printf(3) Library Functions Manual printf(3)
NAME
printf, fprintf, dprintf, sprintf, snprintf, vprintf, vfprintf, vdprintf, vsprintf, vsnprintf - formatted output conversion
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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 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(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
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 ar‐
guments 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 in‐
put 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 com‐
posed of zero or more directives: ordinary characters (not %), which are copied unchanged to the output stream; and conver‐
sion 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:
%[$][flags][width][.precision][length modifier]conversion
The arguments must correspond properly (after type promotion) with the conversion specifier. By default, the arguments are
used in the order given, where each '*' (see Field width and Precision below) and each conversion specifier asks for the
next argument (and it is an error if insufficiently many arguments are given). One can also specify explicitly which argu‐
ment is taken, at each place where an argument is required, by writing "%m$" 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 char‐
acter 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 con‐
versions 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 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 con‐
verted 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.) Note also that the default locale of a C
program is "C" whose locale information indicates no thousands' grouping character. Therefore, without a prior call
to setlocale(3), no thousands' grouping characters will be printed.
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 exam‐
ple, 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 ar‐
gument, or in the m-th argument, respectively, which must be of type int. If the precision is given as just '.', the preci‐
sion 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 charac‐
ters 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 corre‐
sponds 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 fol‐
lowing s conversion corresponds to a pointer to wchar_t argument. On a following a, A, e, E, f, F, g, or G conver‐
sion, 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 corre‐
sponds 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 speci‐
fied 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 preci‐
sion, 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 preci‐
sion; 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 avail‐
able. 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 af‐
ter it is equal to the precision. The default precision suffices for an exact representation of the value if an ex‐
act 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 writ‐
ten. 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 char‐
acters 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 ar‐
gument 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) (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
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.
VERSIONS
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.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
┌──────────────────────────────────────────────────────────────────────────────────────────┬───────────────┬────────────────┐
│Interface │ Attribute │ Value │
├──────────────────────────────────────────────────────────────────────────────────────────┼───────────────┼────────────────┤
│printf(), fprintf(), sprintf(), snprintf(), vprintf(), vfprintf(), vsprintf(), │ Thread safety │ MT-Safe locale │
│vsnprintf() │ │ │
└──────────────────────────────────────────────────────────────────────────────────────────┴───────────────┴────────────────┘
STANDARDS
fprintf(), printf(), sprintf(), snprintf(), vprintf(), vfprintf(), vsprintf(), 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.
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 buf‐
fers 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 de‐
scribed above, since glibc 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 in‐
put, 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;
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 glibc 2.0.6, this is treated as an error instead of being handled grace‐
fully.
SEE ALSO
printf(1), asprintf(3), puts(3), scanf(3), setlocale(3), strfromd(3), wcrtomb(3), wprintf(3), locale(5)
Linux man-pages 6.03 2023-02-05 printf(3)