读取命令结果错误（fgets）

Question

我编写了一个带有按钮的简单可可应用程序。

单击按钮时，会触发计时器：

[NSTimer scheduledTimerWithTimeInterval:1.0f 
                                 target:self 
                               selector:@selector(readCommandResult:)
                               userInfo:nil
                                repeats:YES];

这是计时器：

-(void)readCommandResult:(NSTimer *) timer
{
char ps_cmd[256] = {"ls"};
BOOL isFgetsOK = NO;

FILE *fp = popen(ps_cmd, "r"); 
if (fp) {
    char line[4096];
    while (line == fgets(line, 4096, fp)) {

        isFgetsOK = YES;
    }
    pclose(fp);
}
else {

    NSLog(@"popen error");
}   
if (!isFgetsOK) {
    NSLog(@"fgets error");
}
}

当我运行应用程序时，有时会收到“fgets 错误”， 看起来“line == fgets(line, 4096, fp))”失败了，但我 不知道为什么？

有人可以向我解释一下为什么我在这里收到“fgets 错误”吗 有时随机？？

附： 我尝试更改comamnd（从“ls”到“ps”）； 在终端中，我得到了结果：

  PID TTY           TIME CMD
  744 ttys000    0:00.02 -bash

计时器中的新代码：

 -(void)readCommandResult:(NSTimer *) timer
    {
    char ps_cmd[256] = {"ps"};
    BOOL isFgetsOK = NO;

    FILE *fp = popen(ps_cmd, "r"); 
    if (fp) {
        char line[4096];
        while (line == fgets(line, 4096, fp)) {

                        NSLog(@"length:%d line:%s", strlen(line), line);
            isFgetsOK = YES;
        }
        pclose(fp);
    }
    else {

        NSLog(@"popen error");
    }   
    if (!isFgetsOK) {
        NSLog(@"fgets error");
    }
    }

我得到了日志：

19:56:23.781 TopDemo[856:a0f] length:29 line:  PID TTY           TIME CMD
19:56:23.782 TopDemo[856:a0f] length:31 line:  744 ttys000    0:00.02 -bash
19:56:24.781 TopDemo[856:a0f] length:29 line:  PID TTY           TIME CMD
19:56:24.782 TopDemo[856:a0f] length:31 line:  744 ttys000    0:00.02 -bash
19:56:25.781 TopDemo[856:a0f] length:29 line:  PID TTY           TIME CMD
19:56:25.782 TopDemo[856:a0f] length:31 line:  744 ttys000    0:00.02 -bash
19:56:26.780 TopDemo[856:a0f] length:29 line:  PID TTY           TIME CMD
19:56:26.781 TopDemo[856:a0f] length:31 line:  744 ttys000    0:00.02 -bash
19:56:27.782 TopDemo[856:a0f] fgets error
19:56:28.781 TopDemo[856:a0f] length:29 line:  PID TTY           TIME CMD
19:56:28.782 TopDemo[856:a0f] length:31 line:  744 ttys000    0:00.02 -bash
19:56:29.781 TopDemo[856:a0f] length:29 line:  PID TTY           TIME CMD
19:56:29.782 TopDemo[856:a0f] length:31 line:  744 ttys000    0:00.02 -bash
19:56:30.781 TopDemo[856:a0f] length:29 line:  PID TTY           TIME CMD
19:56:30.782 TopDemo[856:a0f] length:31 line:  744 ttys000    0:00.02 -bash
19:56:31.781 TopDemo[856:a0f] length:29 line:  PID TTY           TIME CMD
19:56:31.782 TopDemo[856:a0f] length:31 line:  744 ttys000    0:00.02 -bash
19:56:32.781 TopDemo[856:a0f] length:29 line:  PID TTY           TIME CMD
19:56:32.782 TopDemo[856:a0f] length:31 line:  744 ttys000    0:00.02 -bash
19:56:33.781 TopDemo[856:a0f] length:29 line:  PID TTY           TIME CMD
19:56:33.782 TopDemo[856:a0f] length:31 line:  744 ttys000    0:00.02 -bash
19:56:34.781 TopDemo[856:a0f] length:29 line:  PID TTY           TIME CMD
19:56:34.782 TopDemo[856:a0f] length:31 line:  744 ttys000    0:00.02 -bash
19:56:35.781 TopDemo[856:a0f] length:29 line:  PID TTY           TIME CMD
19:56:35.782 TopDemo[856:a0f] length:31 line:  744 ttys000    0:00.02 -bash
19:56:36.781 TopDemo[856:a0f] length:29 line:  PID TTY           TIME CMD
19:56:36.782 TopDemo[856:a0f] length:31 line:  744 ttys000    0:00.02 -bash
19:56:37.781 TopDemo[856:a0f] length:29 line:  PID TTY           TIME CMD
19:56:37.783 TopDemo[856:a0f] length:31 line:  744 ttys000    0:00.02 -bash
19:56:38.781 TopDemo[856:a0f] length:29 line:  PID TTY           TIME CMD
19:56:38.782 TopDemo[856:a0f] length:31 line:  744 ttys000    0:00.02 -bash
19:56:39.781 TopDemo[856:a0f] length:29 line:  PID TTY           TIME CMD
19:56:39.782 TopDemo[856:a0f] length:31 line:  744 ttys000    0:00.02 -bash
19:56:40.781 TopDemo[856:a0f] length:29 line:  PID TTY           TIME CMD
19:56:40.782 TopDemo[856:a0f] length:31 line:  744 ttys000    0:00.02 -bash
19:56:41.782 TopDemo[856:a0f] fgets error
19:56:42.783 TopDemo[856:a0f] fgets error
19:56:43.782 TopDemo[856:a0f] fgets error
19:56:44.781 TopDemo[856:a0f] length:29 line:  PID TTY           TIME CMD
19:56:44.782 TopDemo[856:a0f] length:31 line:  744 ttys000    0:00.02 -bash
19:56:45.781 TopDemo[856:a0f] length:29 line:  PID TTY           TIME CMD
19:56:45.782 TopDemo[856:a0f] length:31 line:  744 ttys000    0:00.02 -bash
19:56:46.782 TopDemo[856:a0f] fgets error
19:56:47.781 TopDemo[856:a0f] length:29 line:  PID TTY           TIME CMD
19:56:47.782 TopDemo[856:a0f] length:31 line:  744 ttys000    0:00.02 -bash
19:56:48.781 TopDemo[856:a0f] length:29 line:  PID TTY           TIME CMD
19:56:48.782 TopDemo[856:a0f] length:31 line:  744 ttys000    0:00.02 -bash

我尝试运行应用程序几次，出现“fgets 错误” 随机...

Answer 1

从联机帮助页（http://pubs.opengroup.org/onlinepubs/007908799/xsh/fgets.html）：

成功完成后，fgets()
返回 s.如果流位于
end-of-file，文件结束指示符
设置流并使用 fgets()
返回一个空指针。如果读
发生错误时，错误指示器为
流已设置，fgets() 返回一个
空指针并将 errno 设置为
指出错误。

在文件末尾，它返回 null，如果文件少于 4096 个字符，它将在第一次迭代时达到 EOF，因此将无法到达 isFgetsOK = YES;。 Strike>

抱歉，误解了...如果调用 fgets 时流at EOF，则返回 null，如果在调用过程中达到 EOF，则不会返回 null...^^;