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### 4.45 `printf`, `fprintf`, `asprintf`, `sprintf`, `snprintf`—format output

Synopsis

         #include <stdio.h>

         int printf(const char *format [, arg, ...]);
         int fprintf(FILE *fd, const char *format [, arg, ...]);
         int sprintf(char *str, const char *format [, arg, ...]);
         int asprintf(char **strp, const char *format [, arg, ...]);
         int snprintf(char *str, size_t size, const char *format
             [, arg, ...]);

Description
`printf` accepts a series of arguments, applies to each a format specifier from `*`format, and writes the formatted data to `stdout`, terminated with a null character. The behavior of `printf` is undefined if there are not enough arguments for the format. `printf` returns when it reaches the end of the format string. If there are more arguments than the format requires, excess arguments are ignored.

`fprintf`, `asprintf`, `sprintf` and `snprintf` are identical to `printf`, other than the destination of the formatted output: `fprintf` sends the output to a specified file fd, while `asprintf` stores the output in a dynamically allocated buffer, while `sprintf` stores the output in the specified char array str and `snprintf` limits number of characters written to str to at most size (including terminating `0`). For `sprintf` and `snprintf`, the behavior is undefined if the output `*`str overlaps with one of the arguments. For `asprintf`, strp points to a pointer to char which is filled in with the dynamically allocated buffer. format is a pointer to a charater string containing two types of objects: ordinary characters (other than `%`), which are copied unchanged to the output, and conversion specifications, each of which is introduced by `%`. (To include `%` in the output, use `%%` in the format string.) A conversion specification has the following form:

                %[flags][width][.prec][size][type]

The fields of the conversion specification have the following meanings:

- flags

  an optional sequence of characters which control output justification, numeric signs, decimal points, trailing zeroes, and octal and hex prefixes. The flag characters are minus (`-`), plus (`+`), space ( ), zero (`0`), and sharp (`#`). They can appear in any combination.

  `-`
    The result of the conversion is left justified, and the right is padded with blanks. If you do not use this flag, the result is right justified, and padded on the left.
  `+`
    The result of a signed conversion (as determined by type) will always begin with a plus or minus sign. (If you do not use this flag, positive values do not begin with a plus sign.)
  `" " (space)`
    If the first character of a signed conversion specification is not a sign, or if a signed conversion results in no characters, the result will begin with a space. If the space ( ) flag and the plus (`+`) flag both appear, the space flag is ignored.
  `0`
    If the type character is `d`, `i`, `o`, `u`, `x`, `X`, `e`, `E`, `f`, `g`, or `G`: leading zeroes, are used to pad the field width (following any indication of sign or base); no spaces are used for padding. If the zero (`0`) and minus (`-`) flags both appear, the zero (`0`) flag will be ignored. For `d`, `i`, `o`, `u`, `x`, and `X` conversions, if a precision prec is specified, the zero (`0`) flag is ignored. Note that `0` is interpreted as a flag, not as the beginning of a field width.
  `#`
    The result is to be converted to an alternative form, according to the next character:
    `0`
      increases precision to force the first digit of the result to be a zero.
    `x`
      a non-zero result will have a `0x` prefix.
    `X`
      a non-zero result will have a `0X` prefix.
    `e, E or f`
      The result will always contain a decimal point even if no digits follow the point. (Normally, a decimal point appears only if a digit follows it.) Trailing zeroes are removed.
    `g or G`
      same as `e` or `E`, but trailing zeroes are not removed.
    `all others`
      undefined.
- width

  width is an optional minimum field width. You can either specify it directly as a decimal integer, or indirectly by using instead an asterisk (`*`), in which case an `int` argument is used as the field width. Negative field widths are not supported; if you attempt to specify a negative field width, it is interpreted as a minus (`-`) flag followed by a positive field width.

- prec

  an optional field; if present, it is introduced with ``.`' (a period). This field gives the maximum number of characters to print in a conversion; the minimum number of digits of an integer to print, for conversions with type `d`, `i`, `o`, `u`, `x`, and `X`; the maximum number of significant digits, for the `g` and `G` conversions; or the number of digits to print after the decimal point, for `e`, `E`, and `f` conversions. You can specify the precision either directly as a decimal integer or indirectly by using an asterisk (`*`), in which case an `int` argument is used as the precision. Supplying a negative precision is equivalent to omitting the precision. If only a period is specified the precision is zero. If a precision appears with any other conversion type than those listed here, the behavior is undefined.

- size

  `h`, `l`, and `L` are optional size characters which override the default way that `printf` interprets the data type of the corresponding argument. `h` forces the following `d`, `i`, `o`, `u`, `x` or `X` conversion type to apply to a `short` or `unsigned short`. `h` also forces a following `n` type to apply to a pointer to a `short`. Similarily, an `l` forces the following `d`, `i`, `o`, `u`, `x` or `X` conversion type to apply to a `long` or `unsigned long`. `l` also forces a following `n` type to apply to a pointer to a `long`. `l` with `c`, `s` is equivalent to `C`, `S` respectively. If an `h` or an `l` appears with another conversion specifier, the behavior is undefined. `L` forces a following `e`, `E`, `f`, `g` or `G` conversion type to apply to a `long double` argument. If `L` appears with any other conversion type, the behavior is undefined.

- type

  type specifies what kind of conversion `printf` performs. Here is a table of these:

  `%`
    prints the percent character (`%`)
  `c`
    prints arg as single character
  `C`
    prints wchar_t arg as single multibyte character
  `s`
    prints characters until precision is reached or a null terminator is encountered; takes a string pointer
  `S`
    converts wchar_t characters to multibyte output characters until precision is reached or a null wchar_t terminator is encountered; takes a wchar_t pointer
  `d`
    prints a signed decimal integer; takes an `int` (same as `i`)
  `i`
    prints a signed decimal integer; takes an `int` (same as `d`)
  `o`
    prints a signed octal integer; takes an `int`
  `u`
    prints an unsigned decimal integer; takes an `int`
  `x`
    prints an unsigned hexadecimal integer (using `abcdef` as digits beyond `9`); takes an `int`
  `X`
    prints an unsigned hexadecimal integer (using `ABCDEF` as digits beyond `9`); takes an `int`
  `f`
    prints a signed value of the form `[-]9999.9999`; takes a floating-point number
  `e`
    prints a signed value of the form `[-]9.9999e[+|-]999`; takes a floating-point number
  `E`
    prints the same way as `e`, but using `E` to introduce the exponent; takes a floating-point number
  `g`
    prints a signed value in either `f` or `e` form, based on given value and precision—trailing zeros and the decimal point are printed only if necessary; takes a floating-point number
  `G`
    prints the same way as `g`, but using `E` for the exponent if an exponent is needed; takes a floating-point number
  `n`
    stores (in the same object) a count of the characters written; takes a pointer to `int`
  `p`
    prints a pointer in an implementation-defined format. This implementation treats the pointer as an `unsigned long` (same as `Lu`).

Returns
`sprintf` and `asprintf` return the number of bytes in the output string, save that the concluding `NULL` is not counted. `printf` and `fprintf` return the number of characters transmitted. If an error occurs, `printf` and `fprintf` return `EOF` and `asprintf` returns -1. No error returns occur for `sprintf`.

Portability
The ANSI C standard specifies that implementations must support at least formatted output of up to 509 characters.

Supporting OS subroutines required: `close`, `fstat`, `isatty`, `lseek`, `read`, `sbrk`, `write`.