Skip site navigation (1)Skip section navigation (2)

FreeBSD Man Pages

Man Page or Keyword Search:
Man Architecture
Apropos Keyword Search (all sections) Output format
home | help
PRINTF(3)                  Linux Programmer's Manual                 PRINTF(3)

NAME
       printf, fprintf, sprintf, snprintf, vprintf, vfprintf, vsprintf,
       vsnprintf - 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, ...);

       #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);

DESCRIPTION
       The functions in the printf family produce output according to a format
       as described below. The functions 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.

       The functions vprintf, vfprintf, vsprintf, vsnprintf are equivalent to
       the functions printf, fprintf, sprintf, snprintf, respectively, except
       that they are called with a va_list instead of a variable number of
       arguments. These functions do not call the va_end macro. Consequently,
       the value of ap is undefined after the call. The application should
       call va_end(ap) itself afterwards.

       These eight 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 output.

   Return value
       Upon successful return, these functions return the number of characters
       printed (not including the trailing '\0' used to end output to
       strings).  The functions snprintf and vsnprintf do not write more than
       size bytes (including the trailing '\0').  If the output was truncated
       due to this limit then the return value is the number of characters
       (not including the trailing '\0') which would have been written to the
       final string if enough space had been available. Thus, a return value
       of size or more means that the output was truncated. (See also below
       under NOTES.)  If an output error is encountered, a negative value is
       returned.

   Format of the format string
       The format string is a character string, beginning and ending in its
       initial shift state, if any.  The format string is composed of zero or
       more directives: ordinary 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 %, and ends
       with a conversion specifier.  In between there may be (in this order)
       zero or more flags, an optional minimum field width, an optional
       precision and an optional length modifier.

       The arguments must correspond properly (after type promotion) with the
       conversion specifier. By default, the arguments are used in the order
       given, where each `*' and each conversion specifier asks for the next
       argument (and it is an error if insufficiently many arguments are
       given).  One can also specify explicitly which argument is taken, at
       each place where an argument is required, by writing `%m$' instead of
       `%' and `*m$' instead of `*', where the decimal integer m denotes the
       position in the argument list of the desired argument, indexed starting
       from 1. Thus,
                   printf("%*d", width, num);
       and
                   printf("%2$*1$d", width, num);
       are equivalent. The second style allows repeated references to the same
       argument. The C99 standard does not include the style using `$', which
       comes from the Single Unix Specification.  If the style using `$' is
       used, it must be used throughout for all conversions taking an argument
       and all width and precision arguments, but it may be mixed with `%%'
       formats which do not consume an argument.  There may be no gaps in the
       numbers of arguments specified using `$'; for example, if arguments 1
       and 3 are specified, argument 2 must also be specified somewhere in the
       format string.

       For some numeric conversions a radix character (`decimal point') or
       thousands' grouping character is used. The actual character used
       depends on the LC_NUMERIC part of the locale. The POSIX locale uses `.'
       as radix character, and does not have a grouping character.  Thus,
                   printf("%'.2f", 1234567.89);
       results in `1234567.89' in the POSIX locale, in `1234567,89' in the
       nl_NL locale, and in `1.234.567,89' in the da_DK locale.

   The flag characters
       The character % is followed by zero or more of the following flags:

       #      The value should be converted to an ``alternate form''.  For o
              conversions, the first character of the output string is made
              zero (by prefixing a 0 if it was not zero already).  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.  For other conversions, the
              result is undefined.

       0      The value should be zero padded.  For d, i, o, u, x, X, a, A, e,
              E, f, F, g, and G conversions, the converted value is padded on
              the left with zeros rather than blanks.  If the 0 and - flags
              both appear, the 0 flag is ignored.  If a precision is given
              with a numeric conversion (d, i, o, u, x, and X), the 0 flag is
              ignored.  For other conversions, the behavior is undefined.

       -      The converted value is to be left adjusted on the field
              boundary.  (The default is right justification.) 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.

       ' '    (a space) A blank should be left before a positive number (or
              empty string) produced by a signed conversion.

