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, std::va_list vlist ); (1)
int vfprintf( std::FILE* stream, const char* format, std::va_list (2)
vlist );
int vsprintf( char* buffer, const char* format, std::va_list vlist (3)
);
int vsnprintf( char* buffer, std::size_t buf_size, const char* (4) (since C++11)
format, std::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 byte 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. If the l
c modifier is N/A N/A int wint_t N/A N/A N/A N/A N/A
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
specified,
writes every
s byte up to and N/A N/A char* wchar_t* N/A N/A N/A N/A N/A
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
minimum number
of digits to
d appear. The signed short int long long intmax_t signed ptrdiff_t N/A
i default char long size_t
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 are 0
o the conversion N/A
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
are used. unsigned unsigned
For the X unsigned unsigned unsigned unsigned long long uintmax_t size_t version of
conversion char short int long ptrdiff_t
letters ABCDEF
are used.
Precision
specifies the
minimum number
x of digits to N/A
X 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
minimum number
of digits to
u appear. The N/A
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
after the
f decimal point
F character. The N/A N/A N/A N/A N/A N/A
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
necessary. If
e the value is 0, N/A N/A N/A N/A N/A N/A
E 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
floating-point
value. If the double double(C++11) long
a value is 0, the double
A exponent is also N/A N/A N/A N/A N/A N/A
0. Precision
(C++11) 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 would
g have an exponent
G 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.

The result is signed long signed
n written to the char* short* int* long* long* intmax_t* size_t* ptrdiff_t* N/A
value pointed to
by the argument.
The
specification
may not contain
any flag, field
width, or
precision.
Writes an
implementation
defined
p 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 <cstdarg>
#include <cstdio>
#include <ctime>
#include <vector>

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));
std::va_list args1;
va_start(args1, fmt);
std::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
sprintf buffer
snprintf (function)
(C++11)
vscanf
vfscanf reads formatted input from stdin, a file stream or a
vsscanf buffer
(C++11) using variable argument list
(C++11) (function)
(C++11)
vprint_unicode prints to Unicode capable stdout or a file stream using
vprint_unicode_locking type-erased argument representation
(C++23) (function)
(C++23)
vprint_nonunicode prints to stdout or a file stream using type-erased
vprint_nonunicode_locking argument representation
(C++23) (function)
(C++23)
C documentation for
vprintf,
vfprintf,
vsprintf,
vsnprintf