ungetc - push byte back into input stream
int ungetc(int c, FILE *stream);
[CX] 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.
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] fseeko(), fsetpos(), or rewind()) [CX] or fflush() 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] fseeko(), fsetpos(), or rewind() [CX] (but not fflush()), 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.
Standard I/O Streams, fseek, getc, fsetpos, read, rewind, setbuf
First released in Issue 1. Derived from Issue 1 of the SVID.
POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0687 [87,93], XSH/TC1-2008/0688 , and XSH/TC1-2008/0689  are applied.
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