1 .\" Copyright (c) 1990, 1991, 1993
2 .\" The Regents of the University of California. All rights reserved.
4 .\" This code is derived from software contributed to Berkeley by
5 .\" Chris Torek and the American National Standards Committee X3,
6 .\" on Information Processing Systems.
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32 .\" @(#)printf.3 8.1 (Berkeley) 6/4/93
51 .Nd formatted output conversion
57 .Fn printf "const char * restrict format" ...
59 .Fn fprintf "FILE * restrict stream" "const char * restrict format" ...
61 .Fn sprintf "char * restrict str" "const char * restrict format" ...
63 .Fn snprintf "char * restrict str" "size_t size" "const char * restrict format" ...
65 .Fn asprintf "char **ret" "const char *format" ...
67 .Fn dprintf "int fd" "const char * restrict format" ...
70 .Fn vprintf "const char * restrict format" "va_list ap"
72 .Fn vfprintf "FILE * restrict stream" "const char * restrict format" "va_list ap"
74 .Fn vsprintf "char * restrict str" "const char * restrict format" "va_list ap"
76 .Fn vsnprintf "char * restrict str" "size_t size" "const char * restrict format" "va_list ap"
78 .Fn vasprintf "char **ret" "const char *format" "va_list ap"
80 .Fn vdprintf "int fd" "const char * restrict format" "va_list ap"
84 family of functions produces output according to a
94 the standard output stream;
98 write output to the given output
103 write output to the given file descriptor;
109 write to the character string
115 dynamically allocate a new string with
118 These functions write the output under the control of a
120 string that specifies how subsequent arguments
121 (or arguments accessed via the variable-length argument facilities of
123 are converted for output.
132 to be a pointer to a buffer sufficiently large to hold the formatted string.
133 This pointer should be passed to
135 to release the allocated storage when it is no longer needed.
136 If sufficient space cannot be allocated,
140 will return \-1 and set
153 of the characters printed into the output string
156 character then gets the terminating
158 if the return value is greater than or equal to the
160 argument, the string was too short
161 and some of the printed characters were discarded.
162 The output is always null-terminated, unless
176 The format string is composed of zero or more directives:
181 which are copied unchanged to the output stream;
182 and conversion specifications, each of which results
183 in fetching zero or more subsequent arguments.
184 Each conversion specification is introduced by
188 The arguments must correspond properly (after type promotion)
189 with the conversion specifier.
192 the following appear in sequence:
195 An optional field, consisting of a decimal digit string followed by a
197 specifying the next argument to access.
198 If this field is not provided, the argument following the last
199 argument accessed will be used.
200 Arguments are numbered starting at
202 If unaccessed arguments in the format string are interspersed with ones that
203 are accessed the results will be indeterminate.
205 Zero or more of the following flags:
206 .Bl -tag -width ".So \ Sc (space)"
208 The value should be converted to an
211 .Cm c , d , i , n , p , s ,
214 conversions, this option has no effect.
217 conversions, the precision of the number is increased to force the first
218 character of the output string to a zero.
223 conversions, a non-zero result has the string
229 conversions) prepended to it.
231 .Cm a , A , e , E , f , F , g ,
234 conversions, the result will always contain a decimal point, even if no
235 digits follow it (normally, a decimal point appears in the results of
236 those conversions only if a digit follows).
241 conversions, trailing zeros are not removed from the result as they
243 .It So Cm 0 Sc (zero)
245 For all conversions except
247 the converted value is padded on the left with zeros rather than blanks.
248 If a precision is given with a numeric conversion
249 .Cm ( d , i , o , u , i , x ,
256 A negative field width flag;
257 the converted value is to be left adjusted on the field boundary.
260 conversions, the converted value is padded on the right with blanks,
261 rather than on the left with blanks or zeros.
