我正在尝试将结构体映射为匿名共享内存,以便可以跨多个子进程共享它。但是每当我访问创建它的函数之外的struct时,我都会遇到段错误。该代码源自三位一体模糊器,可在此处找到https://github.com/kernelslacker/trinity。
首先我们有一个标题的摘录,它声明并定义了我们将要使用的结构。注意shm在struct声明下声明为全局。
struct shm_s {
struct childdata **children;
struct stats_s stats;
unsigned int running_childs;
/* rng related state */
unsigned int seed;
#ifdef ARCH_IS_BIARCH
/* Check that 32bit emulation is available. */
unsigned int syscalls32_succeeded;
unsigned int syscalls32_attempted;
#endif
/* pids */
pid_t mainpid;
pid_t last_reaped;
/* various flags. */
enum exit_reasons exit_reason;
bool dont_make_it_fail;
bool spawn_no_more;
bool ready;
bool postmortem_in_progress;
/* Use dispatch queue instead of locks. */
/* main<>watchdog serial queue, for reap_child()
* provides exclusion so they don't both try at the same time. */
dispatch_queue_t reaper_queue;
/* to protect from multiple child processes from
* trying to disable the same syscall at the same time. */
dispatch_queue_t syscalltable_queue;
/* child<>child serial queue, used so only one child spews debug output */
dispatch_queue_t bugQueue;
/* global debug flag.
* This is in the shm so we can do things like gdb to the main pid,
* and have the children automatically take notice.
* This can be useful if for some reason we don't want to gdb to the child.
*/
bool debug;
};
extern struct shm_s *shm;
在c文件中,我们有struct的create和init函数。
struct shm_s *shm;
#define SHM_PROT_PAGES 30
void create_shm(void)
{
void *p;
void *redbefore, *redafter;
unsigned int shm_pages;
unsigned int wholesize;
/* round up shm to nearest page size */
shm_pages = ((sizeof(struct shm_s) + page_size - 1) & PAGE_MASK) / page_size;
wholesize = (SHM_PROT_PAGES + shm_pages + SHM_PROT_PAGES) * page_size;
/* Waste some address space to set up some "protection" near the SHM location. */
p = alloc_shared(wholesize);
redbefore = p;
redafter = p + (SHM_PROT_PAGES + shm_pages) * page_size;
/* set the redzones. */
memset(redbefore, 0x77, SHM_PROT_PAGES * page_size);
memset(redafter, 0x88, SHM_PROT_PAGES * page_size);
/* set the redzones to PROT_NONE */
mprotect(redbefore, SHM_PROT_PAGES * page_size, PROT_NONE);
mprotect(redafter, SHM_PROT_PAGES * page_size, PROT_NONE);
/* clear the whole shm. */
shm = p + (SHM_PROT_PAGES * page_size);
memset(shm, 0, shm_pages * page_size);
printf("shm: redzone:%p. shmdata:%p. redzone:%p end:%p.\n",
redbefore, shm, redafter, p + wholesize);
}
void init_shm(void)
{
unsigned int i;
printf("shm is at %p\n", shm);
shm->stats.total_syscalls_done = 1;
shm->children = zmalloc(max_children * sizeof(struct childdata *));
shm->bugQueue = dispatch_queue_create(NULL, DISPATCH_QUEUE_SERIAL);
shm->reaper_queue = dispatch_queue_create(NULL, DISPATCH_QUEUE_SERIAL);
shm->syscalltable_queue = dispatch_queue_create(NULL, DISPATCH_QUEUE_SERIAL);
for_each_child(i)
{
struct childdata *child;
child = alloc_shared(sizeof(struct childdata));
shm->children[i] = child;
child->pid = EMPTY_PIDSLOT;
child->logfile = NULL;
}
}
这是alloc_shared函数,它实际上创建了共享内存。
void * alloc_shared(unsigned int size)
{
void *ret;
ret = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_ANON | MAP_SHARED, -1, 0);
if (ret == MAP_FAILED) {
printf("mmap %u failure\n", size);
exit(EXIT_FAILURE);
}
/* poison, to force users to set it to something sensible. */
memset(ret, rand(), size);
return ret;
}
然后最后在单独的文件中调用函数create_shm和init_shm。
create_shm();
init_shm();
每当程序到达shm-> stats.total_syscalls_done = 1时,程序就会崩溃;在init_shm中。我打印了shm的地址,它在alloc_shared和init_shm中创建之后是一样的,所以我认为指针没有被破坏。 无论哪种方式,我都不明白为什么我无法访问create_shm()之外的结构。
答案 0 :(得分:2)
根据OP在评论中回答的内容,printf的输出是:
shm: redzone:0x10a0ad000. shmdata:0x10a0cb000. redzone:0x10a0cb000 end:0x10a0e9000
请注意,shmdata
与(redafter
)之后的redzone位于同一地址,均位于0x10a0cb000
。这意味着shm_pages
为零。
这一行很可能是问题所在:
shm_pages = ((sizeof(struct shm_s) + page_size - 1) & PAGE_MASK) / page_size;
我猜测PAGE_MASK
是0xfff
。如果是这样,那么该表达式将始终重新评估为0.正如chmike所指出的那样,因为无论如何都要除以page_size,所以根本不需要使用PAGE_MASK
。在这种情况下,您的代码应为:
shm_pages = (sizeof(struct shm_s) + page_size - 1) / page_size;