printf(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | CONFORMING TO | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON

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

NAME         top

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

SYNOPSIS         top

       #include <stdio.h>

       int printf(const char *restrict format, ...);
       int fprintf(FILE *restrict stream,
                   const char *restrict format, ...);
       int dprintf(int fd,
                   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, ...);

       #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 vdprintf(int fd,
                   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);

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

       snprintf(), vsnprintf():
           _XOPEN_SOURCE >= 500 || _ISOC99_SOURCE
               || /* Glibc <= 2.19: */ _BSD_SOURCE

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

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 function dprintf() is the same as fprintf() except that it
       outputs to a file descriptor, fd, instead of to a stdio(3)
       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 overall syntax of a conversion specification is:

           %[$][flags][width][.precision][length modifier]conversion

       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 or
              unsigned short argument, or a following n conversion
              corresponds to a pointer to a short argument.

       l      (ell) A following integer conversion corresponds to a long
              or unsigned long argument, or a following n conversion
              corresponds to a pointer to a long 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 or unsigned long long argument, or a following
              n conversion corresponds to a pointer to a long long
              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±d;
              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.  The
              exponent always contains at least one digit; if the value
              is zero, the exponent is 0.

       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         top

       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         top

       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         top

       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         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.

EXAMPLES         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 = 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;

           size = (size_t) n + 1;      /* One extra byte for '\0' */
           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         top

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

COLOPHON         top

       This page is part of release 5.13 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                            2021-03-22                      PRINTF(3)

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