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    <p>It is possible for an implementation to meet the interface requirements for <code>fgetc</code> and <code>fputc</code> even if <code>sizeof(int) == 1</code>.</p> <p>The interface for <code>fgetc</code> says that it returns the character read as an <code>unsigned char</code> converted to an <code>int</code>. Nowhere does it say that this value cannot be <code>EOF</code> even though the expectation is clearly that valid reads "usually" return positive values. Of course, <code>fgetc</code> returns <code>EOF</code> on a read failure or end of stream but in these cases the file's error indicator or end-of-file indicator (respectively) is also set.</p> <p>Similarly, nowhere does it say that you can't pass <code>EOF</code> to <code>fputc</code> so long as that happens to coincide with the value of an <code>unsigned char</code> converted to an <code>int</code>.</p> <p>Obviously the programmer has to be very careful on such platforms. This is might not do a full copy:</p> <pre><code>void Copy(FILE *out, FILE *in) { int c; while((c = fgetc(in)) != EOF) fputc(c, out); } </code></pre> <p>Instead, you would have to do something like (not tested!):</p> <pre><code>void Copy(FILE *out, FILE *in) { int c; while((c = fgetc(in)) != EOF || (!feof(in) &amp;&amp; !ferror(in))) fputc(c, out); } </code></pre> <p>Of course, platforms where you will have real problems are those where <code>sizeof(int) == 1</code> and the conversion from <code>unsigned char</code> to <code>int</code> is not an injection. I believe that this would necessarily the case on platforms using sign and magnitude or ones complement for representation of signed integers.</p>
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    1. POCan sizeof(int) ever be 1 on a hosted implementation?
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    1. POCan sizeof(int) ever be 1 on a hosted implementation?
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    1. COPassing `EOF` to `fputc` is completely valid because the argument is converted to `unsigned char` before being written. Thus `fputc(EOF)` is equivalent to `fputc(UCHAR_MAX)`. The conversion in the other direction, however, is undefined behavior if `UCHAR_MAX>INT_MAX`.
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    2. CO@R..: Yes, you're completely correct about passing `EOF` to `fputc`. Then conversion from an `unsigned char` that can't be represented as an `int` value to `int` does not cause _undefined behaviour_, though, it is _implementation defined_. This is important because it allows an implementation to support the `fputc`/`fgetc` round trip.
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    3. COYou are correct. I assumed the behavior was the same as signed arithmetic overflow, but conversion to a signed type is implementation-defined (according to 6.3.1.3 paragraph 3) as you say.
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