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POReading inotify file descriptor fills the buffer with binary garbage
 

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  1. POReading inotify file descriptor fills the buffer with binary garbage
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    <p>I was trying my hand at inotify and the following snippet of a code is what I thought would be a good demo of its capabilities:</p> <pre><code>#include &lt;sys/inotify.h&gt; /* inotify_init(), inotify_add_watch(), IN_* */ #include &lt;stdlib.h&gt; /* EXIT_SUCCESS, EXIT_FAILURE, malloc() */ #include &lt;stdio.h&gt; /* printf(), puts(), perror() */ #include &lt;unistd.h&gt; /* read() */ #include &lt;sys/ioctl.h&gt; /* ioctl, FIONREAD */ #define PTRY(expr) if((expr) == -1) #define PGOTO(label, msg) do{\ perror(msg);\ goto label;\ } while(0) int main(void) { int fd; PTRY(fd = inotify_init()) PGOTO(failure, "inotify_init"); PTRY(inotify_add_watch(fd, ".", IN_ALL_EVENTS)) PGOTO(failure, "inotify_add_watch"); while(1) { unsigned int avail; /* We find out how large a buffer we need */ PTRY(ioctl(fd, FIONREAD, &amp;avail)) PGOTO(failure, "ioctl"); char buffer[avail]; ssize_t count = read(fd, buffer, avail); /* We fill the buffer */ if(avail &gt; 0 &amp;&amp; count &lt; avail) PGOTO(failure, "read"); printf("Avail: %d, Read: %d\n", (int)avail, (int)count); if(avail &gt; 0) puts(""); for(ssize_t i = 0; i &lt; count; ) { /* For each event structure in the buffer */ struct inotify_event* event = (struct inotify_event*)buffer + i; printf("Byte: %d - %d out of %d\n", (int)i, (int)(sizeof(struct inotify_event) + event-&gt;len), (int)count); if(event-&gt;len &gt; 0) { /* We print its contents */ printf("Name: %s\n", event-&gt;name); } printf("Cookie: %d\n", event-&gt;cookie); if(event-&gt;mask &amp; IN_ACCESS) puts("IN_ACCESS"); if(event-&gt;mask &amp; IN_ATTRIB) puts("IN_ATTRIB"); if(event-&gt;mask &amp; IN_CLOSE_WRITE) puts("IN_CLOSE_WRITE"); if(event-&gt;mask &amp; IN_CLOSE_NOWRITE) puts("IN_CLOSE_NOWRITE"); if(event-&gt;mask &amp; IN_CREATE) puts("IN_CREATE"); if(event-&gt;mask &amp; IN_DELETE) puts("IN_DELETE"); if(event-&gt;mask &amp; IN_DELETE_SELF) puts("IN_DELETE_SELF"); if(event-&gt;mask &amp; IN_MODIFY) puts("IN_MODIFY"); if(event-&gt;mask &amp; IN_MOVE_SELF) puts("IN_MOVE_SELF"); if(event-&gt;mask &amp; IN_MOVED_FROM) puts("IN_MOVED_FROM"); if(event-&gt;mask &amp; IN_MOVED_TO) puts("IN_MOVED_TO"); if(event-&gt;mask &amp; IN_OPEN) puts("IN_OPEN"); puts(""); i += sizeof(struct inotify_event) + event-&gt;len; } } return EXIT_SUCCESS; failure: return EXIT_FAILURE; } </code></pre> <p>I am, however, receiving some extraneous records from the <code>read(2)</code> call, filled with binary rubbish. Enclosed is the output of the program upon the invocation of <code>cat notifyTest.c</code> from the shell:</p> <pre><code>Avail: 0, Read: -1 Avail: 64, Read: 64 Byte: 0 - 32 out of 64 Name: notifyTest.c Cookie: 0 IN_OPEN Byte: 32 - 32783 out of 64 Name: ▒▒▒ Cookie: 1726459283 IN_ACCESS IN_ATTRIB IN_CLOSE_WRITE IN_CLOSE_NOWRITE IN_CREATE IN_DELETE IN_DELETE_SELF IN_MODIFY IN_MOVE_SELF IN_MOVED_FROM IN_MOVED_TO IN_OPEN Avail: 0, Read: -1 Avail: 32, Read: 32 Byte: 0 - 32 out of 32 Name: notifyTest.c Cookie: 0 IN_CLOSE_NOWRITE </code></pre> <p>This is potentially code-breaking, since the process would be signaled <code>SIGSEGV</code> and killed while reading <code>event-&gt;name</code> if there weren't any terminating <code>'\0'</code> character within the bounds of the statically allocated buffer.</p> <p>Could this be a quirk, or am I just doing something terribly wrong? My <code>uname -a</code> is:</p> <pre><code>Linux witiko-D830 3.8.0-32-generic #47-Ubuntu SMP Tue Oct 1 22:35:23 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux </code></pre> <p>I'll be grateful for any pointers as to what could have gone amiss.</p>
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