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Wiki MarkupSection 7.19.5.3 of C99 places the following restrictions on update streams \ [[ISO/IEC 9899:1999|AA. Bibliography#ISO/IEC 9899-1999]\]:

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 fflush function or to a file positioning function (fseek, fsetpos, or rewind), 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.

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