       +      A sign (+ or -) always be placed before a number produced by a
              signed conversion.  By default a sign is used only for negative
              numbers. A + overrides a space if both are used.

       The five flag characters above are defined in the C standard.  The
       SUSv2 specifies one further flag character.

       '      For decimal conversion (i, d, u, f, F, g, G) the output is to be
              grouped with thousands' grouping characters if the locale
              information indicates any.  Note that many versions of gcc
              cannot parse this option and will issue a warning.  SUSv2 does
              not include %'F.

       glibc 2.2 adds one further flag character.

       I      For decimal integer conversion (i, d, u) the output uses the
              locale's alternative output digits, if any (for example, Arabic
              digits).  However, it does not include any locale definitions
              with such outdigits defined.

   The field width
       An optional decimal digit string (with nonzero first digit) 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).  Instead of a
       decimal digit string one may write `*' or `*m$' (for some decimal
       integer m) to specify that the field width is given in the next
       argument, or in the m-th argument, respectively, which must be of type
       int.  A negative field width is taken as a `-' flag followed by a
       positive field width.  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.

   The precision
       An optional precision, in the form of a period (`.')  followed by an
       optional decimal digit string.  Instead of a decimal digit string one
       may write `*' or `*m$' (for some decimal integer m) to specify that the
       precision is given in the next argument, or in the m-th argument,
       respectively, which must be of type int.  If the precision is given as
       just `.', or the precision is negative, the precision is taken to be
       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 radix
       character 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 and S conversions.

   The length modifier
       Here, `integer conversion' stands for d, i, o, u, x, or X conversion.

       hh     A following integer conversion corresponds to a signed char or
              unsigned char argument, or a following n conversion corresponds
              to a pointer to a signed char argument.

       h      A following integer conversion corresponds to a short int or
              unsigned short int argument, or a following n conversion
              corresponds to a pointer to a short int argument.

       l      (ell) A following integer conversion corresponds to a long int
              or unsigned long int argument, or a following n conversion
              corresponds to a pointer to a long int argument, or a following
              c conversion corresponds to a wint_t argument, or a following s
              conversion corresponds to a pointer to wchar_t argument.

       ll     (ell-ell).  A following integer conversion corresponds to a long
              long int or unsigned long long int argument, or a following n
              conversion corresponds to a pointer to a long long int argument.

       L      A following a, A, e, E, f, F, g, or G conversion corresponds to
              a long double argument.  (C99 allows %LF, but SUSv2 does not.)

       q      (`quad'. BSD 4.4 and Linux libc5 only. Don't use.)  This is a
              synonym for ll.

       j      A following integer conversion corresponds to an intmax_t or
              uintmax_t argument.

       z      A following integer conversion corresponds to a size_t or
              ssize_t argument. (Linux libc5 has Z with this meaning. Don't
              use it.)

       t      A following integer conversion corresponds to a ptrdiff_t
              argument.

       The SUSv2 only knows about the length modifiers h (in hd, hi, ho, hx,
       hX, hn) and l (in ld, li, lo, lx, lX, ln, lc, ls) and L (in Le, LE, Lf,
       Lg, LG).

   The conversion specifier
       A character that specifies the type of conversion to be applied.  The
       conversion specifiers and their meanings are:

       d,i    The int argument is converted to signed decimal notation.  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. The default precision is 1.  When
              0 is printed with an explicit precision 0, the output is empty.

       o,u,x,X
              The unsigned int argument is converted to 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. The default precision is 1.  When 0 is printed with
              an explicit precision 0, the output is empty.

       e,E    The double argument is rounded and converted in the style
              [-]d.ddde*(Pmdd where there is one digit before the
              decimal-point character and the number of digits after it is
              equal to the precision; 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 exponent.  The exponent always contains at least
              two digits; if the value is zero, the exponent is 00.

       f,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.

