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PRINTF(3) Linux Programmer's Manual PRINTF(3)
NAME
printf, fprintf, sprintf, snprintf, vprintf, vfprintf, vsprintf, vsnprintf - for-
matted output conversion
SYNOPSIS
#include <stdio.h>
int printf(const char *format, ...);
int fprintf(FILE *stream, const char *format, ...);
int sprintf(char *str, const char *format, ...);
int snprintf(char *str, size_t size, const char *format, ...);
#include <stdarg.h>
int vprintf(const char *format, va_list ap);
int vfprintf(FILE *stream, const char *format, va_list ap);
int vsprintf(char *str, const char *format, va_list ap);
int vsnprintf(char *str, size_t size, const char *format, va_list ap);
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 functions vprintf(), vfprintf(), vsprintf(), vsnprintf() are equivalent to the
functions printf(), fprintf(), sprintf(), snprintf(), respectively, except that
they are called with a va_list instead of a variable number of arguments. These
functions do not call the va_end macro. Consequently, the value of ap is undefined
after the call. The application should call va_end(ap) itself afterwards.
These eight functions write the output under the control of a format string that
specifies how subsequent arguments (or arguments accessed via the variable-length
argument facilities of stdarg(3)) are converted for output.
Return value
Upon successful return, these functions return the number of characters printed
(not including the trailing '\0' used to end output to strings). The functions
snprintf() and vsnprintf() do not write more than size bytes (including the trail-
ing '\0'). If the output was truncated due to this limit then the return value is
the number of characters (not including the trailing '\0') which would have been
written to the final string if enough space had been available. Thus, a return
value of size or more means that the output was truncated. (See also below under
NOTES.) If an output error is encountered, a negative value is returned.
Format of the format string
The format string is a character string, beginning and ending in its initial shift
state, if any. The format string is composed of zero or more directives: ordinary
characters (not %), which are copied unchanged to the output stream; and conversion
specifications, each of which results in fetching zero or more subsequent argu-
ments. Each conversion specification is introduced by the character %, and ends
with a conversion specifier. In between there may be (in this order) zero or more
flags, an optional minimum field width, an optional precision and an optional
length modifier.
The arguments must correspond properly (after type promotion) with the conversion
specifier. By default, the arguments are used in the order given, where each '*'
and each conversion specifier asks for the next argument (and it is an error if
insufficiently many arguments are given). One can also specify explicitly which
argument is taken, at each place where an argument is required, by writing '%m$'
instead of '%' and '*m$' instead of '*', where the decimal integer m denotes the
position in the argument list of the desired argument, indexed starting from 1.
Thus,
printf("%*d", width, num);
and
printf("%2$*1$d", width, num);
are equivalent. The second style allows repeated references to the same argument.
The C99 standard does not include the style using '$', which comes from the Single
Unix Specification. If the style using '$' is used, it must be used throughout for
all conversions taking an argument and all width and precision arguments, but it
may be mixed with '%%' formats which do not consume an argument. There may be no
gaps in the numbers of arguments specified using '$'; for example, if arguments 1
and 3 are specified, argument 2 must also be specified somewhere in the format
string.
For some numeric conversions a radix character ('decimal point') or thousands'
grouping character is used. The actual character used depends on the LC_NUMERIC
part of the locale. The POSIX locale uses '.' as radix character, and does not have
a grouping character. Thus,
printf("%'.2f", 1234567.89);
results in '1234567.89' in the POSIX locale, in '1234567,89' in the nl_NL locale,
and in '1.234.567,89' in the da_DK locale.
The flag characters
The character % is followed by zero or more of the following flags:
# The value should be converted to an ''alternate form''. For o conversions,
the first character of the output string is made zero (by prefixing a 0 if
it was not zero already). For x and X conversions, a non-zero 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 conver-
sions, trailing zeros are not removed from the result as they would other-
wise 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 unde-
fined.
- The converted value is to be left adjusted on the field boundary. (The
default is right justification.) Except for n conversions, 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 + over-
rides a space if both are used.
The five flag characters above are defined in the C standard. The SUSv2 specifies
one further flag character.
