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<cstdio>
int vprintf ( const char * format, va_list arg );
Print formatted variable argument list to stdout
Writes to stdout the contents of the format string, expanding the format tags with the value of the argument list arg.
This function behaves exactly as printf except that the variable argument list is passed as a va_list instead of a succession of arguments, which becomes specially useful when the argument list to be passed comes itself from a variable argument list in the calling function.
vprintf does not automatically call the va_end macro.
Parameters
- format
- String that contains the text to be written to stdout.
It can optionally contain embedded format tags that are substituted by the values specified in the arg variable argument list and formatted as requested.
The format tags follow this prototype:
%[flags][width][.precision][length]specifier
Where specifier is the most significant one and defines the type and the interpretation of the value of the coresponding argument:
| specifier | Output | Example |
|---|
| c | Character | a |
| d or i | Signed decimal integer | 392 |
| e | Scientific notation (mantise/exponent) using e character | 3.9265e+2 |
| E | Scientific notation (mantise/exponent) using E character | 3.9265E+2 |
| f | Decimal floating point | 392.65 |
| g | Use the shorter of %e or %f | 392.65 |
| G | Use the shorter of %E or %f | 392.65 |
| o | Signed octal | 610 |
| s | String of characters | sample |
| u | Unsigned decimal integer | 7235 |
| x | Unsigned hexadecimal integer | 7fa |
| X | Unsigned hexadecimal integer (capital letters) | 7FA |
| p | Pointer address | B800:0000 |
| n | Nothing printed. The argument must be a pointer to a signed int, where the number of characters written so far is stored. | |
| % | A % followed by another % character will write % to stdout. |
The tag can also contain flags, width, .precision and modifiers sub-specifiers, which are optional and follow these specifications:
| flags | description |
|---|
| - | Left-justify within the given field width; Right justification is the default (see width sub-specifier). |
| + | Forces to preceed the result with a plus or minus sign (+ or -) even for positive numbers. By default, only negative numbers are preceded with a - sign. |
| (space) | If no sign is going to be written, a blank space is inserted before the value. |
| # | Used with o, x or X specifiers the value is preceeded with 0, 0x or 0X respectively for values different than zero.
Used with e, E and f, it forces the written output to contain a decimal point even if no digits would follow. By default, if no digits follow, no decimal point is written.
Used with g or G the result is the same as with e or E but trailing zeros are not removed. |
| 0 | Left-pads the number with zeroes (0) instead of spaces, where padding is specified (see width sub-specifier). |
| width | description |
|---|
| (number) | Minimum number of characters to be printed. If the value to be printed is shorter than this number, the result is padded with blank spaces. The value is not truncated even if the result is larger. |
| * | The width is not specified in the format string, but as an additional integer value argument preceding the argument that has to be formatted. |
| .precision | description |
|---|
| .number | For integer specifiers (d, i, o, u, x, X): precision specifies the minimum number of digits to be written. If the value to be written is shorter than this number, the result is padded with leading zeros. The value is not truncated even if the result is longer. A precision of 0 means that no character is written for the value 0.
For e, E and f specifiers: this is the number of digits to be printed after de decimal point.
For g and G specifiers: This is the maximum number of significant digits to be printed.
For s: this is the maximum number of characters to be printed. By default all characters are printed until the ending null character is encountered.
For c type: it has no effect.
When no precision is specified, the default is 1. If the period is specified without an explicit value for precision, 0 is assumed. |
| .* | The precision is not specified in the format string, but as an additional integer value argument preceding the argument that has to be formatted. |
| length | description |
|---|
| h | The argument is interpreted as a short int or unsigned short int (only applies to integer specifiers: i, d, o, u, x and X). |
| l | The argument is interpreted as a long int or unsigned long int for integer specifiers (i, d, o, u, x and X), and as a wide character or wide character string for specifiers c and s. |
| L | The argument is interpreted as a long double (only applies to floating point specifiers: e, E, f, g and G). |
- arg
- An object representing the variable arguments list. It shall have already been initialized by the va_start macro defined in <cstdarg>.
Return Value
On success, the total number of characters written is returned.
On failure, a negative number is returned.
Example
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/* vprintf example */
#include <stdio.h>
#include <stdarg.h>
void WriteFormatted (char * format, ...)
{
va_list args;
va_start (args, format);
vprintf (format, args);
va_end (args);
}
int main ()
{
WriteFormatted ("Call with %d variable argument.\n",1);
WriteFormatted ("Call with %d variable %s.\n",2,"arguments");
return 0;
}
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The example illustrates how the WriteFormatted can be called with a different number of arguments, which are on their turn passed to the vprintf function, showing the following output:
Call with 1 variable argument.
Call with 2 variable arguments.
See also
| vfprintf | Write formatted variable argument list to stream (function) |
| vsprintf | Print formatted variable argument list to string (function) |
| printf | Print formatted data to stdout (function) |
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