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PRINTF(3)                BSD Library Functions Manual                PRINTF(3)

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

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <&lt;stdio.h>&gt;

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

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

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

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

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

     #include <&lt;stdarg.h>&gt;

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

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

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

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

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

DESCRIPTION
     The printf() family of functions produces output according to a format as
     described below.  printf() and vprintf() write output to stdout, the
     standard output stream; fprintf() and vfprintf() write output to the
     given output 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.

     These functions return the number of characters printed (not including
     the trailing '\0' used to end output to strings).  If an output error was
     encountered, these functions shall return a negative value.

     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
     return -1 and set ret to be a NULL pointer.

     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, nothing is written and str may be a
     NULL pointer.

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

     The format string is composed of zero or more directives: ordinary char-
     acters (not %), which are copied unchanged to the output stream; and con-
     version 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 promo-
     tion) with the conversion specifier.  After the %, the following appear
     in sequence:

     o   Zero or more of the following flags:

         -   A # character specifying that the value should be converted to an
             ``alternative form''.  For c, d, i, n, p, s, and u conversions,
             this option has no effect.  For o conversions, the precision of
             the number is increased to force the first character of the out-
             put 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, F, g, and G conversions, the result will
             always contain a decimal point, even if no digits follow it (nor-
             mally, a decimal point appears in the results of those conver-
             sions only if a digit follows).  For g and G conversions, trail-
             ing zeros are not removed from the result as they would otherwise
             be.

         -   A zero '0' character specifying zero padding.  For all conver-
             sions 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, i, 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, 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.

     o   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-adjust-
         ment flag has been given) to fill out the field width.

     o   An optional precision, in the form of a period '.' followed by an
         optional 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, f, 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.

     o   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
         argument, or that a following n conversion corresponds to a pointer
         to a short int argument.

     o   The optional character j, specifying that a following d, i, o, u, x,
         or X conversion corresponds to an intmax_t or uintmax_t argument, or
         that a following n conversion corresponds to a pointer to a intmax_t
         argument.

     o   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.

     o   The optional character q, or alternatively two consecutive l (ell)
         characters, specifying that a following d, i, o, u, x, or X conver-
         sion corresponds to a quad_t or u_quad_t argument, or that a follow-
         ing n conversion corresponds to a pointer to a quad_t argument.

     o   The optional character t, specifying that a following d, i, o, u, x,
         or X conversion corresponds to a ptrdiff_t or the corresponding
         unsigned integer type argument, or that a following n conversion cor-
         responds to a pointer to a ptrdiff_t argument.

     o   The optional character z, specifying that a following d, i, o, u, x,
         or X conversion corresponds to a size_t or the corresponding signed
         integer type argument, or that a following n conversion corresponds
         to a pointer to a signed integer type corresponding to size_t argu-
         ment.

     o   The character L specifying that a following e, E, f, F, g, or G con-
         version corresponds to a long double argument.

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

     A field width or precision, or both, may be indicated by an asterisk '*'
     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.

     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.

     fF      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
             decimal point appears, at least one digit appears before it.

             If the double argument represents an infinity it is converted in
             the style [-]inf.  If the double argument represents a NaN it is
             converted in the style [-]nan.  An F conversion produces [-]INF
             and [-]NAN, respectively.

     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 con-
             version uses the letter E (rather than e) to introduce the expo-
             nent.  The exponent always contains at least two digits; if the
             value is zero, the exponent is 00.

             Double arguments representing infinities or NaNs are converted in
             the same styles as in the f and F conversions.

     gG      The double argument is converted in style f or e (or in style F
             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 exponent from its conversion is less than -4 or
             greater than or equal to the precision.  Trailing zeros are
             removed from the fractional part of the result; a decimal point
             appears only if it is followed by at least one digit.

             Double arguments representing infinities or NaNs are converted in
             the same styles as in the f and F conversions.

     c       The int argument is converted to an unsigned char, and the
             resulting character is written.

     s       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 including) a terminating NUL
             character; if a precision is specified, no more than the number
             specified are written.  If a precision is given, no null charac-
             ter need be present; if the precision is not specified, or is
             greater than the size of the array, the array must contain a ter-
             minating NUL character.

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

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

     %       A '%' is written.  No argument is converted.  The complete con-
             version 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
     Upon successful completion printf(), fprintf(), vprintf(), and vfprintf()
     return the number of characters printed.  Otherwise -1 is returned and
     errno is set to indicate the error.

     Upon successful completion sprintf() and vsprintf() return the number of
     characters written to str, excluding the terminating NUL character.  Oth-
     erwise -1 is returned and errno is set to indicate the error.

     Upon successful completion snprintf() and vsnprintf() return the number
     of characters that would have been written to a sufficiently sized str,
     excluding the terminating NUL character.

     Upon successful completion asprintf() and vasprintf() return the number
     of characters written to ret, excluding the terminating NUL character.
     Otherwise -1 is returned, ret is set to NULL, and errno is set to indi-
     cate the error.

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 <stdio.h>
           #include <stdlib.h>
           #include <stdarg.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), printf(9)

STANDARDS
     The fprintf(), printf(), sprintf(), vprintf(), vfprintf(), and vsprintf()
     functions conform to ISO/IEC 9899:1990 (``ISO C90'').  The conversion
     format modifiers %j, %t and %z conform to ISO/IEC 9899:1999
     (``ISO C99'').  The snprintf() and vsnprintf() functions conform to
     ISO/IEC 9899:1999 (``ISO C99'').

HISTORY
     The functions snprintf() and vsnprintf() first appeared in 4.4BSD.  The
     functions asprintf() and vasprintf() are modeled on the ones that first
     appeared in the GNU C library.

CAVEATS
     Because sprintf() and vsprintf() assume an infinitely long string, call-
     ers must be careful not to overflow the actual space; this is often
     impossible to assure.  For safety, programmers should use the snprintf()
     and asprintf() family of interfaces instead.  Unfortunately, the
     snprintf() interfaces are not available on older systems and the
     asprintf() interfaces are not yet 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 your stack, leading to a possible security hole.  This
     holds true even if you have built the string ``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
     you use positional arguments you must 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 your code is non-portable and liable to
     fail.

BUGS
     The conversion formats %D, %O, and are not standard and are provided only
     for backward compatibility.  The effect of padding the 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 conversions, as well as
     other nonsensical combinations such as %Ld, are not standard; such combi-
     nations should be avoided.

BSD                              May 11, 2003                              BSD