std::ungetc(3)

NAME
std::ungetc - std::ungetc

Synopsis
   Defined in header <cstdio>
   int ungetc( int ch, std::FILE *stream );
   If ch does not equal EOF, pushes the character ch (reinterpreted as unsigned char)
   into the input buffer associated with the stream stream in such a manner than
   subsequent read operation from stream will retrieve that character. The external
   device associated with the stream is not modified.
   Stream repositioning operations std::fseek, std::fsetpos, and std::rewind discard
   the effects of ungetc.
   If ungetc is called more than once without an intervening read or repositioning, it
   may fail (in other words, a pushback buffer of size 1 is guaranteed, but any larger
   buffer is implementation-defined). If multiple successful ungetc were performed,
   read operations retrieve the pushed-back characters in reverse order of ungetc
   If ch equals EOF, the operation fails and the stream is not affected.
   A successful call to ungetc clears the end of file status flag std::feof.
   A successful call to ungetc on a binary stream decrements the stream position
   indicator by one (the behavior is indeterminate if the stream position indicator was
   zero).
   A successful call to ungetc on a text stream modifies the stream position indicator
   in unspecified manner but guarantees that after all pushed-back characters are
   retrieved with a read operation, the stream position indicator is equal to its value
   before ungetc.

Parameters
   ch - character to be pushed into the input stream buffer
   stream - file stream to put the character back to

Return value
   On success ch is returned.
   On failure EOF is returned and the given stream remains unchanged.

Notes
   The size of the pushback buffer varies in practice from 4k (Linux, MacOS) to as
   little as 4 (Solaris) or the guaranteed minimum 1 (HPUX, AIX).
   The apparent size of the pushback buffer may be larger if the character that is
   pushed back equals the character existing at that location in the external character
   sequence (the implementation may simply decrement the read file position indicator
   and avoid maintaining a pushback buffer).

Example
   demonstrates the use of std::ungetc in its original purpose: implementing std::scanf // Run this code
 #include <cctype>
 #include <cstdio>
 void demo_scanf(const char* fmt, std::FILE* s)
 {
     while (*fmt != '\0') {
         if (*fmt == '%') {
             switch (*++fmt) {
                 case 'u': {
                     int c{};
                     while (std::isspace(c=std::getc(s))) {}
                     unsigned int num{};
                     while (std::isdigit(c)) {
                        num = num*10 + c-'0';
                        c = std::getc(s);
                     }
                     std::printf("%%u scanned %u\n", num);
                     std::ungetc(c, s);
                     break;
                 }
                 case 'c': {
                     int c = std::getc(s);
                     std::printf("%%c scanned '%c'\n", c);
                     break;
                 }
             }
         } else {
             ++fmt;
         }
     }
 }
 int main()
 {
     if (std::FILE* f = std::fopen("input.txt", "w+")) {
         std::fputs("123x", f);
         std::rewind(f);
         demo_scanf("%u%c", f);
         std::fclose(f);
     }
 }

Output:
 %u scanned 123
 %c scanned 'x'

See also
   fgetc gets a character from a file stream
   getc (function)
   C documentation for
   ungetc