Exercise 4.7 - Function ungets that will push back an entire string onto the input

Question

Write a routine ungets(s) that will push back an entire string onto the input. Should ungets know about buf and bufp, or should it just use ungetch?

/* Write a routine ungets(s) that will push back an entire string onto the input.
    Should ungets(s) know about buf and bufp or should it handle it to ungetch()
*/

#include<stdio.h>
#include<string.h>

#define MAXBUF 100
#define MAXLINE 100

int bufp=0;
int buf[MAXBUF];

int getch(void); 
void ungetch(int c);
void ungets(char s[]);
int mgetline(char line[],int maxline);


int main(void)
{
    char line[MAXLINE];
    int c;

    mgetline(line,MAXLINE);

    ungets(line);

    while((c=getch()) != EOF)
        putchar(c);
    
    return 0;
}

int mgetline(char s[],int lim)
{
    int i,c;
    
    for(i=0;i<lim-1 && (c=getchar())!=EOF && c!='\n';++i)
        s[i]=c;

    if(c=='\n')
        s[i++]=c;

    s[i]='\0';
}

void ungets(char s[])
{
    int i;
    
    i = strlen(s);


    while(i>0)
        ungetch(s[--i]);
}

void ungetch(int c)
{
    if(bufp >= MAXBUF)
        printf("ungetch: too many characters\n");
    else
        buf[bufp++]=c;
}

int getch(void)
{
    return (bufp > 0)?buf[--bufp]:getchar();
}

Run this

Explanation

This program defines ungets(s), which takes a string as an input and and removes one character at a time from the back of the string and puts them into a the buffer BUF. It does this, till all the characters from the input string are placed onto the buffer. It uses the function ungetch to place to the buffer.

When getch() is called, the characters from the buffer are read first and it is output on the screen.

So, when we write something like this.

$ ./a.out
this is a sentence
this is a sentence

The first sentence is read as input and placed in the BUF and the next sentence is read using getch() from the BUF array.

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