              (The SUSv2 does not know about F and says that character string
              representations for infinity and NaN may be made available. The
              C99 standard specifies `[-]inf' or `[-]infinity' for infinity,
              and a string starting with `nan' for NaN, in the case of f
              conversion, and `[-]INF' or `[-]INFINITY' or `NAN*' in the case
              of F conversion.)

       g,G    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.

       a,A    (C99; not in SUSv2) For a conversion, the double argument is
              converted to hexadecimal notation (using the letters abcdef) in
              the style [-]0xh.hhhhp*(Pmd; for A conversion the prefix 0X, the
              letters ABCDEF, and the exponent separator P is used.  There is
              one hexadecimal digit before the decimal point, and the number
              of digits after it is equal to the precision.  The default
              precision suffices for an exact representation of the value if
              an exact representation in base 2 exists and otherwise is
              sufficiently large to distinguish values of type double.  The
              digit before the decimal point is unspecified for non-normalized
              numbers, and nonzero but otherwise unspecified for normalized
              numbers.

       c      If no l modifier is present, the int argument is converted to an
              unsigned char, and the resulting character is written.  If an l
              modifier is present, the wint_t (wide character) argument is
              converted to a multibyte sequence by a call to the wcrtomb
              function, with a conversion state starting in the initial state,
              and the resulting multibyte string is written.

       s      If no l modifier is present: The const 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 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 an l modifier is present: The const wchar_t * argument is
              expected to be a pointer to an array of wide characters.  Wide
              characters from the array are converted to multibyte characters
              (each by a call to the wcrtomb function, with a conversion state
              starting in the initial state before the first wide character),
              up to and including a terminating null wide character. The
              resulting multibyte characters are written up to (but not
              including) the terminating null byte. If a precision is
              specified, no more bytes than the number specified are written,
              but no partial multibyte characters are written. Note that the
              precision determines the number of bytes written, not the number
              of wide characters or screen positions.  The array must contain
              a terminating null wide character, unless a precision is given
              and it is so small that the number of bytes written exceeds it
              before the end of the array is reached.

       C      (Not in C99, but in SUSv2.)  Synonym for lc.  Don't use.

       S      (Not in C99, but in SUSv2.)  Synonym for ls.  Don't use.

       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
              integer indicated by the int * (or variant) pointer argument.
              No argument is converted.

       %      A `%' is written. No argument is converted. The complete
              conversion specification is `%%'.

EXAMPLES
       To print pi to five decimal places:
              #include <math.h>
              #include <stdio.h>
              fprintf(stdout, "pi = %.5f\n", 4 * atan(1.0));

       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);

       Many countries use the day-month-year order.  Hence, an
       internationalized version must be able to print the arguments in an
       order specified by the format:
              #include <stdio.h>
              fprintf(stdout, format,
                   weekday, month, day, hour, min);
       where format depends on locale, and may permute the arguments. With the
       value
              "%1$s, %3$d. %2$s, %4$d:%5$.2d\n"
       one might obtain `Sonntag, 3. Juli, 10:02'.

       To allocate a sufficiently large string and print into it (code correct
       for both glibc 2.0 and glibc 2.1):
              #include <stdio.h>
              #include <stdlib.h>
              #include <stdarg.h>
              char *
              make_message(const char *fmt, ...) {
                 /* Guess we need no more than 100 bytes. */
                 int n, size = 100;
                 char *p;
                 va_list ap;
                 if ((p = malloc (size)) == NULL)
                    return NULL;
                 while (1) {
                    /* Try to print in the allocated space. */
                    va_start(ap, fmt);
                    n = vsnprintf (p, size, fmt, ap);
                    va_end(ap);
                    /* If that worked, return the string. */
                    if (n > -1 && n < size)
                       return p;
                    /* Else try again with more space. */
                    if (n > -1)    /* glibc 2.1 */
                       size = n+1; /* precisely what is needed */
                    else           /* glibc 2.0 */
                       size *= 2;  /* twice the old size */
                    if ((p = realloc (p, size)) == NULL)
                       return NULL;
                 }
              }

NOTES
       The glibc implementation of the functions snprintf and vsnprintf
       conforms to the C99 standard, i.e., behaves as described above, since
       glibc version 2.1. Until glibc 2.0.6 they would return -1 when the
       output was truncated.

