int vprintf( const char* format, va_list vlist );

int vprintf( const char* restrict format, va_list vlist );

int vfprintf( FILE* stream, const char* format, va_list vlist );

int vfprintf( FILE* restrict stream, const char* restrict format,
              va_list vlist );

int vsprintf( char* buffer, const char* format, va_list vlist );

int vsprintf( char* restrict buffer, const char* restrict format,
              va_list vlist );

int vsnprintf( char* restrict buffer, size_t bufsz,
               const char* restrict format, va_list vlist );

int vprintf_s( const char* restrict format, va_list vlist );

int vfprintf_s( FILE* restrict stream, const char* restrict format,
                va_list vlist );

int vsprintf_s( char* restrict buffer, rsize_t bufsz,
                const char* restrict format, va_list vlist );

int vsnprintf_s( char* restrict buffer, rsize_t bufsz,
                 const char* restrict 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.

Parameters

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) 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).
• 0: applicable to floating-point and integer conversions only. Leading zeros are used to pad the field instead of space characters. This flag is ignored if - flag is present. For integer conversions, this flag is ignored if the precision is explicitly specified. For floating-point conversions, this flag is ignored if the value is NaN or infinity. Other conversions using this flag have undefined behavior.
• +: applicable to floating-point and signed integer conversions only. Non-negative numbers are prefixed with a plus character. This flag is ignored by other conversions.
• space: applicable to floating-point and signed integer conversions only. Non-negative numbers are prefixed with a space character. This flag is ignored if + flag is present. This flag is ignored by other conversions.
• #: applicable to floating-point and non-decimal integer conversions only. The alternative form of the conversion is performed. See below for the exact semantics. Other conversions using this flag have undefined behavior.

• (optional) integer value or *: 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, then the - flag is implied and the absolute value used for minimum field width.

• (optional) . followed by non-negative integer number or *: specifies the 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 below for the exact semantics.

• (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.

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

Non-numeric

• %: Writes % character. Has no argument. No modifiers supported, full conversion specification must be %%.
• p: Writes a pointer. Argument is a void*, char*, signed char*, or unsigned char*(since C23). Output format is implementation defined.
• c: Writes a character. Argument type depends on the length modifier:

• (none): Argument is an int, which is converted to unsigned char and copied to the output stream.
• l: Argument is a wint_t, which is converted to a wchar_t, put into a null terminated array, and printed with the semantics of %ls.

• s: Write a string. Argument is a pointer to the initial element of an array of characters. Argument type depends on the length modifier:

• (none): Argument is a char*, signed char*, unsigned char*. Characters are copied to the output stream.
• l: Argument is a wchar_t*. String is converted to a narrow string via wcrtomb (with zero-initialized conversion state), and resulting characters copied to the output stream. For the precision modifier and %n specifier, the number of characters refers to the narrow string, not the converted-from wide string.

The precision specifies the maximum number of characters to write. Characters are copied until the maximum is reached or a null terminator is found. The null terminator does not contribute to the number of characters reported by %n.

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.

Integer

Let SInt and UInt be respective types determined by the length specifier:

• h: short and unsigned short
• (none): int and unsigned
• l: long and unsigned long

Then:

• d/i: Writes a signed integer in decimal representation. Argument is an SInt.
• u: Writes an unsigned integer in decimal representation. Argument is an UInt.
• x/X: Writes an unsigned integer in hexadecimal representation. Argument is an UInt. The alternative form prefixes 0x if the argument is non-zero. If the format specifier is uppercase, the corresponding output will also be uppercase.

• o: Writes an unsigned integer in octal representation. Argument is an UInt. The alternative form increases the precision as much as is needed for the first digit written to be zero.
• n: Returns the number of characters written so far by this call to the function, including minimum field width padding. Argument is an SInt* to write the result to. Only supported modifier is length modifier.

The precision specifies the minimum number of digits to write, leading zeros will be written if necessary. If unspecified, a precision of 1 is used. If precision is 0 and the argument is 0, no digits will be written (the plus or space character is still written if the + or space flag is specified).

The correct conversion specifications for the fixed-width character types (int8_t, etc) are defined in the header <inttypes.h> (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.

Floating-point

The argument type is determined by the length specifier:

• (none): double

• L: long double

Then:

• If the argument is infinity, then inf or infinity is written. Which one is used is implementation defined.
• If the argument is not-a-number, then nan or nan(char_sequence) is written. Which one is used is implementation defined.

Otherwise:

• f/F: Writes a floating-point in decimal notation [-]ddd.ddd. The precision specifies the number of decimal places, trailing zeros will be written if necessary. If unspecified, a precision of 6 is used.
• e/E: Writes a floating-point in decimal exponent notation [-]d.ddde±dd. If the exponent is less than 10, it is written with a leading zero. The precision specifies the number of decimal places, trailing zeros will be written if necessary. If unspecified, a precision of 6 is used.
• g/G: Writes a floating-point in decimal or decimal exponent notation, depending on the argument and precision.

Let P = max(0, precision - 1), or 5 if precision is unspecified.

Let EXP be the exponent written if %e were the specification.

• If -4 ≤ EXP ≤ P, then the argument is written as if the f/F format specifier was used with precision P - EXP.
• Otherwise, the argument is written as if the e/E format specifier was used with precision P.

The alternative form writes a decimal point even if precision is 0. If the format specifier is uppercase, the corresponding output will also be uppercase.

Return value

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.

vsnprintf_s, unlike vsprintf_s, will truncate the result to fit within the array pointed to by buffer.

The implementation of vsnprintf_s in the Microsoft CRT does not conform to the C standard. Microsoft's version has an extra parameter size_t count in third position that contains the maximum amount of characters to be written, not including the null terminator. This parameter is possibly distinct from the buffer size provided via the parameter size_t bufsz.

Example

#include <stdarg.h>
#include <stdio.h>
#include <time.h>

void debug_log(const char* fmt, ...)
{
    struct timespec ts;
    timespec_get(&ts, TIME_UTC);
    char time_buf[100];
    size_t rc = strftime(time_buf, sizeof time_buf, "%D %T", gmtime(&ts.tv_sec));
    snprintf(time_buf + rc, sizeof time_buf - rc, ".%06ld UTC", ts.tv_nsec / 1000);

    va_list args1;
    va_start(args1, fmt);
    va_list args2;
    va_copy(args2, args1);
    char buf[1+vsnprintf(NULL, 0, fmt, args1)];
    va_end(args1);
    vsnprintf(buf, sizeof buf, fmt, args2);
    va_end(args2);

    printf("%s [debug]: %s\n", time_buf, buf);
}

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

02/20/15 21:58:09.072683 UTC [debug]: Logging, 1, 2, 3

References

See also