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MungaSoftwiz / printf

This repository is a group project for the recreation of the C standard library printf function. It was done as part of the ALX Africa curriculum facilitated by Holberton School.

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_printf

A formatted output conversion C program completed as part of the low-level programming and algorithm track at ALX Africa facilitated by Holberton School. The program is a pseudo-recreation of the C standard library function, printf.

Dependencies

The _printf function was coded on an Ubuntu 20.04 LTS machine with gcc version 9.4.0.

Usage

To use the _printf function, assuming the above dependencies have been installed, compile all .c files in the repository and include the header main.h with any main function.

Example main.c:

#include "main.h"

int main(void)
{
    _printf("Hello, World!");

    return (0);
}

Compilation:

$ gcc *.c -o tester

Output:

$ ./tester
Hello, World!
$

Description

The function _printf writes output to standard output. The function writes under the control of a format string that specifies how subsequent arguments (accessed via the variable-length argument facilities of stdarg) are converted for output.

Prototype: int _printf(const char *format, ...);

Return Value

Upon successful return, _printf returns the number of characters printed (excluding the terminating null byte used to end output to strings). If an output error is encountered, the function returns -1.

Format of the Argument String

The format string argument is a constant character string composed of zero or more directives: ordinary characters (not %) which are copied unchanged to the output stream; and conversion specifications, each of which results in fetching zero or more subsequent arguments. Conversion specification is introduced by the character % and ends with a conversion specifier. In between the % character and conversion specifier, there may be (in order) zero or more flags, an optional minimum field width, an optional precision and an optional length modifier. The arguments must correspond with the conversion specifier, and are used in the order given.

Flag Characters

The character % may be followed by zero or more of the following flags:

  • For o conversions, the first character of the output string is prefixed with 0 if it was not zero already.
  • For x converions, 0x is prepended for non-zero numbers.
  • For X conversions, 0X is prepeneded for non-zero numbers.

Example main.c:

int main(void)
{
    _printf("%#x\n", 7);
}

Output:

0x7

(space)

  • A blank is left before a positive number or empty string produced by a signed conversion.

Example main.c:

int main(void)
{
    _printf("% d\n", 7);
}

Output:

 7

+

  • A sign (+ or -) is always placed before a number produced by signed conversion.
  • Overrides a space flag.

Example main.c:

int main(void)
{
    _printf("%+d\n", 7);
}

Output:

+7

0

  • For d, i, o, u, x, and X conversions, the converted value is padded on the left with zeroes rather than blanks.
  • If the 0 flag is provided to a numeric conversion with a specified precision, it is ignored.

Example main.c:

int main(void)
{
    _printf("%05d\n", 7);
}

Output:

00007

-

  • The converted value is left-justified (padded on the right with blanks instead of on the left with blanks or zeroes).
  • Overrides a 0 flag.

Example main.c:

int main(void)
{
    _printf("%-5d7\n", 7);
}

Output:

7    7

Field Width

After flags, a minimum field width may be specified by a decimal digit string The first digit must be non-zero. If the converted value has fewer characters than the provided width, the output is padded on the left or right with spaces (depending on whether the - flag was provided).

Example main.c:

int main(void)
{
    _printf("%7d\n", 7);
}

Ouptut:

      7

Alternatively, width may be provied as an argument using the * character For example, in the following: _printf("%*d\n", 9, 1); the argument 9 is considered the width for the conversion of the decimal 1.

Precision

After any flags or provided width, a precision may be specified by a . followed by a decimal digit string. For d, i, o, u, x, and X conversions, the precision specifies the minimum number of digits to appear. For s and S conversions, the precision specifies the maximum characters to be printed.

Example main.c:

int main(void)
{
    _printf("%.7d\n", 7);
}

Output:

0000007

Alternatively, precision may be provided as an argument using the * character after the .. For example, in the following: _printf("%.*d\n", 9, 1); the argument 9 is considered the precision for the conversion of the decimal 1.

Length Modifiers

After flags, width, and precision and before a conversion specifier, one of the following length modifiers may be provided:

h

Specifies that an integer conversion corresponds to a short int or unsigned short int argument.

Example main.c:

int main(void)
{
    _printf("%hd\n", SHRT_MAX);
}

Output:

32767

l

Specifies that an integer conversion corresponds to a long int or unsigned long int argument.

Example main.c:

int main(void)
{
    _printf("%ld\n", LONG_MAX);
}

Output:

91122337235554775807

Conversion Specifiers

The conversion specifier (introduced by the character %) is a character that specifies the type of conversion to be applied. The _printf function supports the following conversion specifiers:

d, i

The int argument is converted to signed decimal notation.

Example main.c:

int main(void)
{
    _printf("%d\n", 9);
}

Output:

9

b

The unsigned int argument is converted to signed decimal notation.

Example main.c:

int main(void)
{
    _printf("%b\n", 7);
}

Output:

111

o, u, x, X

The unsigned int argument is converted to unsigned octal (o), unsigned decimal (u), or unsigned hexadecimal (x and X). The letters abcdef are used for x conversions and the letters ABCDEF are used for X conversions.

Example main.c:

int main(void)
{
    _printf("%o\n", 77);
}

Output:

115

c

The int argument is converted to an unsigned char.

Example main.c:

int main(void)
{
    _printf("%c\n", 48);
}

Output:

0

s

The const char * argument is expected to be a pointer to a character array (aka. pointer to a string). Characters from the array are written starting from the first element of the array and ending at, but not including, the terminating null byte (\0).

Example main.c:

int main(void)
{
    _printf("%s\n", "Hello, World!");
}

Output:

Hello, World!

S

Identical to the s conversion specifier, except any non-printable characters in the array (ie. characters with an ASCII value < 32 or >= 127) are written as \x followed by the ASCII code value in hexadecimal (upper case, two characters).

Example main.c:

int main(void)
{
    _printf("%S\n", "Hello, World! Π");
}

Output:

Hello, World! \x0FFFFFFFFFFFFFFCE\x0FFFFFFFFFFFFFFA0

r Identical to the s conversion specifier, except characters from the array are written in reverse, starting from, but not including, the terminating null byte (\0) and ending at the first element of the array.

Example main.c:

int main(void)
{
    _printf("r\n", "Hello, World");
}

Output:

dlroW ,olleH

R

Identical to the s conversion specifier, except each character of the array is converted to its corresponding character in ROT13 before being written.

Example main.c:

int main(void)
{
    _printf("%R\n", "Hello, World");
}

Output:

Uryyb, Jbeyq

p

The address of the argument is written. The address is written in hexadecimal with a leading 0x.

Example main.c:

int main(void)
{
    char *str = "Hello, World";

    _printf("%p\n", (void *)str);
}

Output:

0x561a6d7bab5d

%

A % is written. No argument is converted. The complete conversion specification is %%.

Example:

int main(void)
{
    _printf("%%\n");
}

Output:

%

Authors

Acknowledgements

The _printf function emulates functionality of the C standard library function printf. This README borrows from the Linux man page printf(3).

All work contained in this repository was completed as part of the software engineering curriculum at ALX Africa. ALX Africa focuses on preparing students for tech roles using projects based peer learning. For more information, visit this link

About

This repository is a group project for the recreation of the C standard library printf function. It was done as part of the ALX Africa curriculum facilitated by Holberton School.

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