Undefined Behaviour in C

Summary: I tripped over undefined behaviour in C. It's annoying.

I've recently been writing some C code to parse XML quickly. While working on that project, I inadvertently wrote some code which is undefined according to the C language standard. The code compiled and ran fine using Visual Studio, but under gcc (even at -O0) it corrupted memory, sometimes leading to a segfault, but usually just leading to a wrong answer. The code in question was (see full code at GitHub):

d->nodes.nodes[0].nodes = parse_content(d);

To give some context, d is a structure that contains various pieces of state - what the string to be parsed is, how much we have parsed, along with a pointer to the output nodes. The parse_content function parses the bit inside an XML tag, returning the indicies in nodes which it used.

The complication comes from nodes not being a fixed size, but dynamically resized if the number of nodes exceeds the capacity. For big documents that means parse_content will reallocate d->nodes.nodes.

According to the C spec, the compiler can evaluate the LHS and RHS of an assignment in any order. Since gcc computes the location of d->nodes.nodes[0] before calling parse_content it uses the address of the node before reallocation. After reallocation the address will have changed, and the assignment will be made to the wrong location.

I spotted the bug by inserting printf statements, and in doing so, I had to rewrite the code to:

str content = parse_content(d);
d->nodes.nodes[0].nodes = content;

That fixes the issue, since now the evaluation order is strictly defined. As a simplified example of the same issue:

char* array;

char f() {
    array = malloc(42);
    return 'x';    
}

void test() {
    array = malloc(0);
    array[0] = f();
}

Here the line array[0] = f() might assign to either the result of malloc(0) or malloc(42), at the compilers discretion.

I manually checked if I had made any other such mistakes, and I couldn't find any. Naturally, I wanted to find a static checker that could detect such a mistake, so I tried a bunch of them. I wasn't very successful:

• Visual Studio 2015 code analysis made me write assert after each malloc, but nothing further.
• PVS Studio found nothing.
• Clang undefined behaviour found nothing, and seemingly doesn't work on Windows.
• GCC undefined behaviour found nothing, and seemingly doesn't work on Windows.
• RV-Match hit a stack-overflow when running the program.