fprintf
PRINTF(3) Linux Programmer's Manual PRINTF(3)
NAME
printf, fprintf, dprintf, sprintf, snprintf, vprintf, vfprintf, vd-
printf, vsprintf, vsnprintf - formatted output conversion
SYNOPSIS
#include <stdio.h>
int printf(const char *format, ...);
int fprintf(FILE *stream, const char *format, ...);
int dprintf(int fd, 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 vdprintf(int fd, 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);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
snprintf(), vsnprintf():
_XOPEN_SOURCE >= 500 || _ISOC99_SOURCE ||
|| /* Glibc versions <= 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 for-
mat 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 stream.
The functions snprintf() and vsnprintf() write at most size bytes (in-
cluding the terminating null byte ('\0')) to str.
The functions vprintf(), vfprintf(), vdprintf(), vsprintf(), vs-
nprintf() 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 copy-
ing 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 un-
changed to the output stream; and conversion specifications, each of
which results in fetching zero or more subsequent arguments. Each con-
version 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 '*' (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 explic-
itly which argument is taken, at each place where an argument is re-
quired, 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 argu-
ment 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 argu-
ments 1 and 3 are specified, argument 2 must also be specified some-
where in the format string.
For some numeric conversions a radix character ("decimal point") or
thousands' grouping character is used. The actual character used de-
pends 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 bound-
ary. (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 infor-
mation indicates any. (See setlocale(3).) Note that many ver-
sions of gcc(1) cannot parse this option and will issue a warn-
ing. (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 lo-
cale'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 deci-
mal 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, re-
spectively, 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 con-
versions, 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 int or un-
signed short int argument, or a following n conversion corre-
sponds 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 cor-
responds to a pointer to a long int 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 int or unsigned long long int argument, or a following n
conversion corresponds to a pointer to a long long int 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 argu-
ment, 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 compli-
ant 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) nota-
tion. The letters abcdef are used for x conversions; the let-
ters ABCDEF are used for X conversions. The precision, if any,
gives the minimum number of digits that must appear; if the con-
verted 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 pre-
cision 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 expo-
nent 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 rep-
resentations 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 "[-]INFIN-
ITY" 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 signifi-
cant 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+-; 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 preci-
sion. The default precision suffices for an exact representa-
tion 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.
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 ex-
pected 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 preci-
sion 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 ex-
pected 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 char-
acter), 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 speci-
fied, no more bytes than the number specified are written, but
no partial multibyte characters are written. Note that the pre-
cision 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 be-
fore 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 inte-
ger pointed to by the corresponding argument. That argument
shall be an int *, or variant whose size matches the (option-
ally) supplied integer length modifier. No argument is con-
verted. (This specifier is not supported by the bionic C li-
brary.) The behavior is undefined if the conversion specifica-
tion 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 con-
version 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.
ATTRIBUTES
For an explanation of the terms used in this section, see at-
tributes(7).
+------------------------+---------------+----------------+
|Interface | Attribute | Value |
+------------------------+---------------+----------------+
|printf(), fprintf(), | Thread safety | MT-Safe locale |
|sprintf(), snprintf(), | | |
|vprintf(), vfprintf(), | | |
|vsprintf(), vsnprintf() | | |
+------------------------+---------------+----------------+
CONFORMING TO
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 specifica-
tion of snprintf() with C99.
glibc 2.1 adds length modifiers hh, j, t, and z and conversion charac-
ters a and A.
glibc 2.2 adds the conversion character F with C99 semantics, and the
flag character I.
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 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
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 vs-
nprintf() 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 secu-
rity hole.
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 international-
ized 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 size = 0;
char *p = NULL;
va_list ap;
/* Determine required size */
va_start(ap, fmt);
size = vsnprintf(p, size, fmt, ap);
va_end(ap);
if (size < 0)
return NULL;
size++; /* For '\0' */
p = malloc(size);
if (p == NULL)
return NULL;
va_start(ap, fmt);
size = vsnprintf(p, size, fmt, ap);
va_end(ap);
if (size < 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
printf(1), asprintf(3), puts(3), scanf(3), setlocale(3), strfromd(3),
wcrtomb(3), wprintf(3), locale(5)
COLOPHON
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