267 .It So "\ " Sc (space)
268 A blank should be left before a positive number
269 produced by a signed conversion
270 .Cm ( a , A , d , e , E , f , F , g , G ,
274 A sign must always be placed before a
275 number produced by a signed conversion.
278 overrides a space if both are used.
279 .It So "'" Sc (apostrophe)
284 or the integral portion of a floating point conversion
288 should be grouped and separated by thousands using
289 the non-monetary separator returned by
293 An optional decimal digit string specifying a minimum field width.
294 If the converted value has fewer characters than the field width, it will
295 be padded with spaces on the left (or right, if the left-adjustment
296 flag has been given) to fill out
299 An optional precision, in the form of a period
302 optional digit string.
303 If the digit string is omitted, the precision is taken as zero.
304 This gives the minimum number of digits to appear for
305 .Cm d , i , o , u , x ,
308 conversions, the number of digits to appear after the decimal-point for
309 .Cm a , A , e , E , f ,
312 conversions, the maximum number of significant digits for
316 conversions, or the maximum number of characters to be printed from a
321 An optional length modifier, that specifies the size of the argument.
322 The following length modifiers are valid for the
323 .Cm d , i , n , o , u , x ,
327 .Bl -column ".Cm q Em (deprecated)" ".Vt signed char" ".Vt unsigned long long" ".Vt long long *"
328 .It Sy Modifier Ta Cm d , i Ta Cm o , u , x , X Ta Cm n
329 .It Cm hh Ta Vt "signed char" Ta Vt "unsigned char" Ta Vt "signed char *"
330 .It Cm h Ta Vt short Ta Vt "unsigned short" Ta Vt "short *"
331 .It Cm l No (ell) Ta Vt long Ta Vt "unsigned long" Ta Vt "long *"
332 .It Cm ll No (ell ell) Ta Vt "long long" Ta Vt "unsigned long long" Ta Vt "long long *"
333 .It Cm j Ta Vt intmax_t Ta Vt uintmax_t Ta Vt "intmax_t *"
334 .It Cm t Ta Vt ptrdiff_t Ta (see note) Ta Vt "ptrdiff_t *"
335 .It Cm z Ta (see note) Ta Vt size_t Ta (see note)
336 .It Cm q Em (deprecated) Ta Vt quad_t Ta Vt u_quad_t Ta Vt "quad_t *"
342 modifier, when applied to a
346 conversion, indicates that the argument is of an unsigned type
347 equivalent in size to a
351 modifier, when applied to a
355 conversion, indicates that the argument is of a signed type equivalent in
358 Similarly, when applied to an
360 conversion, it indicates that the argument is a pointer to a signed type
361 equivalent in size to a
364 The following length modifier is valid for the
365 .Cm a , A , e , E , f , F , g ,
369 .Bl -column ".Sy Modifier" ".Cm a , A , e , E , f , F , g , G"
370 .It Sy Modifier Ta Cm a , A , e , E , f , F , g , G
371 .It Cm l No (ell) Ta Vt double
372 (ignored, same behavior as without it)
373 .It Cm L Ta Vt "long double"
376 The following length modifier is valid for the
381 .Bl -column ".Sy Modifier" ".Vt wint_t" ".Vt wchar_t *"
382 .It Sy Modifier Ta Cm c Ta Cm s
383 .It Cm l No (ell) Ta Vt wint_t Ta Vt "wchar_t *"
386 A character that specifies the type of conversion to be applied.
389 A field width or precision, or both, may be indicated by
392 or an asterisk followed by one or more decimal digits and a
398 argument supplies the field width or precision.
399 A negative field width is treated as a left adjustment flag followed by a
400 positive field width; a negative precision is treated as though it were
402 If a single format directive mixes positional
404 and non-positional arguments, the results are undefined.
