), sprintf(), snprintf(),
41       respectively, except that they are called with a va_list instead  of  a
42       variable  number  of arguments.  These functions do not call the va_end
43       macro.  Because they invoke the va_arg macro, the value of ap is  unde‐
44       fined after the call.  See stdarg(3).
45
46       These  eight  functions  write the output under the control of a format
47       string that specifies how subsequent arguments (or  arguments  accessed
48       via the variable-length argument facilities of stdarg(3)) are converted
49       for output.
50
51       C99 and POSIX.1-2001 specify that the results are undefined if  a  call
52       to  sprintf(),  snprintf(),  vsprintf(),  or vsnprintf() would cause to
53       copying to take place between objects that overlap (e.g., if the target
54       string  array and one of the supplied input arguments refer to the same
55       buffer).  See NOTES.
56
57   Return value
58       Upon successful return, these functions return the number of characters
59       printed  (not  including  the  trailing  '\0'  used  to  end  output to
60       strings).
61
62       The functions snprintf() and vsnprintf() do not write  more  than  size
63       bytes  (including  the trailing '\0').  If the output was truncated due
64       to this limit then the return value is the number  of  characters  (not
65       including the trailing '\0') which would have been written to the final
66       string if enough space had been available.  Thus,  a  return  value  of
67       size  or  more  means  that  the output was truncated.  (See also below
68       under NOTES.)
69
70       If an output error is encountered, a negative value is returned.
71
72   Format of the format string
73       The format string is a character string, beginning and  ending  in  its
74       initial  shift state, if any.  The format string is composed of zero or
75       more  directives:  ordinary  characters  (not  %),  which  are   copied
76       unchanged  to the output stream; and conversion specifications, each of
77       which results in fetching zero or more subsequent arguments.  Each con‐
78       version specification is introduced by the character %, and ends with a
79       conversion specifier.  In between there may be (in this order) zero  or
80       more  flags, an optional minimum field width, an optional precision and
81       an optional length modifier.
82
83       The arguments must correspond properly (after type promotion) with  the
84       conversion  specifier.  By default, the arguments are used in the order
85       given, where each '*' and each conversion specifier asks for  the  next
86       argument  (and  it  is  an  error  if insufficiently many arguments are
87       given).  One can also specify explicitly which argument  is  taken,  at
88       each  place  where an argument is required, by writing "%m$" instead of
89       '%' and "*m$" instead of '*', where the decimal integer m  denotes  the
90       position in the argument list of the desired argument, indexed starting
91       from 1.  Thus,
92
93           printf("%*d", width, num);
94
95       and
96
97           printf("%2$*1$d", width, num);
98
99       are equivalent.  The second style allows  repeated  references  to  the
100       same  argument.  The C99 standard does not include the style using '$',
101       which comes from the Single Unix Specification.  If the style using '$'
102       is used, it must be used throughout for all conversions taking an argu‐
103       ment and all width and precision arguments, but it may  be  mixed  with
104       "%%" formats which do not consume an argument.  There may be no gaps in
105       the numbers of arguments specified using '$'; for example, if arguments
106       1  and  3 are specified, argument 2 must also be specified somewhere in
107       the format string.
108
109       For some numeric conversions a radix  character  ("decimal  point")  or
110       thousands'  grouping  character  is  used.   The  actual character used
111       depends on the LC_NUMERIC part of the locale.  The  POSIX  locale  uses
112       '.' as radix character, and does not have a grouping character.  Thus,
113
114               printf("%'.2f", 1234567.89);
115
116       results  in  "1234567.89"  in  the POSIX locale, in "1234567,89" in the
117       nl_NL locale, and in "1.234.567,89" in the da_DK locale.
118
119   The flag characters
120       The character % is followed by zero or more of the following flags:
121
122       #      The value should be converted to an  "alternate  form".   For  o
123              conversions,  the  first  character of the output string is made
124              zero (by prefixing a 0 if it was not zero already).  For x and X
125              conversions,  a non-zero result has the string "0x" (or "0X" for
126              X conversions) prepended to it.  For a, A, e, E, f, F, g, and  G
127              conversions,  the  result  will  always contain a decimal point,
128              even if no digits follow it (normally, a decimal  point  appears
129              in  the  results  of those conversions only if a digit follows).
130              For g and G conversions, trailing zeros are not removed from the
131              result  as  they would otherwise be.  For other conversions, the
132              result is undefined.
133
134       0      The value should be zero padded.  For d, i, o, u, x, X, a, A, e,
135              E,  f, F, g, and G conversions, the converted value is padded on
136              the left with zeros rather than blanks.  If the 0  and  -  flags
137              both  appear,  the  0  flag is ignored.  If a precision is given
138              with a numeric conversion (d, i, o, u, x, and X), the 0 flag  is
139              ignored.  For other conversions, the behavior is undefined.
140
141       -      The  converted  value is to be left adjusted on the field bound‐
142              ary.  (The default is right justification.)  Except for  n  con‐
143              versions,  the  converted  value  is  padded  on  the right with
144              blanks, rather than on the left with blanks or zeros.  A - over‐
145              rides a 0 if both are given.
146
