Inline assembly

gcc allows inline assembly in C sources or headers
The code must interact with the optimizer
The syntax is not trivial at all
asm("code" : r-output : r-input : r-clobber);
The "code" string can't use explicit register names
You can use positional names ("%0") or symbolic names ("%[timeout]")
The register lists (out and in) declare their C expressions
The list of clobbered registers can also include
memory: it means memory external to the CPU has been modified
cc: it means "condition code" flags are modified by the asm code
Examples:
#define set_cr(x) /* arm */                             \
        __asm__ __volatile__(                           \
        "mcr    p15, 0, %0, c1, c0, 0   @ set CR"       \
        : : "r" (x) : "cc")

#define mb() __asm__ __volatile__ ("" : : : "memory")

#define rdtsc(low,high) /* x86 */     asm("rdtsc" : "=a" (low), "=d" (high))

All documentation is part of the gcc manual