MirOS Manual: asprintf(3), fprintf(3), printf(3), snprintf(3), sprintf(3), vasprintf(3), vfprintf(3), vprintf(3), vsnprintf(3), vsprintf(3)

PRINTF(3)                  BSD Programmer's Manual                   PRINTF(3)

NAME

     printf, fprintf, sprintf, snprintf, asprintf, vprintf, vfprintf,
     vsprintf, vsnprintf, vasprintf - formatted output conversion

SYNOPSIS

     #include <stdio.h>

     int
     printf(const char *format, ...);

     int
     fprintf(FILE *stream, const char *format, ...);

     int
     sprintf(char *str, const char *format, ...);

     int
     snprintf(char *str, size_t size, const char *format, ...);

     int
     asprintf(char **ret, const char *format, ...);

     #include <stdarg.h>

     int
     vprintf(const char *format, va_list ap);

     int
     vfprintf(FILE *stream, const char *format, va_list ap);

     int
     vsprintf(char *str, const char *format, va_list ap);

     int
     vsnprintf(char *str, size_t size, const char *format, va_list ap);

     int
     vasprintf(char **ret, const char *format, va_list ap);

DESCRIPTION

     The printf() family of functions produce output according to the given
     format as described below. This format may contain "conversion
     specifiers"; the results of such conversions, if any, depend on the argu-
     ments following the format string.

     The printf() and vprintf() functions write output to the standard output
     stream, stdout; fprintf() and vfprintf() write output to the supplied
     stream pointer stream; sprintf(), snprintf(), vsprintf(), and vsnprintf()
     write to the character string str; asprintf() and vasprintf() write to a
     dynamically allocated string that is stored in ret.

     These functions write the output under the control of a format string
     that specifies how subsequent arguments (or arguments accessed via the
     variable-length argument facilities of stdarg(3)) are converted for out-
     put.

     snprintf() and vsnprintf() will write at most size-1 of the characters
     printed into the output string (the size'th character then gets the ter-
     minating '\0'); if the return value is greater than or equal to the size
     argument, the string was too short and some of the printed characters
     were discarded. If size is zero, str may be a null pointer and no charac-
     ters will be written; the number of bytes that would have been written
     excluding the terminating '\0' byte, or -1 on error, will be returned.

     sprintf() and vsprintf() effectively assume an infinite size.

     The format string is composed of zero or more directives: ordinary multi-
     byte characters (not %), which are copied unchanged to the output stream,
     and conversion specifications, each of which results in fetching zero or
     more subsequent arguments. Each conversion specification is introduced by
     the character %. The arguments must correspond properly (after type pro-
     motion) with the conversion specifier. After the %, the following appear
     in sequence:

     •   An optional field, consisting of a decimal digit string followed by a
         $ specifying the next argument to access. If this field is not pro-
         vided, the argument following the last argument accessed will be
         used. Arguments are numbered starting at 1.

     •   Zero or more of the following flags:

         -   A hash '#' character specifying that the value should be convert-
             ed to an "alternate form". For c, d, i, n, p, s, and u conver-
             sions, this option has no effect. For o conversions, the preci-
             sion of the number is increased to force the first character of
             the output string to a zero (except if a zero value is printed
             with an explicit precision of zero). For x and X conversions, a
             non-zero result has the string '0x' (or '0X' for X conversions)
             prepended to it. For e, E, f, g, and G conversions, the result
             will always contain a decimal point, even if no digits follow it
             (normally, a decimal point appears in the results of those
             conversions only if a digit follows). For g and G conversions,
             trailing zeros are not removed from the result as they would oth-
             erwise be.

         -   A zero '0' character specifying zero padding. For all conversions
             except n, the converted value is padded on the left with zeros
             rather than blanks. If a precision is given with a numeric
             conversion (d, i, o, u, x, and X), the '0' flag is ignored.

         -   A negative field width flag '-' indicates the converted value is
             to be left adjusted on the field boundary. Except for n conver-
             sions, the converted value is padded on the right with blanks,
             rather than on the left with blanks or zeros. A '-' overrides a
             '0' if both are given.

         -   A space, specifying that a blank should be left before a positive
             number produced by a signed conversion (d, e, E, f, g, G, or i).

         -   A '+' character specifying that a sign always be placed before a
             number produced by a signed conversion. A '+' overrides a space
             if both are used.

     •   An optional decimal digit string specifying a minimum field width. If
         the converted value has fewer characters than the field width, it
         will be padded with spaces on the left (or right, if the left-
         adjustment flag has been given) to fill out the field width.

     •   An optional precision, in the form of a period '.' followed by an op-
         tional digit string. If the digit string is omitted, the precision is
         taken as zero. This gives the minimum number of digits to appear for
         d, i, o, u, x, and X conversions, the number of digits to appear
         after the decimal-point for e, E, and f conversions, the maximum
         number of significant digits for g and G conversions, or the maximum
         number of characters to be printed from a string for s conversions.

     •   The optional character h, specifying that a following d, i, o, u, x,
         or X conversion corresponds to a short int or unsigned short int ar-
         gument, or that a following n conversion corresponds to a pointer to
         a short int argument.

