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printf, fprintf, sprintf, snprintf, asprintf, vprintf, vfprintf,

 

NAME

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

LIBRARY

      Standard C Library (libc, -lc)
 

SYNOPSIS

      #include <stdio.h>
 
      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 **ret, const char *format, ...);
 
      #include <stdarg.h>
 
      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 **ret, const char *format, va_list ap);
 

DESCRIPTION

      The printf() family of functions produces output according to a format as
      described below.  The printf() and vprintf() functions 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; and asprintf() and
      vasprintf() dynamically allocate a new string with malloc(3).
 
      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) or a negative value if
      an output error occurs, except for snprintf() and vsnprintf(), which
      return the number of characters that would have been printed if the size
      were unlimited (again, not including the final ‘\0’).
 
      The asprintf() and vasprintf() functions set *ret to be a pointer to a
      buffer sufficiently large to hold the formatted string.  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, asprintf()
      and vasprintf() will return -1 and set ret to be a NULL pointer.
 
      The snprintf() and vsnprintf() functions will write at most size-1 of the
      characters printed into the output string (the size’th character then
      gets the terminating ‘\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.  The output is always null-terminated.
 
      The sprintf() and vsprintf() functions 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.  If unaccessed arguments
          in the format string are interspersed with ones that are accessed the
          results will be indeterminate.
 
            Zero or more of the following flags:
 
          ‘#’          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 num‐
                       ber 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 a, A, e,
                       E, f, 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 otherwise be.
 
          ‘0’ (zero)   Zero padding.  For all conversions except n, the con‐
                       verted 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; the converted value is to
                       be left adjusted on the field boundary.  Except for n
                       conversions, 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.
 
          ‘ ’ (space)  A blank should be left before a positive number produced
                       by a signed conversion (a, A, d, e, E, f, F, g, G, or
                       i).
 
          ‘+’          A sign must always be placed before a number produced by
                       a signed conversion.  A + overrides a space if both are
                       used.
 
          ‘     ’          Decimal conversions (d, u, or i) or the integral portion
                       of a floating point conversion (f or F) should be
                       grouped and separated by thousands using the non-mone‐
                       tary separator returned by localeconv(3).
 
            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
          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 a, A, 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 con‐
          versions.
 
            An optional length modifier, that specifies the size of the argument.
          The following length modifiers are valid for the d, i, n, o, u, x, or
          X conversion:
 
          Modifier          d, i           o, u, x, X            n
          hh                signed char    unsigned char         signed char *
          h                 short          unsigned short        short *
          l (ell)           long           unsigned long         long *
          ll (ell ell)      long long      unsigned long long    long long *
          j                 intmax_t       uintmax_t             intmax_t *
          t                 ptrdiff_t      (see note)            ptrdiff_t *
          z                 (see note)     size_t                (see note)
          q (deprecated)    quad_t         u_quad_t              quad_t *
 
          Note: the t modifier, when applied to a o, u, x, or X conversion,
          indicates that the argument is of an unsigned type equivalent in size
          to a ptrdiff_t.  The z modifier, when applied to a d or i conversion,
          indicates that the argument is of a signed type equivalent in size to
          a size_t.  Similarly, when applied to an n conversion, it indicates
          that the argument is a pointer to a signed type equivalent in size to
          a size_t.
 
          The following length modifier is valid for the a, A, e, E, f, F, g,
          or G conversion:
 
          Modifier    a, A, e, E, f, F, g, G
          l (ell)     double (ignored, same behavior as without it)
          L           long double
 
          The following length modifier is valid for the c or s conversion:
 
          Modifier    c         s
          l (ell)     wint_t    wchar_t *
 
            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
              conversions.  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
              exponent.  The exponent always contains at least two digits; if
              the value is zero, the exponent is 00.
 
              For a, A, e, E, f, F, g, and G conversions, positive and negative
              infinity are represented as inf and -inf respectively when using
              the lowercase conversion character, and INF and -INF respectively
              when using the uppercase conversion character.  Similarly, NaN is
              represented as nan when using the lowercase conversion, and NAN
              when using the uppercase conversion.
 
      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.
 
      gG      The double argument is converted in style f or e (or 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.
 
      aA      The double argument is rounded and converted to hexadecimal nota‐
              tion in the style [-]0xh.hhhp[±]d, where the number of digits
              after the hexadecimal-point character is equal to the precision
              specification.  If the precision is missing, it is taken as
              enough to represent the floating-point number exactly, and no
              rounding occurs.  If the precision is zero, no hexadecimal-point
              character appears.  The p is a literal character ‘p’, and the
              exponent consists of a positive or negative sign followed by a
              decimal number representing an exponent of 2.  The A conversion
              uses the prefix “0X” (rather than “0x”), the letters “ABCDEF”
              (rather than “abcdef”) to represent the hex digits, and the let‐
              ter ‘P’ (rather than ‘p’) to separate the mantissa and exponent.
 
