NAME | SYNOPSIS | DESCRIPTION | CONFORMING TO | NOTES | BUGS | EXAMPLE | SEE ALSO | COLOPHON

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

NAME         top

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

SYNOPSIS         top

       #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);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       snprintf(), vsnprintf():
           _BSD_SOURCE || _XOPEN_SOURCE >= 500 || _ISOC99_SOURCE ||
           _POSIX_C_SOURCE >= 200112L;
           or cc -std=c99

DESCRIPTION         top

       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 snprintf() and vsnprintf() write at most size bytes
       (including the terminating null byte ('\0')) to 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.  Because they invoke the va_arg macro, the
       value of ap is undefined after the call.  See stdarg(3).

       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.

       C99 and POSIX.1-2001 specify that the results are undefined if a call
       to sprintf(), snprintf(), vsprintf(), or vsnprintf() would cause
       copying to take place between objects that overlap (e.g., if the
       target string array and one of the supplied input arguments refer to
       the same buffer).  See NOTES.

   Return value
       Upon successful return, these functions return the number of
       characters printed (excluding the null byte used to end output to
       strings).

       The functions snprintf() and vsnprintf() do not write more than size
       bytes (including the terminating null byte ('\0')).  If the output
       was truncated due to this limit, then the return value is the number
       of characters (excluding the terminating null byte) 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 nonzero 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.)  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 -) should 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 C99 standard.  The
       Single UNIX Specification 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(1)
              cannot parse this option and will issue a warning.  (SUSv2 did
              not include %'F, but SUSv3 added it.)

       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,
              since glibc 2.2.3 this will give Arabic-Indic digits in the
              Persian ("fa_IR") locale.

   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 nonexistent 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 '.', the precision is taken to be zero.  A negative precision
       is taken as if the precision were omitted.  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.)  This is a synonym for ll.

       j      A following integer conversion corresponds to an intmax_t or
              uintmax_t argument, or a following n conversion corresponds to
              a pointer to an intmax_t argument.

       z      A following integer conversion corresponds to a size_t or
              ssize_t argument, or a following n conversion corresponds to a
              pointer to a size_t argument.

       t      A following integer conversion corresponds to a ptrdiff_t
              argument, or a following n conversion corresponds to a pointer
              to a ptrdiff_t argument.

       SUSv3 specifies all of the above.  SUSv2 specified only 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±dd 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.

              (SUSv2 does not know about F and says that character string
              representations for infinity and NaN may be made available.
              SUSv3 adds a specification for F.  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, but added in SUSv3) For a conversion, the
              double argument is converted to hexadecimal notation (using
              the letters abcdef) in the style [-]0xh.hhhhp±; 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 nonnormalized 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(3) 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 null byte ('\0'); if a
              precision is specified, no more than the number specified are
              written.  If a precision is given, no null byte need be
              present; if the precision is not specified, or is greater than
              the size of the array, the array must contain a terminating
              null byte.

              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(3) 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 or C11, but in SUSv2, SUSv3, and SUSv4.)  Synonym
              for lc.  Don't use.

       S      (Not in C99 or C11, but in SUSv2, SUSv3, and SUSv4.)  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 pointed to by the corresponding argument.  That
              argument shall be an int * , or variant whose size matches the
              (optionally) supplied integer length modifier.  No argument is
              converted.  The behavior is undefined if the conversion
              specification includes any flags, a field width, or a
              precision.

       m      (Glibc extension.)  Print output of strerror(errno).  No
              argument is required.

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

CONFORMING TO         top

       The fprintf(), printf(), sprintf(), vprintf(), vfprintf(), and
       vsprintf() functions conform to C89 and C99.  The snprintf() and
       vsnprintf() functions conform to C99.

       Concerning the return value of snprintf(), SUSv2 and C99 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.  SUSv3 and later align their
       specification of snprintf() with C99.

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

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

NOTES         top

       Some programs imprudently rely on code such as the following

           sprintf(buf, "%s some further text", buf);

       to append text to buf.  However, the standards explicitly note that
       the results are undefined if source and destination buffers overlap
       when calling sprintf(), snprintf(), vsprintf(), and vsnprintf().
       Depending on the version of gcc(1) used, and the compiler options
       employed, calls such as the above will not produce the expected
       results.

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

BUGS         top

       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(3) and vasprintf(3)).

       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.

EXAMPLE         top

       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, ...)
       {
           int n;
           int size = 100;     /* Guess we need no more than 100 bytes */
           char *p, *np;
           va_list ap;

           p = malloc(size);
           if (p == 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);

               /* Check error code */

               if (n < 0) {
                   free(p);
                   return NULL;
               }

               /* If that worked, return the string */

               if (n < size)
                   return p;

               /* Else try again with more space */

               size = n + 1;       /* Precisely what is needed */

               np = realloc(p, size);
               if (np == NULL) {
                   free(p);
                   return NULL;
               } else {
                   p = np;
               }
           }
       }

       If truncation occurs in glibc versions prior to 2.0.6, this is
       treated as an error instead of being handled gracefully.

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 3.71 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       http://www.kernel.org/doc/man-pages/.

GNU                              2014-07-08                        PRINTF(3)