The Open Group Base Specifications Issue 7, 2018 edition
IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)
Copyright © 2001-2018 IEEE and The Open Group


ungetc - push byte back into input stream


#include <stdio.h>

int ungetc(int
c, FILE *stream);


[CX] [Option Start] The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2017 defers to the ISO C standard. [Option End]

The ungetc() function shall push the byte specified by c (converted to an unsigned char) back onto the input stream pointed to by stream. The pushed-back bytes shall be returned by subsequent reads on that stream in the reverse order of their pushing. A successful intervening call (with the stream pointed to by stream) to a file-positioning function (fseek(), [CX] [Option Start]   fseeko(), [Option End] fsetpos(), or rewind()) [CX] [Option Start]  or fflush() [Option End]  shall discard any pushed-back bytes for the stream. The external storage corresponding to the stream shall be unchanged.

One byte of push-back shall be provided. If ungetc() is called too many times on the same stream without an intervening read or file-positioning operation on that stream, the operation may fail.

If the value of c equals that of the macro EOF, the operation shall fail and the input stream shall be left unchanged.

A successful call to ungetc() shall clear the end-of-file indicator for the stream. The value of the file-position indicator for the stream after all pushed-back bytes have been read, or discarded by calling fseek(), [CX] [Option Start] fseeko(), [Option End] fsetpos(), or rewind() [CX] [Option Start] (but not fflush()), [Option End]  shall be the same as it was before the bytes were pushed back. The file-position indicator is decremented by each successful call to ungetc(); if its value was 0 before a call, its value is unspecified after the call.


Upon successful completion, ungetc() shall return the byte pushed back after conversion. Otherwise, it shall return EOF.


No errors are defined.

The following sections are informative.










Standard I/O Streams, fseek, getc, fsetpos, read, rewind, setbuf

XBD <stdio.h>


First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 7

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0687 [87,93], XSH/TC1-2008/0688 [87], and XSH/TC1-2008/0689 [14] are applied.

End of informative text.


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