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Ungetc

Item Value Header filestdio.h Declarationint ungetc(int ch, FILE *stream); FunctionPut the character ch back to the input stream stream. This character will be obtained by the next read operation on stream. Returnreturns ch on success or EOF on failure. A call to fflush(), fseek(), or rewind() discards the character. You may not unget an EOF. #include <stdio.h> int main () {   FILE * fp;   int c;   char buffer [256];   fp = fopen ("test.txt","rt");     if (fp==NULL)     perror ("Error opening file");   else {     while (!feof (fp))     {       c=getc (fp);       if (c == '#')         ungetc ('@',fp);       else         ungetc (c,fp);       fgets (buffer,255,fp);       fputs (buffer,stdout);     }   }   return 0;  }