Always initialize c_options=NULL

[MichaelChirico]

This was flagged by clang's -Wsometimes-uninitialized.

[krlmlr]

Thanks for raising this. Could the problem be not the c_options but that Rz_SEXP_to_igraph_arpack_options() doesn't fully initialize the whole struct?

Do we need a specific version of clang to replicate this?

[MichaelChirico]

This fails to compile for me:

// reprex.c
#include <stddef.h>

extern void consume_pointer(void* ptr);
extern int is_null(void* arg);

void test_pagerank_options(void* options, int algo) {
    void* c_options; // Declared without initialization

    if (!is_null(options)) {
        if (algo == 1) {
            c_options = options;
        } else {
            c_options = NULL;
        }
    }

    // When is_null(options) is true, c_options is read uninitialized here:
    consume_pointer(c_options);
}

clang -Wsometimes-uninitialized -c reprex.c

output:

reprex.c:9:9: warning: variable 'c_options' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
    if (!is_null(options)) {
        ^~~~~~~~~~~~~~~~~
reprex.c:18:21: note: uninitialized use occurs here
    consume_pointer(c_options);
                    ^~~~~~~~~
reprex.c:9:5: note: remove the 'if' if its condition is always true
    if (!is_null(options)) {
    ^~~~~~~~~~~~~~~~~~~~~~~
reprex.c:7:20: note: initialize the variable 'c_options' to silence this warning
    void* c_options; // Declared without initialization
                   ^
                    = NULL
1 warning generated.

clang --version
# Ubuntu clang version 14.0.0-1ubuntu1.1
# Target: x86_64-pc-linux-gnu
# Thread model: posix
# InstalledDir: /usr/bin

[MichaelChirico]

It does have the feel of a false positive to it, since it doesn't cause any "real" issues, but there is still UB here as summarized by Gemini:

"Algorithmatically, it acts like a false positive because PRPACK ignores the options argument entirely—so it won't cause runtime errors or crashes. However, for Clang and C static analyzers, it is a true positive: in C, loading an uninitialized variable to pass it as a function argument at the call site is an uninitialized read, regardless of whether the callee dereferences it later. Initializing to NULL satisfies C evaluation rules and prevents compiler/sanitizer traps on strict build toolchains."

[krlmlr]

Of course, thanks for the simple example. Will adapt the code generator.

[krlmlr]

Thanks, included in #2792.