ungetc() in C/C++

int ungetc(int char, FILE *stream)

• char: specifies the int promotion of the character to be put back. The value is internally converted to an unsigned char when put back.
• stream: specifies the pointer to a FILE object that identifies an input stream.

• On success, the ungetc() function returns the character ch.
• On failure, EOF is returned without changing the stream.

1. The ungetc() function pushes the byte specified by char (converted to an unsigned char) back onto the input stream pointed to by stream.
2. The pushed-back bytes is returned by subsequent reads on that stream in the reverse order of their pushing.
3. A successful intervening call (with the stream pointed to by stream) to a file-positioning function ( fseek(), fsetpos(), or rewind()) discards any pushed-back bytes for the stream.
4. The external storage corresponding to the stream shall be unchanged.
5. A successful call to ungetc() clears the end-of-file indicator for the stream.
6. The value of the file-position indicator for the stream after reading or discarding all pushed-back bytes shall be the same as it was before the bytes were pushed back.
7. The file-position indicator is decremented by each successful call to ungetc(), if its value was 0 before a call, its value is unspecified after the call.

#include <stdio.h>

int main()
{
    FILE* f;
    int char;
    char buffer[256];

    // read a file
    f = fopen("use1.txt", "r");

    // when no data
    if (f == NULL) {
        printf("Error in opening file");
        return (-1);
    }

    // read lines till end
    while (!feof(f)) {

        // get line
        char = getc(f);
        // replace ! with +
        if (char == '!') {
            ungetc('+', f);
        }
        // if not
        else {
            ungetc(c, f);
        }
        fgets(buffer, 255, f);
        fputs(buffer, stdout);
    }
    return 0;
}

Input: !c standard library
       !library function stdio.h-ungetc()
Output: +c standard library
        +library function stdio.h-ungetc()

// C program for taking input till we
// get 1 at the input 
#include <stdio.h>
int main()
{
    int ch;

    // reads characters from the stdin and show
    // them on stdout until encounters '1'
    while ((ch = getchar()) != '1')
        putchar(ch);

    // ungetc() returns '1' previously
    // read back to stdin
    ungetc(ch, stdin);

    // getchar() attempts to read
    // next character from stdin
    // and reads character '1' returned
    // back to the stdin by ungetc()
    ch = getchar();

    // putchar() displays character
    putchar(ch);
    return 0;
}