FCNTL returns errno - 无效参数

Question

我不明白为什么我的 fcntl 一直说 "Invalid argument"。请帮忙。如果我理解正确，我应该用指向我要检查的字节的数据填充我的羊群（我传递给 fcntl 的那个）。

  int byteno;
  printf(ANSI_COLOR_BLUE "Insert byte number: " ANSI_COLOR_RESET);
  scanf("%d\n", &byteno);

  struct flock* fl = malloc(sizeof(flock));
  fl -> l_type = F_GETLK;
  fl -> l_whence = SEEK_SET;
  fl -> l_start = byteno;
  fl -> l_len = 1;
  fl -> l_pid = getpid();

  if(fcntl(descr, F_GETLK, fl) == -1){
    perror(ANSI_COLOR_RED "Error while getting info about locks" ANSI_COLOR_RESET);
    free(fl);
    continue;
  }

  if(!(fl -> l_type == F_UNLCK)){
    printf(ANSI_COLOR_YELLOW "Other proccess has a lock here. PID = %d" ANSI_COLOR_RESET, fl -> l_pid);
    continue;
  }

Answer 1

F_GETLK (struct flock *)

On input to this call, lock describes a lock we would like to place on the file. If the lock could be placed, fcntl() does not actually place it, but returns F_UNLCK in the l_type field of lock and leaves the other fields of the structure unchanged. If one or more incompatible locks would prevent this lock being placed, then fcntl() returns details about one of these locks in the l_type, l_whence, l_start, and l_len fields of lock and sets l_pid to be the PID of the process holding that lock.

F_GETLK 不用于获取有关锁的信息，而是用于“测试应用指定的锁是否有效”。

试试下面的例子。运行 5 秒内两次 window。

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <string.h>

void main(void) {
    int fd;
    struct flock fl_info;
    int ret;

    fd = open("./test.txt", O_RDWR | O_CREAT, 0600);
    if (fd == -1) {
        perror("open()");
        return;
    }

    memset(&fl_info, 0, sizeof(fl_info));

    /* try to get a WRITE lock on the entire file (l_len == zero) */
    fl_info.l_type = F_WRLCK;
    fl_info.l_whence = SEEK_SET;
    fl_info.l_start = 0;
    fl_info.l_len = 0;

    ret = fcntl(fd, F_GETLK, &fl_info);
    if (ret == -1) {
        perror("fcntl(F_GETLK)");
        return;
    }

    if (fl_info.l_type != F_UNLCK) {
        printf("Too bad... it's already locked... by pid=%d\n", fl_info.l_pid);
        return;
    }

    /* try to apply a write lock */
    fl_info.l_type = F_WRLCK;
    ret = fcntl(fd, F_SETLK, &fl_info);
    if (ret == -1) {
        perror("fcntl(F_SETLK)");
        return;
    }

    printf("Success!\n");

    sleep(5);

    printf("I'm done, bye!\n")

    return;
}

另请注意，malloc() 存储 struct flock 是不必要的。

FCNTL returns errno - 无效参数

FCNTL returns errno - Invalid argument

c

unix

operating-system

fcntl