1 // 2 // ungetc.cpp 3 // 4 // Copyright (c) Microsoft Corporation. All rights reserved. 5 // 6 // Defines ungetc(), which pushes a character back into a stream. 7 // 8 #include <corecrt_internal_stdio.h> 9 10 11 12 // Pushes a character ("ungets" it) back into a stream. It is possible to push 13 // back one character. It may not be possible to push back more than one 14 // character in a row. Returns the pushed-back character on success; returns 15 // EOF on failure. Ungetting EOF is expressly forbidden. 16 extern "C" int __cdecl ungetc(int const c, FILE* const stream) 17 { 18 _VALIDATE_RETURN(stream != nullptr, EINVAL, EOF); 19 20 int return_value = EOF; 21 22 _lock_file(stream); 23 __try 24 { 25 return_value = _ungetc_nolock(c, stream); 26 } 27 __finally 28 { 29 _unlock_file(stream); 30 } 31 __endtry 32 33 return return_value; 34 } 35 36 37 38 extern "C" int __cdecl _ungetc_nolock(int const c, FILE* public_stream) 39 { 40 __crt_stdio_stream const stream(public_stream); 41 42 _VALIDATE_STREAM_ANSI_RETURN(stream, EINVAL, EOF); 43 44 // Ungetting EOF is expressly forbidden: 45 if (c == EOF) 46 return EOF; 47 48 // The stream must either be open in read-only mode, or must be open in 49 // read-write mode and must not currently be in write mode: 50 bool const is_in_read_only_mode = stream.has_all_of(_IOREAD); 51 bool const is_in_rw_write_mode = stream.has_all_of(_IOUPDATE | _IOWRITE); 52 53 if (!is_in_read_only_mode && !is_in_rw_write_mode) 54 return EOF; 55 56 // If the stream is currently unbuffered, buffer it: 57 if (stream->_base == nullptr) 58 __acrt_stdio_allocate_buffer_nolock(stream.public_stream()); 59 60 // At this point, we know that _base is not null, since the file is buffered. 61 62 if (stream->_ptr == stream->_base) 63 { 64 // If we've already buffered a pushed-back character, there's no room for 65 // another, and there's nothing we can do: 66 if (stream->_cnt) 67 return EOF; 68 69 ++stream->_ptr; 70 } 71 72 // If the stream is string-backed (and not file-backed), do not modify the buffer: 73 if (stream.is_string_backed()) 74 { 75 --stream->_ptr; 76 if (*stream->_ptr != static_cast<char>(c)) 77 { 78 ++stream->_ptr; 79 return EOF; 80 } 81 } 82 else 83 { 84 --stream->_ptr; 85 *stream->_ptr = static_cast<char>(c); 86 } 87 88 ++stream->_cnt; 89 stream.unset_flags(_IOEOF); 90 stream.set_flags(_IOREAD); 91 92 return c & 0xff; 93 } 94