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Section 7.19.5.3 of C99 places the following restrictions on update streams: \[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\]
<blockquote><p>When a file is opened with update mode both input and output may be performed on the associated stream. However, output shall not be directly followed by input without an intervening call to the <code>fflush</code> function or to a file positioning function (<code>fseek</code>, <code>fsetpos</code>, or <code>rewind</code>), and input shall not be directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end-of-file. Opening (or creating) a text file with update mode may instead open (or create) a binary stream in some implementations.</p></blockquote>Receiving input from a stream directly following an output to that stream  without an intervening call to {{fflush()}}, {{fseek()}}, {{fsetpos()}}, or {{rewind()}}, or outputting to a stream after receiving input from it without a call to {{fseek()}}, {{fsetpos()}}, {{rewind()}} if the file is not at end-of-file results in [undefined behavior|BB. Definitions#undefined behavior] (see also [undefined behavior 143| CC. Undefined Behavior#ub_143] in Annex J of C99). Consequently, a call to {{fseek()}}, {{fflush()}} or {{fsetpos()}} is necessary between input and output to the same stream (see [FIO07-C. Prefer fseek() to rewind()|FIO07-C. Prefer fseek() to rewind()]).

\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] Section 7.19.5.3, "The {{fopen}} function"