c11

I am going through the assembly generated by GCC for an ARM Cortex M4, and noticed that atomic_compare_exchange_weak gets two DMB instructions inserted around the condition (compiled with GCC 4.9 using -std=gnu11 -O2 ): // if (atomic_compare_exchange_weak(&address, &x, y)) dmb sy ldrex r0, [r3] cmp r0, r2 itt eq strexeq lr, r1, [r3] cmpeq.w lr, #0 dmb sy bne.n ... Since the programming guide to barrier instructions for ARM Cortex M4 states that: Omitting the DMB or DSB instruction in the examples in Figure 41 and Figure 42 would not cause any error because the Cortex-M processors: do not re

The following example is given in the C11 standard, 6.5.2.3 The following is not a valid fragment (because the union type is not visible within function f): struct t1 { int m; }; struct t2 { int m; }; int f(struct t1 *p1, struct t2 *p2) { if (p1->m < 0) p2->m = -p2->m; return p1->m; } int g() { union { struct t1 s1; struct t2 s2; } u; /* ... */ return f(&u.s1, &u.s2); } Why does it matter that the union type is visible to the function f? In reading through the relevant section a couple times, I could not see anything in the containing section disallowing this. It matters because of 6.5.2.3

According to N1570 (C11 draft) 6.5.6/8 Additive operators : Moreover, if the expression P points to the last element of an array object, the expression (P)+1 points one past the last element of the array object , and if the expression Q points one past the last element of an array object, the expression (Q)-1 points to the last element of the array object Subclause 6.5.6/9 also contains: Moreover, if the expression P points either to an element of an array object or one past the last element of an array object, and the expression Q points to the last element of the same array object, the

C11 introduces the _Thread_local storage class specifier that can be used in combination with the static and extern storage class specifiers to declare a variable as thread local. The GNU C compiler suite implements a storage class specifier __thread with the same same semantics. Unfortunately I did not find any compiler (I tried gcc, clang and SUN studio) that actually implements the _Thread_local keywords. I currently use the following construct to declare a keyword thread_local : /* gcc doesn't know _Thread_local from C11 yet */ #ifdef __GNUC__ # define thread_local __thread #elif __STDC

The malloc() function returns a pointer of type void* . It allocates memory in bytes according to the size_t value passed as argument to it. The resulting allocation is raw bytes which can be used with any data type in C(without casting). Can an array with type char declared within a function that returns void * , be used with any data type like the resulting allocation of malloc ? For example, #include <stdio.h> void *Stat_Mem(); int main(void) { //size : 10 * sizeof(int) int buf[] = { 1,2,3,4,5,6,7,8,9,10 }; int *p = Stat_Mem(); memcpy(p, buf, sizeof(buf)); for (int n = 0; n < 10; n++) {