A simple ANSI-C I/O demonstration. Reads a username and password from stdin, performs tab/newline replacement on the username/password.

username-pass-io.c

/*
  ANSI-C C90: -std=c99 -c -Wall -ansi -pedantic-errors -fstack-protector-all -Werror -Wshadow
  
  Build: gcc username-pass-io.c -o io.o
*/

/*
  Produced 2020
  By https://amattu.com/links/github
  Copy Alec M.
  License GNU Affero General Public License v3.0
*/

/*
  Description:
  - Read a username and password from stdin
  - Perform simple whitespace replacement
  - Display username and password
*/

/* Files */
#include <stdio.h>
#include <string.h>
#include <stdlib.h>

/* Input Config */
#define MAX_USER_LEN 25
#define MAX_PASS_LEN 45

/* Function prototypes */
static char* trim_whitespace(char *input);

/* Main Program */
int main() {
  /* Variables */
  char *username = NULL,
    *password = NULL;

  /* Allocate Variable Spacing */
  if (!(username = malloc(MAX_USER_LEN)))
    return 1;
  if (!(password = malloc(MAX_PASS_LEN)))
    return 1;

  /* Read Username */
  while (!username || strlen(username) <= 0 || strlen(username) > MAX_USER_LEN) {
    printf("Please enter your username: ");
    fgets(username, MAX_USER_LEN, stdin);
    username = trim_whitespace(username);
  }

  /* Read Password */
  while (!password || strlen(password) <= 0 || strlen(password) > MAX_PASS_LEN) {
    printf("Please enter your password: ");
    fgets(password, MAX_PASS_LEN, stdin);
    password = trim_whitespace(password);
  }

  /* Display results */
  printf("\n---------->[ Results ]<----------\n");
  printf("Username [%03d]: %s\nPassword [%03d]: %s\n", (int) strlen(username),
    username, (int) strlen(password), password);

  /* Default */
  return 0;
}

/* Remove tabs/newlines from char string
  returns NULL on errors */
static char* trim_whitespace(char *input) {
  /* Variables */
  char *output;
  int i = 0;
  int nsi = 0;

  if (!(output = malloc(strlen(input))))
    return NULL;

  /* Loops */
  while (input[i]) {
    if (input[i] != '\t' && input[i] != '\n')
      output[nsi++] = input[i];
    i++;
  }

  /* Return */
  return output;
}