printf, fprintf, sprintf---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, ...]);

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 and sprintf are identical to printf, other than the destination of the formatted output: fprintf sends the output to a specified file fd, while sprintf stores the output in the specified char array str. For sprintf, the behavior is also undefined if the output *str overlaps with one of the arguments. 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. 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

  s

  prints characters until precision is reached or a null terminator is encountered; takes a string 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 returns 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. 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.

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