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copied!<p>That's pretty bad. Don't ever make a char buffer and cast it to a struct, because the alignment will be wrong (ie, the char buffer is going to have some random starting address because strings can start anywhere, but ints need/should have addresses multiples of four on most architectures).</p> <p>The solution is not to do nasty casts like that. Make a proper union that will have the alignment of the most restrictive of its members, or use a special element to force the alignment you need if you have to (see the definition of sockaddr_storage in your /usr/include/sys/socket.h or similar).</p> <h3>Illustration</h3> <p>You create a buffer on the stack and read some data into it:</p> <pre><code>char buf[1024]; int nread = read(fd, &buf, sizeof(buf)); </code></pre> <p>Now you pretend the buffer was the struct:</p> <pre><code>CHECK(nread >= sizeof(struct icmphdr)); struct icmphdr* hdr = (struct icmphdr*)buf; hdr->u.gateway; // probable SIGSEGV on eg Itanium! </code></pre> <p>By reinterpreting the buffer as a struct, we bypassed the compiler's checks. If we're unlucky, <code>&hdr->u.gateway</code> won't be a multiple of four, and accessing it as an integer will barf on some platforms.</p> <h3>Illustration of solution</h3> <pre><code>strut iphdr hdr; int nread = read(fd, &hdr, sizeof(hdr)); CHECK(nread == sizeof(hdr)); hdr.u.gateway; // OK </code></pre> <p>Let the compiler help you. Don't do grotty casts. When you make a buffer, tell the compiler what you're going to use the buffer for so it can put it in the correct place in memory for you.</p>

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