I finally trace down a typo bug, which is something similar to the following code. But shouldn't the compiler detect this (by default options)?
#include <stdio.h>
int main()
{
int c = c;
return printf("%d\n", c);
}
$ gcc --version
gcc (Ubuntu 4.4.3-4ubuntu5.1) 4.4.3
I don't see why it wouldn't compile. Definition happens prior to initialization. Of course this initialization is pointless, however, there's no reason it wouldn't work from the compilers stand point.
C does not have the same types of protections that more modern languages like C# have. The C# compiler would give an error that you're using an unassigned variable. C doesn't care. It will not protect you from yourself.
int x = initialize_and_register(&x). (That kind of invocation would typically be packaged in a macro.)
x in an expression initializing x is actually useful, so it's not (only) about C not protecting you from yourself. And if you wish to add that info to the answer, that's what the "Edit" button is for. :)
It's perfectly legitimate to use a variable in its own initializer. Consider a linked list:
#include <stdio.h>
struct node { struct node *prev, *next; int value; };
int main() {
struct node l[] = {{0, l + 1, 42}, {l, l + 2, 5}, {l, 0, 99}};
for (struct node *n = l; n; n = n->next)
printf("%d\n", n->value);
return 0;
}
In general, diagnosing when a value is used uninitialised is a difficult problem; although some compilers can detect it in some cases it doesn't make sense to require it to happen.
l decays into a pointer &[0] in this context.
gcc -O3 -g -std=c99 -Wall -Wextra -Wmissing-prototypes -Wstrict-prototypes -Wold-style-definition -c x.cand the complaints were about 'not a prototype' and 'old-style function definition' and not about the variable initialization. OTOH,clangdiagnosed:x.c:5:13: warning: variable 'c' is uninitialized when used within its own initialization [-Wuninitialized](but adding-Wuninitializedto the GCC options didn't add to the messages —clanghas better diagnostics here).-Winit-selfflag.-Wall -Wextradoes not include-Winit-selfin GCC 4.9.3.