              Note that there may be multiple valid ways to represent floating-
              point numbers in this hexadecimal format.  For example,
              0x3.24p+0, 0x6.48p-1 and 0xc.9p-2 are all equivalent.  The format
              chosen depends on the internal representation of the number, but
              the implementation guarantees that the length of the mantissa
              will be minimized.  Zeroes are always represented with a mantissa
              of 0 (preceded by a ‘-’ if appropriate) and an exponent of +0.
 
      C       Treated as c with the l (ell) modifier.
 
      c       The int argument is converted to an unsigned char, and the
              resulting character is written.
 
              If the l (ell) modifier is used, the wint_t argument shall be
              converted to a wchar_t, and the (potentially multi-byte) sequence
              representing the single wide character is written, including any
              shift sequences.  If a shift sequence is used, the shift state is
              also restored to the original state after the character.
 
      S       Treated as s with the l (ell) modifier.
 
      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.
 
              If the l (ell) modifier is used, the wchar_t * argument is
              expected to be a pointer to an array of wide characters (pointer
              to a wide string).  For each wide character in the string, the
              (potentially multi-byte) sequence representing the wide character
              is written, including any shift sequences.  If any shift sequence
              is used, the shift state is also restored to the original state
              after the string.  Wide characters from the array are written up
              to (but not including) a terminating wide NUL character; if a
              precision is specified, no more than the number of bytes speci‐
              fied are written (including shift sequences).  Partial characters
              are never written.  If a precision is given, no null character
              need be present; if the precision is not specified, or is greater
              than the number of bytes required to render the multibyte repre‐
              sentation of the string, the array must contain a terminating
              wide 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 ‘%%’.
 
      The decimal point character is defined in the program’s locale (category
      LC_NUMERIC).
 
      In no case does a non-existent or small field width cause truncation of a
      numeric 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 π 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);
            }
      The sprintf() and vsprintf() functions are easily misused in a manner
      which enables malicious users to arbitrarily change a running program’s
      functionality through a buffer overflow attack.  Because sprintf() and
      vsprintf() assume an infinitely long string, callers must be careful not
      to overflow the actual space; this is often hard to assure.  For safety,
      programmers should use the snprintf() interface instead.  For example:
 
      void
      foo(const char *arbitrary_string, const char *and_another)
      {
              char onstack[8];
 
      #ifdef BAD
              /*
               * This first sprintf is bad behavior.  Do not use sprintf!
               */
              sprintf(onstack, "%s, %s", arbitrary_string, and_another);
      #else
              /*
               * The following two lines demonstrate better use of
               * snprintf().
               */
              snprintf(onstack, sizeof(onstack), "%s, %s", arbitrary_string,
                  and_another);
      #endif
      }
 
      The printf() and sprintf() family of functions are also easily misused in
      a manner allowing malicious users to arbitrarily change a running pro‐
      gram’s functionality by either causing the program to print potentially
      sensitive data “left on the stack”, or causing it to generate a memory
      fault or bus error by dereferencing an invalid pointer.
 
      %n can be used to write arbitrary data to potentially carefully-selected
      addresses.  Programmers are therefore strongly advised to never pass
      untrusted strings as the format argument, as an attacker can put format
      specifiers in the string to mangle your stack, leading to a possible
      security hole.  This holds true even if the string was built using a
      function like snprintf(), as the resulting string may still contain user-
      supplied conversion specifiers for later interpolation by printf().
 
      Always use the proper secure idiom:
 
            snprintf(buffer, sizeof(buffer), "%s", string);
 

ERRORS

      In addition to the errors documented for the write(2) system call, the
      printf() family of functions may fail if:
 
      [EILSEQ]           An invalid wide character code was encountered.
 
      [ENOMEM]           Insufficient storage space is available.
      printf(1), fmtcheck(3), scanf(3), setlocale(3), wprintf(3)
 

STANDARDS

      Subject to the caveats noted in the BUGS section below, the fprintf(),
      printf(), sprintf(), vprintf(), vfprintf(), and vsprintf() functions con‐
      form to ANSI X3.159-1989 (“ANSI C”) and ISO/IEC 9899:1999 (“ISO C99”).
      With the same reservation, the snprintf() and vsnprintf() functions con‐
      form to ISO/IEC 9899:1999 (“ISO C99”).
 

HISTORY

      The functions asprintf() and vasprintf() first appeared in the GNU C
      library.  These were implemented by Peter Wemm 〈peter@FreeBSD.org〉 in
      FreeBSD 2.2, but were later replaced with a different implementation from
      Todd C. Miller 〈Todd.Miller@courtesan.com〉 for OpenBSD 2.3.
 

BUGS

      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 printf family of functions do not correctly handle multibyte charac‐
      ters in the format argument.
 

Sections

Based on BSD UNIX
FreeBSD is an advanced operating system for x86 compatible (including Pentium and Athlon), amd64 compatible (including Opteron, Athlon64, and EM64T), UltraSPARC, IA-64, PC-98 and ARM architectures. It is derived from BSD, the version of UNIX developed at the University of California, Berkeley. It is developed and maintained by a large team of individuals. Additional platforms are in various stages of development.