147       ' '    (a  space)  A  blank should be left before a positive number (or
148              empty string) produced by a signed conversion.
149
150       +      A sign (+ or -) should always be placed before a number produced
151              by a signed conversion.  By default a sign is used only for neg‐
152              ative numbers.  A + overrides a space if both are used.
153
154       The five flag characters above are defined  in  the  C  standard.   The
155       SUSv2 specifies one further flag character.
156
157       '      For decimal conversion (i, d, u, f, F, g, G) the output is to be
158              grouped with thousands' grouping characters if the locale infor‐
159              mation  indicates any.  Note that many versions of gcc(1) cannot
160              parse this option and will issue  a  warning.   SUSv2  does  not
161              include %'F.
162
163       glibc 2.2 adds one further flag character.
164
165       I      For  decimal  integer  conversion  (i, d, u) the output uses the
166              locale's alternative output digits, if any.  For example,  since
167              glibc  2.2.3  this  will give Arabic-Indic digits in the Persian
168              ("fa_IR") locale.
169
170   The field width
171       An optional decimal digit string (with non-zero first digit) specifying
172       a  minimum  field  width.   If the converted value has fewer characters
173       than the field width, it will be padded with spaces  on  the  left  (or
174       right, if the left-adjustment flag has been given).  Instead of a deci‐
175       mal digit string one may write "*" or "*m$" (for some  decimal  integer
176       m) to specify that the field width is given in the next argument, or in
177       the m-th argument, respectively, which must be of type int.  A negative
178       field  width is taken as a '-' flag followed by a positive field width.
179       In no case does a nonexistent or small field width cause truncation  of
180       a  field;  if the result of a conversion is wider than the field width,
181       the field is expanded to contain the conversion result.
182
183   The precision
184       An optional precision, in the form of a period ('.')   followed  by  an
185       optional  decimal  digit string.  Instead of a decimal digit string one
186       may write "*" or "*m$" (for some decimal integer m) to specify that the
187       precision  is  given  in  the  next  argument, or in the m-th argument,
188       respectively, which must be of type int.  If the precision is given  as
189       just  '.',  or  the precision is negative, the precision is taken to be
190       zero.  This gives the minimum number of digits to appear for d,  i,  o,
191       u, x, and X conversions, the number of digits to appear after the radix
192       character for a, A, e, E, f, and F conversions, the maximum  number  of
193       significant  digits  for  g and G conversions, or the maximum number of
194       characters to be printed from a string for s and S conversions.
195
196   The length modifier
197       Here, "integer conversion" stands for d, i, o, u, x, or X conversion.
198
199       hh     A following integer conversion corresponds to a signed  char  or
200              unsigned  char argument, or a following n conversion corresponds
201              to a pointer to a signed char argument.
202
203       h      A following integer conversion corresponds to  a  short  int  or
204              unsigned  short int argument, or a following n conversion corre‐
205              sponds to a pointer to a short int argument.
206
207       l      (ell) A following integer conversion corresponds to a  long  int
208              or  unsigned long int argument, or a following n conversion cor‐
209              responds to a pointer to a long int argument, or a  following  c
210              conversion  corresponds  to  a wint_t argument, or a following s
211              conversion corresponds to a pointer to wchar_t argument.
212
213       ll     (ell-ell).  A following integer conversion corresponds to a long
214              long  int  or  unsigned long long int argument, or a following n
215              conversion corresponds to a pointer to a long long int argument.
216
217       L      A following a, A, e, E, f, F, g, or G conversion corresponds  to
218              a long double argument.  (C99 allows %LF, but SUSv2 does not.)
219
220       q      ("quad".  4.4BSD  and  Linux libc5 only.  Don't use.)  This is a
221              synonym for ll.
222
223       j      A following integer conversion corresponds  to  an  intmax_t  or
224              uintmax_t argument.
225
226       z      A  following  integer  conversion  corresponds  to  a  size_t or
227              ssize_t argument.  (Linux libc5 has Z with this meaning.   Don't
228              use it.)
229
230       t      A  following integer conversion corresponds to a ptrdiff_t argu‐
231              ment.
232
233       The SUSv2 only knows about the length modifiers h (in hd, hi,  ho,  hx,
234       hX, hn) and l (in ld, li, lo, lx, lX, ln, lc, ls) and L (in Le, LE, Lf,
235       Lg, LG).
236
237   The conversion specifier
238       A character that specifies the type of conversion to be  applied.   The
239       conversion specifiers and their meanings are:
240
241       d, i   The  int  argument is converted to signed decimal notation.  The
242              precision, if any, gives the minimum number of digits that  must
243              appear;  if  the  converted  value  requires fewer digits, it is
244              padded on the left with zeros.   The  default  precision  is  1.
245              When  0  is  printed with an explicit precision 0, the output is
246              empty.
247
248       o, u, x, X
249              The unsigned int argument is converted to  unsigned  octal  (o),
250              unsigned  decimal  (u),  or unsigned hexadecimal (x and X) nota‐
251              tion.  The letters abcdef are used for x conversions;  the  let‐
252              ters  ABCDEF are used for X conversions.  The precision, if any,
253              gives the minimum number of digits that must appear; if the con‐
254              verted  value  requires  fewer  digits, it is padded on the left