' For decimal conversion (i, d, u, f, F, g, G) the output is to be grouped
with thousands' grouping characters if the locale information indicates any.
Note that many versions of gcc(1) cannot parse this option and will issue a
warning. SUSv2 does not include %'F.
glibc 2.2 adds one further flag character.
I For decimal integer conversion (i, d, u) the output uses the locale's alter-
native output digits, if any. For example, since glibc 2.2.3 this will give
Arabic-Indic digits in the Persian ('fa_IR') locale.
The field width
An optional decimal digit string (with non-zero 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 argu-
ment, 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 non-existent or small field width cause truncation of a field; if the result
of a conversion is wider than the field width, the field is expanded to contain the
conversion result.
The precision
An optional precision, in the form of a period ('.') followed by an optional deci-
mal 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 '.', or the precision is negative, the precision is
taken to be zero. This gives the minimum number of digits to appear for d, i, o,
u, x, and X conversions, the number of digits to appear after the radix character
for a, A, e, E, f, and F conversions, the maximum number of significant digits for
g and G conversions, or the maximum number of characters to be printed from a
string for s and S conversions.
The length modifier
Here, 'integer conversion' stands for d, i, o, u, x, or X conversion.
hh A following integer conversion corresponds to a signed char or unsigned char
argument, or a following n conversion corresponds to a pointer to a signed
char argument.
h A following integer conversion corresponds to a short int or unsigned short
int argument, or a following n conversion corresponds to a pointer to a
short int argument.
l (ell) A following integer conversion corresponds to a long int or unsigned
long int argument, or a following n conversion corresponds to a pointer to a
long int argument, or a following c conversion corresponds to a wint_t argu-
ment, or a following s conversion corresponds to a pointer to wchar_t argu-
ment.
ll (ell-ell). A following integer conversion corresponds to a long long int or
unsigned long long int argument, or a following n conversion corresponds to
a pointer to a long long int argument.
L A following a, A, e, E, f, F, g, or G conversion corresponds to a long dou-
ble argument. (C99 allows %LF, but SUSv2 does not.)
q ('quad'. 4.4BSD and Linux libc5 only. Don't use.) This is a synonym for ll.
j A following integer conversion corresponds to an intmax_t or uintmax_t argu-
ment.
z A following integer conversion corresponds to a size_t or ssize_t argument.
(Linux libc5 has Z with this meaning. Don't use it.)
t A following integer conversion corresponds to a ptrdiff_t argument.
The SUSv2 only knows about the length modifiers h (in hd, hi, ho, hx, hX, hn) and l
(in ld, li, lo, lx, lX, ln, lc, ls) and L (in Le, LE, Lf, Lg, LG).
The conversion specifier
A character that specifies the type of conversion to be applied. The conversion
specifiers and their meanings are:
d,i The int argument is converted to signed decimal notation. The precision, if
any, gives the minimum number of digits that must appear; if the converted
value requires fewer digits, it is padded on the left with zeros. The
default precision is 1. When 0 is printed with an explicit precision 0, the
output is empty.
o,u,x,X
The unsigned int argument is converted to unsigned octal (o), unsigned deci-
mal (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 pre-
cision 0, the output is empty.
e,E The double argument is rounded and converted in the style [-]d.ddde?dd where
there is one digit before the decimal-point character and the number of dig-
its 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 charac-
ter 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.
(The SUSv2 does not know about F and says that character string representa-
tions for infinity and NaN may be made available. 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 conver-
sions). 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) 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 separa-
tor 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 preci-
sion suffices for an exact representation of the value if an exact represen-
tation in base 2 exists and otherwise is sufficiently large to distinguish
values of type double. The digit before the decimal point is unspecified
for non-normalized numbers, and non-zero but otherwise unspecified for nor-
malized 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() 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() func-
tion, 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 writ-
ten. 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, but in SUSv2.) Synonym for lc. Don't use.
S (Not in C99, but in SUSv2.) 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 indicated
by the int * (or variant) pointer argument. No argument is converted.
m (Glibc extension.) Print output of strerror(errno). No argument is
required.
% A '%' is written. No argument is converted. The complete conversion specifi-
cation is '%%'.