406 The conversion specifiers and their meanings are:
407 .Bl -tag -width ".Cm diouxX"
411 (or appropriate variant) argument is converted to signed decimal
419 or unsigned hexadecimal
428 conversions; the letters
433 The precision, if any, gives the minimum number of digits that must
434 appear; if the converted value requires fewer digits, it is padded on
439 argument is converted to signed decimal, unsigned octal, or unsigned
440 decimal, as if the format had been
445 These conversion characters are deprecated, and will eventually disappear.
449 argument is rounded and converted in the style
451 .Oo \- Oc Ar d Li \&. Ar ddd Li e \(+- Ar dd
453 where there is one digit before the
454 decimal-point character
455 and the number of digits after it is equal to the precision;
456 if the precision is missing,
457 it is taken as 6; if the precision is
458 zero, no decimal-point character appears.
461 conversion uses the letter
465 to introduce the exponent.
466 The exponent always contains at least two digits; if the value is zero,
470 .Cm a , A , e , E , f , F , g ,
473 conversions, positive and negative infinity are represented as
477 respectively when using the lowercase conversion character, and
481 respectively when using the uppercase conversion character.
482 Similarly, NaN is represented as
484 when using the lowercase conversion, and
486 when using the uppercase conversion.
490 argument is rounded and converted to decimal notation in the style
492 .Oo \- Oc Ar ddd Li \&. Ar ddd ,
494 where the number of digits after the decimal-point character
495 is equal to the precision specification.
496 If the precision is missing, it is taken as 6; if the precision is
497 explicitly zero, no decimal-point character appears.
498 If a decimal point appears, at least one digit appears before it.
502 argument is converted in style
513 The precision specifies the number of significant digits.
514 If the precision is missing, 6 digits are given; if the precision is zero,
518 is used if the exponent from its conversion is less than \-4 or greater than
519 or equal to the precision.
520 Trailing zeros are removed from the fractional part of the result; a
521 decimal point appears only if it is followed by at least one digit.
525 argument is rounded and converted to hexadecimal notation in the style
527 .Oo \- Oc Li 0x Ar h Li \&. Ar hhhp Oo \(+- Oc Ar d ,
529 where the number of digits after the hexadecimal-point character
530 is equal to the precision specification.
531 If the precision is missing, it is taken as enough to represent
532 the floating-point number exactly, and no rounding occurs.
533 If the precision is zero, no hexadecimal-point character appears.
536 is a literal character
538 and the exponent consists of a positive or negative sign
539 followed by a decimal number representing an exponent of 2.
542 conversion uses the prefix
550 to represent the hex digits, and the letter
554 to separate the mantissa and exponent.
556 Note that there may be multiple valid ways to represent floating-point
557 numbers in this hexadecimal format.
559 .Li 0x1.92p+1 , 0x3.24p+0 , 0x6.48p-1 ,
564 and later always prints finite non-zero numbers using
566 as the digit before the hexadecimal point.
567 Zeroes are always represented with a mantissa of 0 (preceded by a
569 if appropriate) and an exponent of
580 argument is converted to an
581 .Vt "unsigned char" ,
582 and the resulting character is written.
586 (ell) modifier is used, the
588 argument shall be converted to a
590 and the (potentially multi-byte) sequence representing the
591 single wide character is written, including any shift sequences.
592 If a shift sequence is used, the shift state is also restored
593 to the original state after the character.
603 argument is expected to be a pointer to an array of character type (pointer
605 Characters from the array are written up to (but not including)
609 if a precision is specified, no more than the number specified are
611 If a precision is given, no null character
612 need be present; if the precision is not specified, or is greater than
613 the size of the array, the array must contain a terminating
619 (ell) modifier is used, the
621 argument is expected to be a pointer to an array of wide characters
622 (pointer to a wide string).
623 For each wide character in the string, the (potentially multi-byte)
624 sequence representing the
625 wide character is written, including any shift sequences.
626 If any shift sequence is used, the shift state is also restored
627 to the original state after the string.
628 Wide characters from the array are written up to (but not including)
632 if a precision is specified, no more than the number of bytes specified are
633 written (including shift sequences).
