NAME¶

std::vprintf,std::vfprintf,std::vsprintf,std::vsnprintf - std::vprintf,std::vfprintf,std::vsprintf,std::vsnprintf

Synopsis¶

Defined in header <cstdio>
int vprintf( const char* format, va_list vlist ); (1)
int vfprintf( std::FILE* stream, const char* format, va_list vlist (2)
);
int vsprintf( char* buffer, const char* format, va_list vlist ); (3)
int vsnprintf( char* buffer, std::size_t buf_size, const char* (4) (since C++11)
format, va_list vlist );

Loads the data from the locations, defined by vlist, converts them to character
string equivalents and writes the results to a variety of sinks.

1) Writes the results to stdout.
2) Writes the results to a file stream stream.
3) Writes the results to a character string buffer.
4) Writes the results to a character string buffer. At most buf_size-1 characters
are written. The resulting character string will be terminated with a null
character, unless buf_size is zero. If buf_size is zero, nothing is written and
buffer may be a null pointer, however the return value (number of bytes that would
be written not including the null terminator) is still calculated and returned.

Parameters¶

stream - output file stream to write to
buffer - pointer to a character string to write to
buf_size - maximum number of characters to write
format - pointer to a null-terminated character string specifying how to interpret
the data
vlist - variable argument list containing the data to print.

The format string consists of ordinary multibyte characters (except %), which are
copied unchanged into the output stream, and conversion specifications. Each
conversion specification has the following format:

* introductory % character

* (optional) one or more flags that modify the behavior of the
conversion:

* -: the result of the conversion is left-justified within
the field (by default it is right-justified)
* +: the sign of signed conversions is always prepended to
the result of the conversion (by default the result is
preceded by minus only when it is negative)
* space: if the result of a signed conversion does not start
with a sign character, or is empty, space is prepended to
the result. It is ignored if + flag is present.
* # : alternative form of the conversion is performed. See
the table below for exact effects otherwise the behavior
is undefined.
* 0 : for integer and floating point number conversions,
leading zeros are used to pad the field instead of space
characters. For integer numbers it is ignored if the
precision is explicitly specified. For other conversions
using this flag results in undefined behavior. It is
ignored if - flag is present.

* (optional) integer value or * that specifies minimum field width. The
result is padded with space characters (by default), if required, on
the left when right-justified, or on the right if left-justified. In
the case when * is used, the width is specified by an additional
argument of type int, which appears before the argument to be converted
and the argument supplying precision if one is supplied. If the value
of the argument is negative, it results with the - flag specified and
positive field width. (Note: This is the minimum width: The value is
never truncated.)

* (optional) . followed by integer number or *, or neither that specifies
precision of the conversion. In the case when * is used, the precision
is specified by an additional argument of type int, which appears
before the argument to be converted, but after the argument supplying
minimum field width if one is supplied. If the value of this argument
is negative, it is ignored. If neither a number nor * is used, the
precision is taken as zero. See the table below for exact effects of
precision.

* (optional) length modifier that specifies the size of the argument (in
combination with the conversion format specifier, it specifies the type
of the corresponding argument)

* conversion format specifier

The following format specifiers are available:

Conversion Explanation Expected Specifier Argument Type
Length hh ll j z t
Modifier→ h (none) l L
(C++11) (C++11) (C++11) (C++11) (C++11)
writes literal %.
The full
% conversion N/A N/A N/A N/A N/A N/A N/A N/A N/A
specification
must be %%.
writes a
single
character.

The argument is
first converted
to unsigned char.
c If the l modifier N/A N/A int wint_t N/A N/A N/A N/A N/A
is used, the
argument is first
converted to a
character string
as if by %ls with
a wchar_t[2]
argument.
writes a
character
string

The argument must
be a pointer to
the initial
element of an
array of
characters.
Precision
specifies the
maximum number of
bytes to be
written. If
Precision is not
s specified, writes N/A N/A char* wchar_t* N/A N/A N/A N/A N/A
every byte up to
and not including
the first null
terminator. If
the l specifier
is used, the
argument must be
a pointer to the
initial element
of an array of
wchar_t, which is
converted to char
array as if by a
call to wcrtomb
with
zero-initialized
conversion state.
converts a
signed integer
into decimal
representation
[-]dddd.

Precision
specifies the
d minimum number of signed long signed
i digits to appear. char short int long long intmax_t size_t ptrdiff_t N/A
The default
precision is 1.
If both the
converted value
and the precision
are 0 the
conversion
results in no
characters.
converts an
unsigned
integer into
octal
representation
oooo.

Precision
specifies the
minimum number of
digits to appear.
The default
precision is 1.
If both the
converted value
and the precision
o are 0 the N/A
conversion
results in no
characters. In
the alternative
implementation
precision is
increased if
necessary, to
write one leading
zero. In that
case if both the
converted value
and the precision
are 0,
single 0 is
written.
converts an
unsigned
integer into
hexadecimal
representation
hhhh.

For the x
conversion
letters abcdef unsigned unsigned unsigned unsigned unsigned unsigned
are used. char short int long long uintmax_t size_t version of
For the X long ptrdiff_t
conversion
letters ABCDEF
are used.
Precision
x specifies the N/A
X minimum number of
digits to appear.
The default
precision is 1.
If both the
converted value
and the precision
are 0 the
conversion
results in no
characters. In
the alternative
implementation 0x
or 0X is prefixed
to results if the
converted value
is nonzero.
converts an
unsigned
integer into
decimal
representation
dddd.

