How to Write Your Own printf() in C?

In this article, we will learn how to implement your own custom printf() function in C language.

The printf() function is present in <stdio.h> header and uses a const char* and variable length arguments as its parameters. Hence we would be using <stdarg.h> header file which provides operations available on variable arguments passed to a function.

Prerequisites: Variadic Functions in C, Strings in C

Problem Statement: Write your own printf() function in C

Example Outputs:

Code 1: myprintf("Hello world %i",404);
Output: Hello world 404.
Code 2: char s[]="Multiverse is real";
        myprintf("%s Hello %.3f",s,2.3);
Output: Multiverse is real Hello 2.30

Algorithm:

1. Create a function int myprintf() with const char* and ‘…’ as its parameters. (‘. . .’ enables the function to receive any number of arguments).
2. Initialize a pointer of type va_list to work with variable arguments. Pass the pointer to va_start.
3. Process the passed string with a delimiter as ‘%’ or ‘\0’, and store it in a token string.
4. For every token starting with ‘%’ check if its next characters are a format specifier defined in C. It can be ‘i’,’s’,’lf’.
5. Pass the token string and value retrieved from va_arg() to fprintf() in order to print the required value to the console.
6. Once the whole string is processed, end the va_arg pointer.
7. Return 0.

Code:

C

// C program to implement your own printf() function
#include <stdarg.h>
#include <stdio.h>

// your own printf
int myprintf(const char* str, ...)
{
    // initializing list pointer
    va_list ptr;
    va_start(ptr, str);

    // char array to store token
    char token[1000];
    // index of where to store the characters of str in
    // token
    int k = 0;

    // parsing the formatted string
    for (int i = 0; str[i] != '\0'; i++) {
        token[k++] = str[i];

        if (str[i + 1] == '%' || str[i + 1] == '\0') {
            token[k] = '\0';
            k = 0;
            if (token[0] != '%') {
                fprintf(
                    stdout, "%s",
                    token); // printing the whole token if
                            // it is not a format specifier
            }
            else {
                int j = 1;
                char ch1 = 0;

                // this loop is required when printing
                // formatted value like 0.2f, when ch1='f'
                // loop ends
                while ((ch1 = token[j++]) < 58) {
                }
                // for integers
                if (ch1 == 'i' || ch1 == 'd' || ch1 == 'u'
                    || ch1 == 'h') {
                    fprintf(stdout, token,
                            va_arg(ptr, int));
                }
                // for characters
                else if (ch1 == 'c') {
                    fprintf(stdout, token,
                            va_arg(ptr, int));
                }
                // for float values
                else if (ch1 == 'f') {
                    fprintf(stdout, token,
                            va_arg(ptr, double));
                }
                else if (ch1 == 'l') {
                    char ch2 = token[2];

                    // for long int
                    if (ch2 == 'u' || ch2 == 'd'
                        || ch2 == 'i') {
                        fprintf(stdout, token,
                                va_arg(ptr, long));
                    }

                    // for double
                    else if (ch2 == 'f') {
                        fprintf(stdout, token,
                                va_arg(ptr, double));
                    }
                }
                else if (ch1 == 'L') {
                    char ch2 = token[2];

                    // for long long int
                    if (ch2 == 'u' || ch2 == 'd'
                        || ch2 == 'i') {
                        fprintf(stdout, token,
                                va_arg(ptr, long long));
                    }

                    // for long double
                    else if (ch2 == 'f') {
                        fprintf(stdout, token,
                                va_arg(ptr, long double));
                    }
                }

                // for strings
                else if (ch1 == 's') {
                    fprintf(stdout, token,
                            va_arg(ptr, char*));
                }

                // print the whole token
                // if no case is matched
                else {
                    fprintf(stdout, "%s", token);
                }
            }
        }
    }

    // ending traversal
    va_end(ptr);
    return 0;
}

// driver code
int main()
{
    myprintf("Integer: %d\nString: %s\nFloat: %0.2f\n", 10,
             "GeeksforGeeks", 12.2548);
    return 0;
}

Output

Integer: 10
String: GeeksforGeeks
Float: 12.25

Time Complexity:

The time complexity of the above code is: O(n * L), where,

• n: length of format specifier. For example, If %0.5f then n=3.
• L: Length of the output.

In most practical cases, the time complexity is O(1).