The ungetc Function in C

If a character read from an input stream is not appropriate for the current operation, we can push it back to the stream using the ungetc function. A subsequent character read operation (such as fgetc or getc) on that stream will return this character. The prototype of the ungetc function is given below.

int ungetc ( int c, FILE *fp ) ;

This function converts the character specified by argument c to unsigned char before pushing it to stream fp. If the operation is successful, the function returns the character pushed to the stream; otherwise, it returns an EOF.

The ungetc function is useful in input scanning operations. Consider that we wish to read an integer number from a stream one character at a time (using fgetc or getc). Let us also assume that a number is followed by either a whitespace or a non-numeric character.

For example, each line given below contains two integer numbers: 123 and 1234.

      123 2345

      123,2345

Note that we have no information about the number of digits in the number being read. Only when we read a non-numeric character, will we come to know that the digits in the number are over. However, we have already read the non-numeric character that follows the number being read. This character may be a part of subsequent data item. Hence, we push it back to the stream and it will be read by a subsequent read operation on that stream. ·

The following points may be noted about the ungetc function:

1. C language guarantees the pushback of only one character. If we perform more than one ungetc operation on a stream without an intervening read or file positioning operation on that stream (such as fseek, fsetpos or rewind), the ungetc function may fail.

2. The character pushed to a stream is actually stored in the buffer associated with the stream. This character is discarded by file positioning operations.