255              with zeros.  The default precision is 1.  When 0 is printed with
256              an explicit precision 0, the output is empty.
257
258       e, E   The  double  argument  is  rounded  and  converted  in the style
259              [-]d.ddde±dd where there is one digit before  the  decimal-point
260              character and the number of digits after it is equal to the pre‐
261              cision; if the precision is missing, it is taken as  6;  if  the
262              precision  is  zero,  no  decimal-point character appears.  An E
263              conversion uses the letter E (rather than e)  to  introduce  the
264              exponent.   The exponent always contains at least two digits; if
265              the value is zero, the exponent is 00.
266
267       f, F   The double argument is rounded and converted to decimal notation
268              in  the  style  [-]ddd.ddd, where the number of digits after the
269              decimal-point character is equal to the precision specification.
270              If  the precision is missing, it is taken as 6; if the precision
271              is explicitly zero, no decimal-point character  appears.   If  a
272              decimal point appears, at least one digit appears before it.
273
274              (The  SUSv2 does not know about F and says that character string
275              representations for infinity and NaN may be made available.  The
276              C99  standard  specifies "[-]inf" or "[-]infinity" for infinity,
277              and a string starting with "nan" for NaN, in the case of f  con‐
278              version,  and "[-]INF" or "[-]INFINITY" or "NAN*" in the case of
279              F conversion.)
280
281       g, G   The double argument is converted in style f or e (or F or E  for
282              G  conversions).  The precision specifies the number of signifi‐
283              cant digits.  If the precision is missing, 6 digits  are  given;
284              if  the  precision is zero, it is treated as 1.  Style e is used
285              if the exponent from its conversion is less than -4  or  greater
286              than or equal to the precision.  Trailing zeros are removed from
287              the fractional part of the result; a decimal point appears  only
288              if it is followed by at least one digit.
289
290       a, A   (C99;  not  in  SUSv2)  For a conversion, the double argument is
291              converted to hexadecimal notation (using the letters abcdef)  in
292              the  style  [-]0xh.hhhhp±d;  for A conversion the prefix 0X, the
293              letters ABCDEF, and the exponent separator P is used.  There  is
294              one  hexadecimal  digit before the decimal point, and the number
295              of digits after it is equal to the precision.  The default  pre‐
296              cision  suffices  for an exact representation of the value if an
297              exact representation in base 2 exists and  otherwise  is  suffi‐
298              ciently  large  to distinguish values of type double.  The digit
299              before the decimal point is unspecified for non-normalized  num‐
300              bers, and non-zero but otherwise unspecified for normalized num‐
301              bers.
302
303       c      If no l modifier is present, the int argument is converted to an
304              unsigned  char, and the resulting character is written.  If an l
305              modifier is present, the wint_t  (wide  character)  argument  is
306              converted  to  a  multibyte sequence by a call to the wcrtomb(3)
307              function, with a conversion state starting in the initial state,
308              and the resulting multibyte string is written.
309
310       s      If  no  l  modifier  is  present:  The  const char * argument is
311              expected to be a pointer to an array of character type  (pointer
312              to  a string).  Characters from the array are written up to (but
313              not including) a terminating null byte ('\0'); if a precision is
314              specified,  no more than the number specified are written.  If a
315              precision is given, no null byte need be present; if the  preci‐
316              sion is not specified, or is greater than the size of the array,
317              the array must contain a terminating null byte.
318
319              If an l modifier is present: The const  wchar_t  *  argument  is
320              expected  to  be a pointer to an array of wide characters.  Wide
321              characters from the array are converted to multibyte  characters
322              (each  by  a  call to the wcrtomb(3) function, with a conversion
323              state starting in the initial state before the first wide  char‐
324              acter),  up  to and including a terminating null wide character.
325              The resulting multibyte characters are written up  to  (but  not
326              including)  the terminating null byte.  If a precision is speci‐
327              fied, no more bytes than the number specified are  written,  but
328              no partial multibyte characters are written.  Note that the pre‐
329              cision determines the number of bytes written, not the number of
330              wide  characters  or screen positions.  The array must contain a
331              terminating null wide character, unless a precision is given and
332              it  is  so  small  that  the  number of bytes written exceeds it
333              before the end of the array is reached.
334
335       C      (Not in C99, but in SUSv2.)  Synonym for lc.  Don't use.
336
337       S      (Not in C99, but in SUSv2.)  Synonym for ls.  Don't use.
338
339       p      The void * pointer argument is printed in hexadecimal (as if  by
340              %#x or %#lx).
341
342       n      The number of characters written so far is stored into the inte‐
343              ger indicated by the int * (or variant)  pointer  argument.   No
344              argument is converted.
345
346       m      (Glibc  extension.)   Print output of strerror(errno).  No argu‐
347              ment is required.
348
349       %      A '%' is written.  No argument is converted.  The complete  con‐
350              version specification is '%%'.
351