634 Partial characters are never written.
635 If a precision is given, no null character
636 need be present; if the precision is not specified, or is greater than
637 the number of bytes required to render the multibyte representation of
638 the string, the array must contain a terminating wide
644 pointer argument is printed in hexadecimal (as if by
649 The number of characters written so far is stored into the
650 integer indicated by the
652 (or variant) pointer argument.
653 No argument is converted.
658 No argument is converted.
659 The complete conversion specification
665 character is defined in the program's locale (category
668 In no case does a non-existent or small field width cause truncation of
669 a numeric field; if the result of a conversion is wider than the field
671 field is expanded to contain the conversion result.
673 These functions return the number of characters printed
674 (not including the trailing
676 used to end output to strings),
681 which return the number of characters that would have been printed if the
684 (again, not including the final
686 These functions return a negative value if an error occurs.
688 To print a date and time in the form
689 .Dq Li "Sunday, July 3, 10:02" ,
694 are pointers to strings:
695 .Bd -literal -offset indent
697 fprintf(stdout, "%s, %s %d, %.2d:%.2d\en",
698 weekday, month, day, hour, min);
702 to five decimal places:
703 .Bd -literal -offset indent
706 fprintf(stdout, "pi = %.5f\en", 4 * atan(1.0));
709 To allocate a 128 byte string and print into it:
710 .Bd -literal -offset indent
714 char *newfmt(const char *fmt, ...)
718 if ((p = malloc(128)) == NULL)
721 (void) vsnprintf(p, 128, fmt, ap);
727 The conversion formats
732 are provided only for backward compatibility.
733 The effect of padding the
735 format with zeros (either by the
737 flag or by specifying a precision), and the benign effect (i.e., none)
744 conversions, as well as other
745 nonsensical combinations such as
747 are not standard; such combinations
750 In addition to the errors documented for the
754 family of functions may fail if:
757 An invalid wide character code was encountered.
759 Insufficient storage space is available.
765 or the return value would be too large to be represented by an
775 Subject to the caveats noted in the
790 With the same reservation, the
807 first appeared in the
810 These were implemented by
811 .An Peter Wemm Aq Mt peter@FreeBSD.org
814 but were later replaced with a different implementation
818 .An Todd C. Miller Aq Mt Todd.Miller@courtesan.com .
823 functions were added in
828 family of functions do not correctly handle multibyte characters in the
831 .Sh SECURITY CONSIDERATIONS
836 functions are easily misused in a manner which enables malicious users
837 to arbitrarily change a running program's functionality through
838 a buffer overflow attack.
843 assume an infinitely long string,
844 callers must be careful not to overflow the actual space;
845 this is often hard to assure.
846 For safety, programmers should use the
852 foo(const char *arbitrary_string, const char *and_another)
858 * This first sprintf is bad behavior. Do not use sprintf!
860 sprintf(onstack, "%s, %s", arbitrary_string, and_another);
863 * The following two lines demonstrate better use of
866 snprintf(onstack, sizeof(onstack), "%s, %s", arbitrary_string,
876 family of functions are also easily misused in a manner
877 allowing malicious users to arbitrarily change a running program's
878 functionality by either causing the program
879 to print potentially sensitive data
880 .Dq "left on the stack" ,
881 or causing it to generate a memory fault or bus error
882 by dereferencing an invalid pointer.
885 can be used to write arbitrary data to potentially carefully-selected
887 Programmers are therefore strongly advised to never pass untrusted strings
890 argument, as an attacker can put format specifiers in the string
891 to mangle your stack,
892 leading to a possible security hole.
893 This holds true even if the string was built using a function like
895 as the resulting string may still contain user-supplied conversion specifiers
896 for later interpolation by
899 Always use the proper secure idiom:
901 .Dl "snprintf(buffer, sizeof(buffer), \*q%s\*q, string);"