c - C 中的链表丢失头部信息

Question

作为开发简单 shell 项目的一部分，我已经在 C 中实现了一个基本链表 - 它通过维护 shell 在后台运行的 pid 列表来支持进程后台运行。代码如下。使用 queue_process(some_pid) 插入第一次工作正常，但随后链表就像列表中从来没有任何节点一样(即“没有现有进程”总是由调试功能打印).我已经检查了这个文件和调用这些函数的文件是否有任何会重置头指针无用的东西。我是否在链表逻辑中遗漏了什么？

调用 queue_process EDIT: 发生在由 shell 启动的子进程中，看起来像这样/EDIT: queue_process(getpid())

谢谢!

    void queue_process(pid_t pid_to_insert)
{
    pmesg(2, "In queue_process.\n");
    if (head == NULL)
    {
        pmesg(3, "No existing processes.\n");
        head = malloc(sizeof(struct xssh_process));
        head->pid = pid_to_insert;
        head->next = NULL;
    }
    else
    {
        pmesg(3, "There are existing processes.\n");
        struct xssh_process *new_process = malloc(sizeof(struct xssh_process));
        new_process->next= head;
        head = new_process;
    }
    print_processes();
}

void print_processes()
{
    pmesg(2, "In print_processes.\n");
    struct xssh_process *at_node = head;
    if (head == NULL) { pmesg(2, "There are currently no background processes.\n"); return; }
    pmesg(2, "Process IDs from head (most recently executed) to tail: %i -> ", at_node->pid);
    while (at_node != NULL)
    {
        pmesg(2, "%i ->", at_node->pid);
        at_node = at_node->next;
    }
    pmesg(3, "Head's pid in print is %i.\n", head->pid);
}

Answer 1

对于您遇到的错误没有任何帮助，但您的代码让我觉得过于复杂:

pmesg(2, "In queue_process.\n");
if (head == NULL)
{
    pmesg(3, "No existing processes.\n");
    head = malloc(sizeof(struct xssh_process));
    head->pid = pid_to_insert;
    head->next = NULL;
    tail = malloc(sizeof(struct xssh_process));
    tail = head;
}
else
{
    pmesg(3, "There are existing processes.\n");
    struct xssh_process *new_process = malloc(sizeof(struct xssh_process));
    new_process->next= head;
    head = new_process;
}

这可以简化很多。由于无论如何您都是在列表的头部插入，因此您不需要为空列表单独的逻辑:

void queue_process(pid_t pid_to_insert) { 
   struct xssh_process *new_process = malloc(sizeof(*new_process));
   new_process->pid = pid_to_insert;
   new_process->next = head;
   head = new_process;
}

同样，print_processes 可以稍微精简一下:

void print_processes() { 
    struct xssh_process *p;
    for (p=head; p!=NULL; p=p->next)
        printf("%d\n", p->pid);
}

OTOH，链表让我觉得是一个糟糕的选择——假设指针至少和 PID 一样大，至少 50% 的内存是指针的开销。