CONFORMING TO

353       The   fprintf(),   printf(),   sprintf(),  vprintf(),  vfprintf(),  and
354       vsprintf() functions conform  to  C89  and  C99.   The  snprintf()  and
355       vsnprintf() functions conform to C99.
356
357       Concerning  the  return  value  of snprintf(), SUSv2 and C99 contradict
358       each other: when snprintf() is called with size=0 then SUSv2 stipulates
359       an  unspecified  return  value  less than 1, while C99 allows str to be
360       NULL in this case, and gives the return value (as always) as the number
361       of  characters  that  would have been written in case the output string
362       has been large enough.
363
364       Linux libc4 knows about the five C standard flags.  It knows about  the
365       length  modifiers  h, l, L, and the conversions c, d, e, E, f, F, g, G,
366       i, n, o, p, s, u, x, and X, where F is a synonym for f.   Additionally,
367       it  accepts  D, O, and U as synonyms for ld, lo, and lu.  (This is bad,
368       and caused serious bugs later, when support for  %D  disappeared.)   No
369       locale-dependent  radix  character,  no thousands' separator, no NaN or
370       infinity, no "%m$" and "*m$".
371
372       Linux libc5 knows about the five C  standard  flags  and  the  '  flag,
373       locale,  "%m$" and "*m$".  It knows about the length modifiers h, l, L,
374       Z, and q, but accepts L and q both for long double and  for  long  long
375       int  (this is a bug).  It no longer recognizes F, D, O, and U, but adds
376       the conversion character m, which outputs strerror(errno).
377
378       glibc 2.0 adds conversion characters C and S.
379
380       glibc 2.1 adds length modifiers hh, j, t, and z and conversion  charac‐
381       ters a and A.
382
383       glibc  2.2  adds the conversion character F with C99 semantics, and the
384       flag character I.
385

