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

NAME
     printf, fprintf, sprintf, snprintf, asprintf, vprintf, vfprintf,
     vsprintf, vsnprintf, vasprintf, vansprintf - 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 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 giv-
     en output stream; sprintf(), snprintf(), vsprintf() and vsnprintf() write
     to the character string str; asprintf() and vasprintf() write to a dynam-
     ically 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 and set ret to be a NULL pointer.

     If asprintf() or vasprintf() are given a size of 0, they 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.

     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:

     -   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 # character specifying that the value should be converted 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 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, g, and G, conversions, the result will al-
             ways 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, 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-adjust-
         ment 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 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.

     -   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 l (ell) specifying that a following d, i, o,
         u, x, or X conversion applies to a pointer to a long int or unsigned
         long int argument, or that a following n conversion corresponds to a
         pointer to a long int argument.

     -   The optional character q, specifying that a following d, i, o, u, x,
         or X conversion corresponds to a quad int or unsigned quad int argu-
         ment, or that a following n conversion corresponds to a pointer to a
         quad int argument.

     -   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 and Tahoe compilers).

     -   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 re-
             sulting 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 ar-
             ray are written up to (but not including) a terminating NUL char-
             acter; if a precision is specified, no more than the number spec-
             ified 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 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.

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)

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

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

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

     Because sprintf() and vsprintf() assume an infinitely long string,
     callers 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() interface is not available on older systems and the asprintf()
     interface is not portable.

     Never print a user-supplied string directly as a format without using %s,
     as an attacker can put format specifiers in that string to mangle your
     stack. 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.

OpenBSD 2.6                      June 4, 1993                                5