EXAMPLE
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, ...) {
/* Guess we need no more than 100 bytes. */
int n, size = 100;
char *p, *np;
va_list ap;
if ((p = malloc (size)) == NULL)
return NULL;
while (1) {
/* Try to print in the allocated space. */
va_start(ap, fmt);
n = vsnprintf (p, size, fmt, ap);
va_end(ap);
/* If that worked, return the string. */
if (n > -1 && n < size)
return p;
/* Else try again with more space. */
if (n > -1) /* glibc 2.1 */
size = n+1; /* precisely what is needed */
else /* glibc 2.0 */
size *= 2; /* twice the old size */
if ((np = realloc (p, size)) == NULL) {
free(p);
return NULL;
} else {
p = np;
}
}
}
NOTES
The glibc implementation of the functions snprintf() and vsnprintf() conforms to
the C99 standard, i.e., behaves as described above, since glibc version 2.1. Until
glibc 2.0.6 they would return -1 when the output was truncated.
CONFORMING TO
The fprintf(), printf(), sprintf(), vprintf(), vfprintf(), and vsprintf() functions
conform to C89 and C99. The snprintf() and vsnprintf() functions conform to C99.
Concerning the return value of snprintf(), SUSv2 and C99 contradict each other:
when snprintf() is called with size=0 then SUSv2 stipulates an unspecified return
value less than 1, while C99 allows str to be NULL in this case, and gives the
return value (as always) as the number of characters that would have been written
in case the output string has been large enough.
Linux libc4 knows about the five C standard flags. It knows about the length modi-
fiers h,l,L, and the conversions cdeEfFgGinopsuxX, where F is a synonym for f.
Additionally, it accepts D,O,U as synonyms for ld,lo,lu. (This is bad, and caused
serious bugs later, when support for %D disappeared.) No locale-dependent radix
character, no thousands' separator, no NaN or infinity, no %m$ and *m$.
Linux libc5 knows about the five C standard flags and the ' flag, locale, %m$ and
*m$. It knows about the length modifiers h,l,L,Z,q, but accepts L and q both for
long doubles and for long long integers (this is a bug). It no longer recognizes
FDOU, but adds the conversion character m, which outputs strerror(errno).
glibc 2.0 adds conversion characters C and S.
glibc 2.1 adds length modifiers hh,j,t,z and conversion characters a,A.
glibc 2.2 adds the conversion character F with C99 semantics, and the flag charac-
ter I.
HISTORY
Unix V7 defines the three routines printf(), fprintf(), sprintf(), and has the flag
-, the width or precision *, the length modifier l, and the conversions doxfegcsu,
and also D,O,U,X as synonyms for ld,lo,lu,lx. This is still true for 2.9.1BSD, but
2.10BSD has the flags #, + and <space> and no longer mentions D,O,U,X. 2.11BSD has
vprintf(), vfprintf(), vsprintf(), and warns not to use D,O,U,X. 4.3BSD Reno has
the flag 0, the length modifiers h and L, and the conversions n, p, E, G, X (with
current meaning) and deprecates D,O,U. 4.4BSD introduces the functions snprintf()
and vsnprintf(), and the length modifier q. FreeBSD also has functions asprintf()
and vasprintf(), that allocate a buffer large enough for sprintf(). In glibc there
are functions dprintf() and vdprintf() that print to a file descriptor instead of a
stream.
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 pre-
dict. Use snprintf() and vsnprintf() instead (or asprintf() and vasprintf).
Linux libc4.[45] does not have a snprintf(), but provides a libbsd that contains an
snprintf() equivalent to sprintf(), i.e., one that ignores the size argument.
Thus, the use of snprintf() with early libc4 leads to serious security problems.
Code such as printf(foo); often indicates a bug, since foo may contain a % charac-
ter. If foo comes from untrusted user input, it may contain %n, causing the
printf() call to write to memory and creating a security hole.
SEE ALSO
printf(1), asprintf(3), dprintf(3), scanf(3), setlocale(3), wcrtomb(3), wprintf(3),
locale(5)
Linux Manpage 2000-10-16 PRINTF(3)
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