NOTES

387       Some programs imprudently rely on code such as the following
388
389           sprintf(buf, "%s some further text", buf);
390
391       to append text to buf.  However, the standards explicitly note that the
392       results  are  undefined  if source and destination buffers overlap when
393       calling sprintf(), snprintf(), vsprintf(), and vsnprintf().   Depending
394       on the version of gcc(1) used, and the compiler options employed, calls
395       such as the above will not produce the expected results.
396
397       The glibc implementation of the functions  snprintf()  and  vsnprintf()
398       conforms  to  the  C99  standard,  that is, behaves as described above,
399       since glibc version 2.1.  Until glibc 2.0.6 they would return  -1  when
400       the output was truncated.
401

BUGS

403       Because  sprintf()  and  vsprintf()  assume an arbitrarily long string,
404       callers must be careful not to overflow the actual space; this is often
405       impossible  to assure.  Note that the length of the strings produced is
406       locale-dependent  and  difficult  to  predict.   Use   snprintf()   and
407       vsnprintf() instead (or asprintf(3) and vasprintf(3)).
408
409       Linux libc4.[45] does not have a snprintf(), but provides a libbsd that
410       contains an snprintf() equivalent  to  sprintf(),  that  is,  one  that
411       ignores  the  size  argument.   Thus,  the use of snprintf() with early
412       libc4 leads to serious security problems.
413
414       Code such as printf(foo); often indicates a bug, since foo may  contain
415       a  % character.  If foo comes from untrusted user input, it may contain
416       %n, causing the printf() call to write to memory and creating  a  secu‐
417       rity hole.
418

EXAMPLE

420       To print pi to five decimal places:
421
422           #include <math.h>
423           #include <stdio.h>
424           fprintf(stdout, "pi = %.5f\n", 4 * atan(1.0));
425
426       To  print  a  date  and time in the form "Sunday, July 3, 10:02", where
427       weekday and month are pointers to strings:
428
429           #include <stdio.h>
430           fprintf(stdout, "%s, %s %d, %.2d:%.2d\n",
431                   weekday, month, day, hour, min);
432
433       Many countries use the day-month-year order.  Hence, an  international‐
434       ized  version must be able to print the arguments in an order specified
435       by the format:
436
437           #include <stdio.h>
438           fprintf(stdout, format,
439                   weekday, month, day, hour, min);
440
441       where format depends on locale, and may permute  the  arguments.   With
442       the value:
443
444           "%1$s, %3$d. %2$s, %4$d:%5$.2d\n"
445
446       one might obtain "Sonntag, 3. Juli, 10:02".
447
448       To allocate a sufficiently large string and print into it (code correct
449       for both glibc 2.0 and glibc 2.1):
450
451       #include <stdio.h>
452       #include <stdlib.h>
453       #include <stdarg.h>
454
455       char *
456       make_message(const char *fmt, ...)
457       {
458           /* Guess we need no more than 100 bytes. */
459           int n, size = 100;
460           char *p, *np;
461           va_list ap;
462
463           if ((p = malloc(size)) == NULL)
464               return NULL;
465
466           while (1) {
467               /* Try to print in the allocated space. */
468               va_start(ap, fmt);
469               n = vsnprintf(p, size, fmt, ap);
470               va_end(ap);
471               /* If that worked, return the string. */
472               if (n > -1 && n < size)
473                   return p;
474               /* Else try again with more space. */
475               if (n > -1)    /* glibc 2.1 */
476                   size = n+1; /* precisely what is needed */
477               else           /* glibc 2.0 */
478                   size *= 2;  /* twice the old size */
479               if ((np = realloc (p, size)) == NULL) {
480                   free(p);
481                   return NULL;
482               } else {
483                   p = np;
484               }
485           }
486       }
487

SEE ALSO

489       printf(1), asprintf(3), dprintf(3), scanf(3), setlocale(3), wcrtomb(3),
490       wprintf(3), locale(5)
491

COLOPHON

493       This  page  is  part of release 3.22 of the Linux man-pages project.  A
494       description of the project, and information about reporting  bugs,  can
495       be found at http://www.kernel.org/doc/man-pages/.
496
497
498
499GNU                               2008-12-19                         PRINTF(3)
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