Ungetc() Not Ungetting Character

Question

My code is extremely simple, but I figure that's best to figure out what's going on in my program. For some reason, I can't seem to get ungetc() to work properly, nor can I figure out it's purpose. Normally, ungetc() should return a character back into the stream or, at least, I think it should. For example, if I type ABC, getting three characters through getchar(), I would expect ungetc() to return one of the letters getchar() had returned. If I later call putchar(), I would expect B to be returned, since C was tossed back into the stream, but C was returned. I've tried both getchar() as well as getc() and run into the same issue.

Condensed version, I typed "ABC" into my program, expecting ungetc() to cause the putc() to return 'B', but all I get is C again. Why is this happening?

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
int main(void)
{
    char c;
    c=getchar();
    c=getchar();
    c=getchar();
    ungetc(c, stdout);
    putchar(c);
}

ungetc is an INPUT function. Use ungetc(c, stdin);. BTW, all that does is put c back into the input buffer, so the next getchar will get that again. It doesn't change c. — Tim Roberts
– Tim Roberts, Commented Aug 21, 2024 at 3:00
Thanks for the response, it was very helpful. I tested ungetchar() by using it before each c=getchar() and it seems work. If I don't use unget() char, it cycles through the input stream. If I use unget() char after each getchar(), it stays on A, meaning A (or the first character) is going back on the buffer. — Ratdude
– Ratdude, Commented Aug 21, 2024 at 21:36

score 3 · Accepted Answer · 2024-08-21 03:52:14Z

3

Removed the two unused headers.
The type of the variable c should be int not char to match the functions used. It serves no purpose to assign values to c if they are not used. That said you can eliminate the variable entirely (point-free style).
The program segfaults as it should be push c to stdin instead of stdout.
Call getchar() after the ungetc() to demonstrate the property of interest otherwise you just print the previous read value.

#include <stdio.h>

int main(void) {
    getchar();
    getchar();
    ungetc(getchar(), stdin);
    putchar(getchar());
}

and example run:

ABC
C

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