     •   The optional character sequence hh, specifying that a following d, i,
         o, u, x, or X conversion corresponds to a signed char or unsigned
         char argument, or that a following n conversion corresponds to a
         pointer to a signed char argument.

     •   The optional character j, specifying that a following d, i, o, u, x,
         or X conversion corresponds to a intmax_t or uintmax_t argument, or
         that a following n conversion corresponds to a pointer to a intmax_t
         argument. (In the current implementation, this is a quad integer.)

     •   The optional character l (ell) specifying that a following d, i, o,
         u, x, or X conversion corresponds to a long int or unsigned long int
         argument, or that a following n conversion corresponds to a pointer
         to a long int argument, or that a following c, or s conversion
         corresponds to a wide character argument.

     •   The optional character sequence ll, specifying that a following d, i,
         o, u, x, or X conversion corresponds to a quad int or unsigned quad
         int argument, or that a following n conversion corresponds to a
         pointer to a quad int argument. The use of q has been deprecated as
         conversion character.

     •   The optional character t, specifying that a following d, i, o, u, x,
         or X conversion corresponds to a ptrdiff_t or intptr_t argument, or
         that a following n conversion corresponds to a pointer to a ptrdiff_t
         argument.

     •   The optional character z, specifying that a following d, i, o, u, x,
         or X conversion corresponds to a size_t or ssize_t argument, or that
         a following n conversion corresponds to a pointer to a ssize_t argu-
         ment.

     •   The character L specifying that a following e, E, f, g, or G conver-
         sion corresponds to a long double argument.

     •   A character that specifies the type of conversion to be applied.

     A field width or precision, or both, may be indicated by an asterisk '*'
     or an asterisk followed by one or more decimal digits and a '$' instead
     of a digit string. In this case, an int argument supplies the field width
     or precision. A negative field width is treated as a left adjustment flag
     followed by a positive field width; a negative precision is treated as
     though it were missing. If a single format directive mixes positional
     (nn$) and non-positional arguments, the results are undefined.

     The conversion specifiers and their meanings are:

     diouxX  The int (or appropriate variant) argument is converted to signed
             decimal (d and i), unsigned octal (o), unsigned decimal (u), or
             unsigned hexadecimal (x and X) notation. The letters abcdef are
             used for x conversions; the letters ABCDEF are used for X conver-
             sions. The precision, if any, gives the minimum number of digits
             that must appear; if the converted value requires fewer digits,
             it is padded on the left with zeros.

     DOU     The long int argument is converted to signed decimal, unsigned
             octal, or unsigned decimal, as if the format had been ld, lo, or
             lu respectively. These conversion characters are deprecated, and
             will eventually disappear.

     eE      The double argument is rounded and converted in the style
             [-]d.ddde+-dd where there is one digit before the decimal-point
             character and the number of digits after it is equal to the pre-
             cision; if the precision is missing, it is taken as 6; if the
             precision is zero, no decimal-point character appears. An E
             conversion uses the letter E (rather than e) to introduce the ex-
             ponent. The exponent always contains at least two digits; if the
             value is zero, the exponent is 00.

     f       The double argument is rounded and converted to decimal notation
             in the style [-]ddd.ddd, where the number of digits after the
             decimal-point character is equal to the precision specification.
             If the precision is missing, it is taken as 6; if the precision
             is explicitly zero, no decimal-point character appears. If a de-
             cimal point appears, at least one digit appears before it.

     gG      The double argument is converted in style f or e (or E for G
             conversions). The precision specifies the number of significant
             digits. If the precision is missing, 6 digits are given; if the
             precision is zero, it is treated as 1. Style e is used if the ex-
             ponent from its conversion is less than -4 or greater than or
             equal to the precision. Trailing zeros are removed from the frac-
             tional part of the result; a decimal point appears only if it is
             followed by at least one digit.

     C       Deprecated alias for lc. Do not use.

     c       If the l specifier is not given, the int argument is converted to
             an unsigned char, and the resulting character is written. Other-
             wise, the wint_t argument is converted to an wchar_t, and the
             resulting wide character is written as multibyte string according
             to the rules of the ls conversion.

     S       Deprecated alias for ls. Do not use.

     s       If the l specifier is not given, the char * argument is expected
             to be a pointer to an array of character type (pointer to a
             string). Characters from the array are written up to (but not in-
             cluding) a terminating NUL character; if a precision is speci-
             fied, no more than the number specified are written. If a preci-
             sion is given, no NUL character need be present; if the precision
             is not specified, or is greater than the size of the array, the
             array must contain a terminating NUL character.

             If the l specifier is given, the wchar_t * argument is expected
             to be a pointer to an array of wide character type (pointer to a
             wide string). The wide character string must be NUL terminated.
             The wide string is then converted to a multibyte string; if the
             conversion fails due to an invalid wide character, the function
             fails with EILSEQ. Otherwise, the output is truncated to at most
             the number of octets (bytes) corresponding to the field preci-
             sion; however, no incomplete multibyte characters are written.
             Then, it is padded to the field width if specified.

     p       The void * pointer argument is printed in hexadecimal (as if by
             '%#x' or '%#lx').

     m       The equivalent of strerror(errno) is written. This is a non-
             portable extension, don't use.

     n       The number of characters written so far is stored into the in-
             teger indicated by the int * (or variant) pointer argument. No
             argument is converted.

