PRINTF(3)               System Library Functions Manual              PRINTF(3)

NAME
     asprintf, vasprintf - formatted output conversion

SYNOPSIS
     #include 

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

     #include 

     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
     arguments following the format string.  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),

     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.  The value of ret in this situation is implementation-depen-
     dent (on OpenBSD, ret will be set to the null pointer, but this behavior
     should not be relied upon).

     The arguments must correspond properly (after type promotion) with the
     conversion specifier.  After the %, the following appear in sequence:

     o   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.

     o   Zero or more of the following flags:

         -   A hash `#' character specifying that the value should be con-
             verted to an ``alternate 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 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 con-
             versions 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 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, 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.

     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, and f conversions, the maximum num-
         ber 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 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 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.

     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, g, or G conver-
         sion corresponds to a long double argument (but note that long double
         values are not currently supported by the VAX compiler).

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

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

     gG      The double argument is converted in style f or e (or E for G con-
             versions).  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.

     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 charac-
             ter; if a precision is specified, no more than the number speci-
             fied are written.  If a precision is given, no null 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.

     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.

SEE ALSO
     printf(1), printf(3), strlcat(3), strlcpy(3,) scanf(3)

STANDARDS
     The functions asprintf() and vasprintf() first appeared in the GNU C
     library.  This implementation first appeared in OpenBSD 2.3.

CAVEATS
     The conversion formats %D, %O, 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 conversions, as
     well as other nonsensical combinations such as %Ld, are not standard;
     such combinations should be avoided.  The asprintf() and vasprintf()
     interfaces are 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 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.

Interix                       September 21, 2004                       Interix