1UNGETC(3P) POSIX Programmer's Manual UNGETC(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 ungetc — push byte back into input stream
13
15 #include <stdio.h>
16 int ungetc(int c, FILE *stream);
18
20 The functionality described on this reference page is aligned with the
21 ISO C standard. Any conflict between the requirements described here
22 and the ISO C standard is unintentional. This volume of POSIX.1‐2017
23 defers to the ISO C standard.
24 The ungetc() function shall push the byte specified by c (converted to
26 an unsigned char) back onto the input stream pointed to by stream. The
27 pushed-back bytes shall be returned by subsequent reads on that stream
28 in the reverse order of their pushing. A successful intervening call
29 (with the stream pointed to by stream) to a file-positioning function
30 (fseek(), fseeko(), fsetpos(), or rewind()) or fflush() shall discard
31 any pushed-back bytes for the stream. The external storage correspond‐
32 ing to the stream shall be unchanged.
33 One byte of push-back shall be provided. If ungetc() is called too many
35 times on the same stream without an intervening read or file-position‐
36 ing operation on that stream, the operation may fail.
37 If the value of c equals that of the macro EOF, the operation shall
39 fail and the input stream shall be left unchanged.
40 A successful call to ungetc() shall clear the end-of-file indicator for
42 the stream. The value of the file-position indicator for the stream
43 after all pushed-back bytes have been read, or discarded by calling
44 fseek(), fseeko(), fsetpos(), or rewind() (but not fflush()), shall be
45 the same as it was before the bytes were pushed back. The file-position
46 indicator is decremented by each successful call to ungetc(); if its
47 value was 0 before a call, its value is unspecified after the call.
48
50 Upon successful completion, ungetc() shall return the byte pushed back
51 after conversion. Otherwise, it shall return EOF.
52
54 No errors are defined.
55 The following sections are informative.
57
59 None.
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62 None.
63
65 None.
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68 None.
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71 Section 2.5, Standard I/O Streams, fseek(), getc(), fsetpos(), read(),
72 rewind(), setbuf()
73 The Base Definitions volume of POSIX.1‐2017, <stdio.h>
75
77 Portions of this text are reprinted and reproduced in electronic form
78 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
79 table Operating System Interface (POSIX), The Open Group Base Specifi‐
80 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
81 Electrical and Electronics Engineers, Inc and The Open Group. In the
82 event of any discrepancy between this version and the original IEEE and
83 The Open Group Standard, the original IEEE and The Open Group Standard
84 is the referee document. The original Standard can be obtained online
85 at http://www.opengroup.org/unix/online.html .
86 Any typographical or formatting errors that appear in this page are
88 most likely to have been introduced during the conversion of the source
89 files to man page format. To report such errors, see https://www.ker‐
90 nel.org/doc/man-pages/reporting_bugs.html .
91IEEE/The Open Group 2017 UNGETC(3P)