printf(3) — Linux manual page

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | STANDARDS | HISTORY | CAVEATS | BUGS | EXAMPLES | SEE ALSO | COLOPHON

printf(3) Library Functions Manual printf(3)

NAME top

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

LIBRARY top

 Standard C library (libc, -lc)

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(size_t size;
 char str[restrict size], size_t size,
 const char *restrict format, ...);
 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(size_t size;
 char str[restrict size], 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円')) 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 CAVEATS.
 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:
 %[argument$][flags][width][.precision][length modifier]conversion
 The arguments must correspond properly (after type promotion) with
 the conversion specifier. By default, the arguments are used in
 the order given, where each '*' (see Field width and Precision
 below) and each conversion specifier asks for the next argument
 (and it is an error if insufficiently many arguments are given).
 One can also specify explicitly which argument is taken, at each
 place where an argument is required, by writing "%m$" 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 m, if errno  contains a
 valid error code, the output of strerrorname_np(errno) is
 printed; otherwise, the value stored in errno  is printed as
 a decimal number. 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 an integer 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.
 POSIX 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 as a non-
 monetary quantity. Misleadingly, this isn't necessarily
 every thousand: for example Karbi ("mjw_IN"), groups its
 digits into 3 once, then 2 repeatedly. Compare locale(7)
 grouping and thousands_sep, contrast with
 mon_grouping/mon_thousands_sep and strfmon(3). This is a
 no-op in the default "C" locale.
 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. On a following a, A, e, E, f, F, g, or G
 conversion, this length modifier is ignored (C99; not in
 SUSv2).
 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 radix 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 radix point is unspecified for
 nonnormalized numbers, and nonzero but otherwise
 unspecified for normalized numbers. The exponent, d, is
 the appropriate exponent of 2 expressed as a decimal
 integer; it 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円'); 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, and on
 Android from API level 29.) Print output of
 strerror(errno) (or strerrorname_np(errno) in the alternate
 form). 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 bytes
 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円')). 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 CAVEATS.)
 On error, a negative value is returned, and errno  is set to
 indicate the error.

ERRORS top

 See write(2) and putwc(3). In addition, the following error may
 occur:
 EOVERFLOW
 The value to be returned is greater than INT_MAX.
 The dprintf() function may fail additionally if:
 EBADF The fd argument is not a valid file descriptor.

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() │ │ │
 └───────────────────────────────┴───────────────┴────────────────┘

STANDARDS top

 fprintf()
 printf()
 sprintf()
 vprintf()
 vfprintf()
 vsprintf()
 snprintf()
 vsnprintf()
 C11, POSIX.1-2008.
 dprintf()
 vdprintf()
 GNU, POSIX.1-2008.

HISTORY top

 fprintf()
 printf()
 sprintf()
 vprintf()
 vfprintf()
 vsprintf()
 C89, POSIX.1-2001.
 snprintf()
 vsnprintf()
 SUSv2, C99, POSIX.1-2001.
 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.
 dprintf()
 vdprintf()
 GNU, POSIX.1-2008.
 Issue 4 of the X/Open Portability Guide (SUSv1, 1994) adds '.
 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.
 glibc 2.35 gives a meaning to the alternate form (#) of the m
 conversion specifier, that is %#m.

CAVEATS 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 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円' */
 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 glibc 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 the man-pages (Linux kernel and C library
 user-space interface documentation) project. Information about
 the project can be found at 
 ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
 for this manual page, see
 ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
 This page was obtained from the tarball man-pages-6.15.tar.gz
 fetched from
 ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
 2025年08月11日. If you discover any rendering problems in this HTML
 version of the page, or you believe there is a better or more up-
 to-date source for the page, or you have corrections or
 improvements to the information in this COLOPHON (which is not
 part of the original manual page), send a mail to
 man-pages@man7.org
Linux man-pages 6.15 2025年07月19日 printf(3)

Pages that refer to this page: bash(1), find(1), gawk(1), hexdump(1), printf(1), ps(1), time(1), perf_event_open(2), asprintf(3), avc_init(3), curs_printw(3x), curs_termcap(3x), ecvt(3), ecvt_r(3), err(3), errno(3), error(3), FILE(3type), form_field_validation(3x), fwide(3), gcvt(3), intmax_t(3type), intN_t(3type), intptr_t(3type), lber-encode(3), localeconv(3), lttng-ust(3), lttng_ust_tracef(3), lttng_ust_tracelog(3), numa(3), pmnotifyerr(3), pmnumberstr(3), pmprintf(3), pmsprintf(3), printf.h(3head), ptrdiff_t(3type), qecvt(3), rpc(3), scanf(3), sd_bus_error(3), sd_bus_message_new_method_error(3), sd-daemon(3), sd-id128(3), sd_id128_to_string(3), sd_journal_print(3), sd_journal_stream_fd(3), selinux_set_callback(3), setbuf(3), size_t(3type), sscanf(3), stdarg(3), stdin(3), stdio(3), strfmon(3), strfromd(3), strftime(3), syslog(3), uuid_parse(3), void(3type), wprintf(3), slapd.conf(5), slapd-config(5), terminfo(5), feature_test_macros(7), locale(7), signal-safety(7), system_data_types(7), btrfstune(8), rpm(8), slappasswd(8)



HTML rendering created 2025年09月06日 by Michael Kerrisk, author of The Linux Programming Interface.

For details of in-depth Linux/UNIX system programming training courses that I teach, look here.

Hosting by jambit GmbH.

Cover of TLPI

Web Analytics Made Easy - StatCounter

AltStyle によって変換されたページ (->オリジナル) /