printf() - Custom Implementation

In this project we will implement a printf() function of the following prototype: int _printf(const char *format, ...);

The function is variadic and thus takes any amount of arguments following the format pointer.

Goals 🎉

• Implement custom printf() function similar to that of the standard library
• Use variadic functions to allow a custom number of arguments
• Parse a format string to identify custom print options

Use & Examples ☕️

Use - General:_printf("format string", var1, var2, ...);

Examples:

• Basic String: _printf("%s World", "Hello");

  • Output: Hello World

• Print integers: _printf("Here is an array element: arr[%d]:%c", 32, arr[32]);

  • Output: Here is an array element arr[32]:A

You can do much more, by combining specifiers like string or integer (full list of specifiers below) with flags, length modifiers, width, and precision.

Return value of _printf() _printf() returns negative one (-1) upon failure, and the number of bytes (as an int) written to standard output upon success.

Format specifiers follow this pattern: %[flags][width][.precision][length]specifier

• Example: _printf("%+.22ld\n", LONG_MAX)
  • Output: +0009223372036854775807

Specifiers 🔤

The specifier character at the end is the most significant component, since it defines the type and the interpretation of its corresponding argument:

Specifier	Output	Examples
d or i	Signed integer	1024, -1024
u	Unsigned integer	1024
o	Unsigned octal	432
x	Unsigned hexadecimal integer	3ca
X	Unsigned hexadecimal integer (uppercase)	3CA
b	Binary Representation of unsigned integer	01010110
p	Pointer address	0x403212
s	String of characters	Hello World
c	Character	y
r	Reversed string of characters	dlroW olleH
R	ROT13 Translation of string	Uryyb
S	String with hex-ascii value replacing special chars	\x0A\x0A
The format specifier can also contain sub-specifiers: flags, width, .precision and modifiers (in that order), which are optional and follow these specifications:

Flags 🏳️🏴

Flag	Description
-	Left-justify the output within the field width that was given; Right justification is the default (see width sub-specifier).
+	Preceeds 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.
0	Left-pads the number with zeroes (0) instead of spaces when padding is specified (see width sub-specifier).

Width 📏

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 📐

.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 s: this is the maximum number of characters to be printed. By default all characters are printed until the ending null character is encountered. 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 Modifiers 🤥

The length sub-specifier modifies the length of the data type. This is a chart showing the types used to interpret the corresponding arguments with and without length specifier (if a different type is used, the proper type promotion or conversion is performed, if allowed

Modifier/Specifier	d & i	u, o, x, X	c	s	p
none	int	unsigned int	int	char pointer	void pointer
h	short int	unsigned short int
l	long int	unsigned long int

All together they allow you to manipulate output for various formatting techniques. Although printf does essentially the same thing, we have replicated it to further deepen our understanding of C.

Files / Directories 📂

Name	Information	Relevant Files
testing	Directory containing files relevant to testing, such as main files.	main.c
holberton.h	Header file containing struct and public function definitions.	*.c
_printf.c	Main printf function file. Setups buffer and calls other functions as the format string is traversed.	char_checkers.c, printf_helper_funcs.c, get_field_funcs.c, buffer_functions.c
_bases.c	Contains generic base calculation functions. Gets size of buffer needed to hold conversion from int[base]->string, and fills the buffer with the proper conversion (seperate functions).	int_spec_funcs.c, get_hex_n_funcs.c, custom_spec_funcs.c
_strings.c	Custom functions for string operations, such as strlen and strcpy.	flag_spec_funcs.c, get_field_funcs.c, custom_spec_funcs.c, char_spec_funcs.c, get_prec_funcs.c, hex_spec_funcs.c
buffer_functions.c	Functions used by _printf.c and printf_helper_funcs.c to create, copy, and print the buffer so only one write call is needed.	_printf.c, printf_helper_funcs.c
get_string_funcs.c	Function that gets the proper string function for a given type specifier	_printf.c
char_checkers.c	Functions that check characters, such as if it is a flag, specifier, alpha, printable or in an array.	print_f.c, custom_spec_funcs.c, _strings.c, get_flag_funcs.c, get_prec_funcs.c
char_spec_funcs.c	Character specific functions returned by get_string_func(). Creates memory and copies variable type into a string usable by buffer. Also deals with insertion of null byte character into buffer directly.	get_string_funcs.c, printf_helper_funcs.c
custom_spec_funcs.c	Custom specifier functions such as reversing a string, or getting binary representation of a number. The strings are returned by get_string_func().	get_string_funcs.c
flag_spec_funcs.c	Contain functions that append various characacters ('+', ' ', '0x', etc.) to strings of valid specifiers.	get_flag_funcs.c, printf_helper_funcs.c
int_spec_funcs.c	Integer specific functions returned by get_string_func(). Creates memory and copies variable type into a string, doing proper base conversion.	_bases.c, get_string_funcs.c
hex_spec_funcs.c	Hexadecimal specific functions returned by get_string_func(). Creates memory and copies variable type into string, doing proper base conversion for hex (16).	_bases.c, get_string_funcs.c

References 📖

• “Printf.” Cplusplus.com, www.cplusplus.com/reference/cstdio/printf/.