musl/src/process/fdop.h

#define FDOP_CLOSE 1
#define FDOP_DUP2 2
#define FDOP_OPEN 3
#define FDOP_CHDIR 4
#define FDOP_FCHDIR 5

struct fdop {
	struct fdop *next, *prev;
	int cmd, fd, srcfd, oflag;
	mode_t mode;
	char path[];
};

#define malloc __libc_malloc
#define calloc __libc_calloc
#define realloc undef
#define free __libc_free