     %       A '%' is written. No argument is converted. The complete conver-
             sion specification is '%%'.

     In no case does a non-existent or small field width cause truncation of a
     field; if the result of a conversion is wider than the field width, the
     field is expanded to contain the conversion result.

RETURN VALUES

     The printf(), fprintf(), sprintf(), vprintf(), vfprintf(), vsprintf(),
     and vasprintf() functions return the number of characters printed (not
     including the trailing '\0' used to end output to strings).

     The snprintf() and vsnprintf() functions return the number of characters
     that would have been output if the size were unlimited
     (again, not including the final '\0'). If an output or encoding error oc-
     curs, a negative value is returned instead.

     asprintf() and vasprintf() return a pointer to a buffer sufficiently
     large to hold the string in the ret argument. This pointer should be
     passed to free(3) to release the allocated storage when it is no longer
     needed. If sufficient space cannot be allocated, these functions will re-
     turn -1. The value of ret in this situation is implementation-dependent
     (on MirOS, ret will be set to the null pointer, but this behavior should
     not be relied upon).

EXAMPLES

     To print a date and time in the form `Sunday, July 3, 10:02', where
     weekday and month are pointers to strings:

           #include <stdio.h>

           fprintf(stdout, "%s, %s %d, %.2d:%.2d\n",
               weekday, month, day, hour, min);

     To print pi to five decimal places:

           #include <math.h>
           #include <stdio.h>

           fprintf(stdout, "pi = %.5f\n", 4 * atan(1.0));

     To allocate a 128-byte string and print into it:

           #include <stdarg.h>
           #include <stdio.h>
           #include <stdlib.h>

           char *
           newfmt(const char *fmt, ...)
           {
                   char *p;
                   va_list ap;

                   if ((p = malloc(128)) == NULL)
                           return (NULL);
                   va_start(ap, fmt);
                   (void) vsnprintf(p, 128, fmt, ap);
                   va_end(ap);
                   return (p);
           }

SEE ALSO

     printf(1), scanf(3)

STANDARDS

     The fprintf(), printf(), sprintf(), vprintf(), vfprintf(), and vsprintf()
     functions conform to ANSI X3.159-1989 ("ANSI C").

     Some of the ISO/IEC 9899:1999 ("ISO C99") extensions have been implement-
     ed.

HISTORY

     The functions snprintf() and vsnprintf() first appeared in 4.4BSD.

     The %m conversion first appeared in the GNU/Linux libc5 library. This im-
     plementation, which processes width and precision too, first appeared in
     MirOS #10.

     The functions asprintf() and vasprintf() first appeared in the GNU C li-
     brary. This implementation first appeared in OpenBSD 2.3.

     The %C and %S conversions appear in Plan 9 as well as GNU libc 2.0, and
     were first implemented for MirOS #10.

CAVEATS

     The conversion formats %C, %D, %O, %S, and %U are not standard and are
     provided only for backward compatibility. The effect of padding the %p
     format with zeros (either by the '0' flag or by specifying a precision),
     and the benign effect (i.e., none) of the '#' flag on %n and %p conver-
     sions, as well as other nonsensical combinations such as %Ld, are not
     standard; such combinations should be avoided.

     Because sprintf() and vsprintf() assume an infinitely long string, call-
     ers must be careful not to overflow the actual space; this is often im-
     possible to assure. For safety, programmers should use the snprintf() and
     asprintf() family of interfaces instead. Unfortunately, the snprintf()
     interface is not available on older systems and the asprintf() interface
     is not portable.

     It is important never to pass a string with user-supplied data as a for-
     mat without using '%s'. An attacker can put format specifiers in the
     string to mangle the stack, leading to a possible security hole. This
     holds true even if the string has been built "by hand" using a function
     like snprintf(), as the resulting string may still contain user-supplied
     conversion specifiers for later interpolation by printf().

     Be sure to use the proper secure idiom:

           snprintf(buffer, sizeof(buffer), "%s", string);

     There is no way for printf() to know the size of each argument passed. If
     positional arguments are used, care must be taken to ensure that all
     parameters, up to the last positionally specified parameter, are used in
     the format string. This allows for the format string to be parsed for
     this information. Failure to do this will mean the code is non-portable
     and liable to fail.

     On MirOS, wide characters are always in UCS-2 Unicode, and multibyte
     characters are always in OPTU-8 (CESU-8, UTF-8), for both the source and
     execution character sets; however, this is not portable. Other operating
     systems might not implement locale at all (even some of the other BSDs)
     or on a non-Unicode basis (e.g. the Citrus I18N extension for BSD) or re-
     quire calls to set an appropriate locale.

MirOS BSD #10-current          February 9, 2014                              5

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