Can't dereference a char* array passed through a function

Question

The code is the following. The reason I'm using void * args is because this function has to be used in a threaded program. It was tested with and without any threads, and it doesn't work in any of both workflows. I'm pretty new to C and this might be obvious to the average programmer but even with gdb I can't find the problem.

void arg(const void* args) {
    char* arr = (char*) args;
    printf("%s", arr[0]);
    printf("%s", arr[1]);
    return;
}

int main (void) {
    char* a[2] = {"Hello", "World!"};
    arg((void*)a);
    return 0;
}

The above code will segfault when dereferencing args.

Answer 1

The expressions arr[0] and arr[1] have the type char after the casting

char* arr = (char*) args;

So at least using the conversion specifier s with objects of the type char

printf("%s", arr[0]);
printf("%s", arr[1]);

is incorrect.

It seems you mean the following

void arg(const void* args) {
    const char** arr = args;
    printf("%s", arr[0]);
    printf("%s", arr[1]);
}

Pay attention to that the array a

char* a[2] = {"Hello", "World!"};

used in expressions (with rare exceptions) is implicitly converted to a pointer of the type char **.

In C++ the array of string literals shall be declared with the qualifier const

const char* a[2] = {"Hello", "World!"};

And you need explicitly casting the pointer of the type const void to the type const char ** like for example

const char*const* arr = static_cast<const char * const *>( args );