printf

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



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

SYNOPSIS
       #include <stdio.h>

       int printf(const char *format, ...);
       int fprintf(FILE *stream, const char *format, ...);
       int dprintf(int fd, 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 vdprintf(int fd, 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():
           _XOPEN_SOURCE >= 500 || _ISOC99_SOURCE ||
               || /* Glibc versions <= 2.19: */ _BSD_SOURCE

       dprintf(), vdprintf():
           Since glibc 2.10:
               _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:
               _GNU_SOURCE

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 function dprintf() is the same as fprintf() except that it outputs to
       a file descriptor, fd, instead of to a stdio stream.

       The functions snprintf() and vsnprintf() write at most size bytes
       (including the terminating null byte ('\0')) to str.

       The functions vprintf(), vfprintf(), vdprintf(), vsprintf(), vsnprintf()
       are equivalent to the functions printf(), fprintf(), dprintf(),
       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).

       All of 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
       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.

   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 '*' (see Field width and Precision below) 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.  (See setlocale(3).)  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.

   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.  (See setlocale(3).)  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.

   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.

   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.

   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.

       q      A synonym for ll.  This is a nonstandard extension, derived from
              BSD; avoid its use in new code.

       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.)

       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.

       Z      A nonstandard synonym for z that predates the appearance of z.  Do
              not use in new code.

       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, except for those modifiers explicitly
       noted as being nonstandard extensions.  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).

       As a nonstandard extension, the GNU implementations treats ll and L as
       synonyms, so that one can, for example, write llg (as a synonym for the
       standards-compliant Lg) and Ld (as a synonym for the standards compliant
       lld).  Such usage is nonportable.

   Conversion specifiers
       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 (which is nonzero if the
              argument is nonzero) 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.  (This
              specifier is not supported by the bionic C library.)  The behavior
              is undefined if the conversion specification includes any flags, a
              field width, or a precision.

       m      (Glibc extension; supported by uClibc and musl.)  Print output of
              strerror(errno).  No argument is required.

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

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.

ATTRIBUTES
       For an explanation of the terms used in this section, see attributes(7).

       ┌────────────────────────┬───────────────┬────────────────┐
       │Interface               Attribute     Value          │
       ├────────────────────────┼───────────────┼────────────────┤
       │printf(), fprintf(),    │ Thread safety │ MT-Safe locale │
       │sprintf(), snprintf(),  │               │                │
       │vprintf(), vfprintf(),  │               │                │
       │vsprintf(), vsnprintf() │               │                │
       └────────────────────────┴───────────────┴────────────────┘

CONFORMING TO
       fprintf(), printf(), sprintf(), vprintf(), vfprintf(), vsprintf():
       POSIX.1-2001, POSIX.1-2008, C89, C99.

       snprintf(), vsnprintf(): POSIX.1-2001, POSIX.1-2008, C99.

       The dprintf() and vdprintf() functions were originally GNU extensions
       that were later standardized in POSIX.1-2008.

       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.  POSIX.1-2001 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
       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
       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.

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, ...)
       {
           int n = 0;
           size_t size = 0;
           char *p = NULL;
           va_list ap;

           /* Determine required size */

           va_start(ap, fmt);
           n = vsnprintf(p, size, fmt, ap);
           va_end(ap);

           if (n < 0)
               return NULL;

           /* One extra byte for '\0' */

           size = (size_t) n + 1;
           p = malloc(size);
           if (p == NULL)
               return NULL;

           va_start(ap, fmt);
           n = vsnprintf(p, size, fmt, ap);
           va_end(ap);

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

           return p;
       }

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

SEE ALSO
       printf(1), asprintf(3), puts(3), scanf(3), setlocale(3), strfromd(3),
       wcrtomb(3), wprintf(3), locale(5)

COLOPHON
       This page is part of release 5.08 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
       https://www.kernel.org/doc/man-pages/.



GNU                                2020-06-09                          PRINTF(3)