PRINTF(3) Linux Programmer's Manual PRINTF(3)
NAME
printf, fprintf, sprintf, snprintf, vprintf, vfprintf, vsprintf, vsnprintf - formatted
output conversion
SYNOPSIS
#include
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
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 facili-
ties 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 trailing '\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 arguments. Each conversion
specification is introduced by the character %, and ends with a conversion specifier. In
between there may be (in this order) zero or more flags, an optional minimum field width,
an optional precision and an optional length modifier.
The arguments must correspond properly (after type promotion) with the conversion speci-
fier. By default, the arguments are used in the order given, where each '*' and each con-
version 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 '$', which comes from the Single Unix Specifica-
tion. If the style using '$' is used, it must be used throughout for all conversions tak-
ing an argument and all width and precision arguments, but it may be mixed with '%%' for-
mats 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 conver-
sions, 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.) 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) pro-
duced by a signed conversion.
+ A sign (+ or -) 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 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 thou-
sands' grouping characters if the locale information indicates any. Note that many
versions of gcc 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 alternative
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 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 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 decimal
digit string. Instead of a decimal digit string one may write '*' or '*m$' (for some dec-
imal 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 min-
imum 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 num-
ber 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 argu-
ment, or a following n conversion corresponds to a pointer to a signed char argu-
ment.
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 argu-
ment.
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 argu-
ment, or a following c conversion corresponds to a wint_t argument, or a following
s conversion corresponds to a pointer to wchar_t argument.
ll (ell-ell). A following integer conversion corresponds to a long long int or
unsigned long long int argument, or a following n conversion corresponds to a
pointer to a long long int argument.
L A following a, A, e, E, f, F, g, or G conversion corresponds to a long double argu-
ment. (C99 allows %LF, but SUSv2 does not.)
q ('quad'. BSD 4.4 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 argument.
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 speci-
fiers 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 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.
(The SUSv2 does not know about F and says that character string representations 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 conver-
sion.)
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) For a conversion, the double argument is converted to hexadeci-
mal notation (using the letters abcdef) in the style [-]0xh.hhhhp?d; for A conver-
sion 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 other-
wise is sufficiently large to distinguish values of type double. The digit before
the decimal point is unspecified for non-normalized numbers, and nonzero but other-
wise unspecified for normalized numbers.
c If no l modifier is present, the int argument is converted to an unsigned char, and
the resulting character is written. If an l modifier is present, the wint_t (wide
character) argument is converted to a multibyte sequence by a call to the wcrtomb
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 NUL character; if a precision is
specified, no more than the number specified are written. If a precision is given,
no null character need be present; if the precision is not specified, or is greater
than the size of the array, the array must contain a terminating NUL character.
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 con-
verted to multibyte characters (each by a call to the wcrtomb function, with a con-
version 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, 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.
% A '%' is written. No argument is converted. The complete conversion specification
is '%%'.
EXAMPLES
To print pi to five decimal places:
#include
#include
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
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
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
#include
#include
char *
make_message(const char *fmt, ...) {
/* Guess we need no more than 100 bytes. */
int n, size = 100;
char *p;
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 ((p = realloc (p, size)) == NULL)
return NULL;
}
}
NOTES
The glibc implementation of the functions snprintf and vsnprintf conforms to the C99 stan-
dard, 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 ANSI
X3.159-1989 (''ANSI C'') and ISO/IEC 9899:1999 (''ISO C99''). The snprintf and vsnprintf
functions conform to ISO/IEC 9899:1999.
Concerning the return value of snprintf, the SUSv2 and the C99 standard 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 modifiers
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 a new con-
version 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 character 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 BSD 2.9.1, but BSD 2.10 has the flags #,
+ and and no longer mentions D,O,U,X. BSD 2.11 has vprintf, vfprintf, vsprintf,
and warns not to use D,O,U,X. BSD 4.3 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. BSD 4.4
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 predict. 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 % 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.
SEE ALSO
printf(1), asprintf(3), dprintf(3), wcrtomb(3), wprintf(3), scanf(3), locale(5)
Linux Manpage 2000-10-16 PRINTF(3)
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