CONFORMING TO
       The fprintf, printf, sprintf, vprintf, vfprintf, and vsprintf functions
       conform to ANSI X3.159-1989 (``ANSI C'') and ISO/IEC 9899:1999 (``ISO
       C99'').  The snprintf and vsnprintf functions conform to ISO/IEC
       9899:1999.

       Concerning the return value of snprintf, the SUSv2 and the C99 standard
       contradict each other: when snprintf is called with size=0 then SUSv2
       stipulates an unspecified return value less than 1, while C99 allows
       str to be NULL in this case, and gives the return value (as always) as
       the number of characters that would have been written in case the
       output string has been large enough.

       Linux libc4 knows about the five C standard flags.  It knows about the
       length modifiers h,l,L, and the conversions cdeEfFgGinopsuxX, where F
       is a synonym for f.  Additionally, it accepts D,O,U as synonyms for
       ld,lo,lu.  (This is bad, and caused serious bugs later, when support
       for %D disappeared.) No locale-dependent radix character, no thousands'
       separator, no NaN or infinity, no %m$ and *m$.

       Linux libc5 knows about the five C standard flags and the ' flag,
       locale, %m$ and *m$.  It knows about the length modifiers h,l,L,Z,q,
       but accepts L and q both for long doubles and for long long integers
       (this is a bug).  It no longer recognizes FDOU, but adds a new
       conversion character m, which outputs strerror(errno).

       glibc 2.0 adds conversion characters C and S.

       glibc 2.1 adds length modifiers hh,j,t,z and conversion characters a,A.

       glibc 2.2 adds the conversion character F with C99 semantics, and the
       flag character I.

HISTORY
       Unix V7 defines the three routines printf, fprintf, sprintf, and has
       the flag -, the width or precision *, the length modifier l, and the
       conversions doxfegcsu, and also D,O,U,X as synonyms for ld,lo,lu,lx.
       This is still true for BSD 2.9.1, but BSD 2.10 has the flags #, + and
       <space> and no longer mentions D,O,U,X.  BSD 2.11 has vprintf,
       vfprintf, vsprintf, and warns not to use D,O,U,X.  BSD 4.3 Reno has the
       flag 0, the length modifiers h and L, and the conversions n, p, E, G, X
       (with current meaning) and deprecates D,O,U.  BSD 4.4 introduces the
       functions snprintf and vsnprintf, and the length modifier q.  FreeBSD
       also has functions asprintf and vasprintf, that allocate a buffer large
       enough for sprintf.  In glibc there are functions dprintf and vdprintf
       that print to a file descriptor instead of a stream.

BUGS
       Because sprintf and vsprintf assume an arbitrarily long string, callers
       must be careful not to overflow the actual space; this is often
       impossible to assure. Note that the length of the strings produced is
       locale-dependent and difficult to predict.  Use snprintf and vsnprintf
       instead (or asprintf and vasprintf).

       Linux libc4.[45] does not have a snprintf, but provides a libbsd that
       contains an snprintf equivalent to sprintf, i.e., one that ignores the
       size argument.  Thus, the use of snprintf with early libc4 leads to
       serious security problems.

       Code such as printf(foo); often indicates a bug, since foo may contain
       a % character.  If foo comes from untrusted user input, it may contain
       %n, causing the printf call to write to memory and creating a security
       hole.

SEE ALSO
       printf(1), asprintf(3), dprintf(3), wcrtomb(3), wprintf(3), scanf(3),
       locale(5)

Linux Manpage                     2000-10-16                         PRINTF(3)

NAME | SYNOPSIS | DESCRIPTION | EXAMPLES | NOTES | CONFORMING TO | HISTORY | BUGS | SEE ALSO

Want to link to this manual page? Use this URL:
<https://www.freebsd.org/cgi/man.cgi?query=sprintf&sektion=3&manpath=Red+Hat+Linux%2fi386+9>

home | help