Precision
specifies the
u minimum number of N/A
digits to appear.
The default
precision is 1.
If both the
converted value
and the precision
are 0 the
conversion
results in no
characters.
converts
floating-point
number to the
decimal
notation in
the style
[-]ddd.ddd.

Precision
specifies the
exact number of
digits to appear
f after the decimal
F point character. N/A N/A N/A N/A N/A N/A
The default
precision is 6.
In the
alternative
implementation
decimal point
character is
written even if
no digits follow
it. For infinity
and not-a-number
conversion style
see notes.
converts
floating-point
number to the
decimal
exponent
notation.

For the e
conversion style
[-]d.ddde±dd is
used.
For the E
conversion style
[-]d.dddE±dd is
used.
The exponent
contains at least
two digits, more
digits are used
only if
e necessary. If the
E value is 0, N/A N/A N/A N/A N/A N/A
the exponent is
also 0.
Precision
specifies the
exact number of
digits to appear
after the decimal
point character.
The default
precision is 6.
In the
alternative
implementation
decimal point
character is
written even if
no digits follow
it. For infinity
and not-a-number
conversion style
see notes.
converts
floating-point
number to the
hexadecimal
exponent
notation.

For the a
conversion style
[-]0xh.hhhp±d is
used.
For the A
conversion style
[-]0Xh.hhhP±d is
used.
The first
hexadecimal digit
is not 0 if the
argument is a
normalized double double long
floating point (C++11) double
a value. If the
A value is 0,
the exponent is N/A N/A N/A N/A N/A N/A
(C++11) also 0.
Precision
specifies the
exact number of
digits to appear
after the
hexadecimal point
character. The
default precision
is sufficient for
exact
representation of
the value. In the
alternative
implementation
decimal point
character is
written even if
no digits follow
it. For infinity
and not-a-number
conversion style
see notes.
converts
floating-point
number to
decimal or
decimal
exponent
notation
depending on
the value and
the precision.

For the g
conversion style
conversion with
style e or f will
be performed.
For the G
conversion style
conversion with
style E or F will
be performed.
Let P equal the
precision if
nonzero, 6 if the
precision is not
specified, or 1
if the precision
is 0. Then,
if a conversion
with style E
g would have an
G exponent of X: N/A N/A N/A N/A N/A N/A

* if P > X ≥
−4, the
conversion is
with style f
or F and
precision P
− 1 − X.
* otherwise,
the
conversion is
with style e
or E and
precision P
− 1.

Unless
alternative
representation is
requested the
trailing zeros
are removed, also
the decimal point
character is
removed if no
fractional part
is left. For
infinity and
not-a-number
conversion style
see notes.
returns the
number of
characters
written so far
by this call
to the
function.
signed long signed
n The result is char* short* int* long* long* intmax_t* size_t* ptrdiff_t* N/A
written to the
value pointed to
by the argument.
The specification
may not contain
any flag, field
width, or
precision.
writes an
implementation
p defined character N/A N/A void* N/A N/A N/A N/A N/A N/A
sequence defining
a pointer.

The floating point conversion functions convert infinity to inf or infinity. Which
one is used is implementation defined.

Not-a-number is converted to nan or nan(char_sequence). Which one is used is
implementation defined.

The conversions F, E, G, A output INF, INFINITY, NAN instead.

Even though %c expects int argument, it is safe to pass a char because of the
integer promotion that takes place when a variadic function is called.

The correct conversion specifications for the fixed-width character types (int8_t,
etc) are defined in the header <cinttypes> (although PRIdMAX, PRIuMAX, etc is
synonymous with %jd, %ju, etc).

The memory-writing conversion specifier %n is a common target of security exploits
where format strings depend on user input and is not supported by the bounds-checked
printf_s family of functions.

There is a sequence point after the action of each conversion specifier; this
permits storing multiple %n results in the same variable or, as an edge case,
printing a string modified by an earlier %n within the same call.

If a conversion specification is invalid, the behavior is undefined.

Return value¶

1-3) Number of characters written if successful or negative value if an error
occurred.
4) Number of characters written if successful or negative value if an error
occurred. If the resulting string gets truncated due to buf_size limit, function
returns the total number of characters (not including the terminating null-byte)
which would have been written, if the limit was not imposed.

Notes¶

All these functions invoke va_arg at least once, the value of arg is indeterminate
after the return. These functions do not invoke va_end, and it must be done by the
caller.

Example¶

// Run this code

#include <vector>
#include <cstdio>
#include <cstdarg>
#include <ctime>

void debug_log(const char *fmt, ...)
{
std::time_t t = std::time(nullptr);
char time_buf[100];
std::strftime(time_buf, sizeof time_buf, "%D %T", std::gmtime(&t));
va_list args1;
va_start(args1, fmt);
va_list args2;
va_copy(args2, args1);
std::vector<char> buf(1+std::vsnprintf(nullptr, 0, fmt, args1));
va_end(args1);
std::vsnprintf(buf.data(), buf.size(), fmt, args2);
va_end(args2);
std::printf("%s [debug]: %s\n", time_buf, buf.data());
}

int main()
{
debug_log("Logging, %d, %d, %d", 1, 2, 3);
}

Output:¶

04/13/15 15:09:18 [debug]: Logging, 1, 2, 3

See also¶

printf
fprintf prints formatted output to stdout, a file stream or a buffer
sprintf (function)
snprintf
(C++11)
vscanf
vfscanf reads formatted input from stdin, a file stream or a buffer
vsscanf using variable argument list
(C++11) (function)
(C++11)
(C++11)