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POConditionals using filehandles in C++
 

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  1. POConditionals using filehandles in C++
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    <p>I'm working with a C++ library which makes extensive use of constructs like:</p> <pre><code>FILE *out_file1, *out_file2 ... *out_fileN; //for some output files, but not all: out_file1 = fopen( filename, "w" ) //later if( out_file1 ) fprintf( ... ) if( out_file2 ) fprintf( ... ) </code></pre> <p>This seems to work OK under g++ on OS X. When I run it on linux, however, I get segfaults. Checking through the code, out_file is often initialised to non zero values.</p> <p>I've tried adding</p> <pre><code>out_file = NULL </code></pre> <p>but this doesn't seem to help - in fact, according to the debugger, it doesn't change the value of out_file.</p> <p>Can anyone help as to:</p> <p>Is this a recognised and sensible way to do file IO (i.e. using the file pointers in conditionals)</p> <p>Why is the value of the pointer not being set to null?</p> <p>How can I set it to null?</p> <p>Just to be clear - I'm trying to change the code as little as possible, as I'm coding a wrapper to someone else's library. So, even if the general structure is a strange way to do things, I'd rather find a workaround which doesn't change it if possible.</p> <p><strong>EDIT:</strong> Since this seems to be a reasonable, if outdated way to do conditional file IO, I can narrow the scope of my question to the second two out of three, i.e.</p> <pre><code>class IO { private: FILE* opFile IO() { //At this point, opFile == 0x40 opFile = NULL; //At this point opFile is still 0x40 } } </code></pre> <p>So obviously, if it comes out of the constructor with a non-null value, anything like:</p> <pre><code>if( opFile ) fprintf( ... ) </code></pre> <p>will fail. But how is it managing to come out of the constructor with a non-null value?</p> <p>And in case it helps, this works "as expected" under gcc on OSX, but not g++-4.3 or g++4.4 on Ubuntu.</p>
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    1. COAccording to the documentation for fopen() it should be returning NULL if the open fails, so the conditional should work as is. You probably have a bug elsewhere in the code which is causing undefined behaviour. Can you make a minimal working example that still shows this problem?
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    2. COJust nitpicking but it seems you're dealing with C code, not C++...
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    3. CORight, but fopen is not always being called - the plan is that files which are not opened are null, so it doesn't try to write to them.
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