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我用一个按钮写了一个简单的可可应用程序。

单击按钮时，将触发计时器：

[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，有时我会收到“fgets error”，似乎“line == fgets(line, 4096, fp))”失败但我不知道为什么？

有人可以向我解释一下为什么我有时会在这里随机出现“fgets错误”吗？

PS：我试图改变命令（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

我尝试多次运行该应用程序，并且“fgets错误”随机出现......

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1 回答 1

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从手册页（http://pubs.opengroup.org/onlinepubs/007908799/xsh/fgets.html）：

成功完成后，fgets() 返回 s。如果流位于文件结尾，则设置流的文件结尾指示符，并且 fgets() 返回一个空指针。如果发生读取错误，则设置流的错误指示符，fgets() 返回空指针并设置 errno 以指示错误。

在文件结束时，它返回 null，如果文件少于 4096 个字符，它将在第一次迭代时命中 EOF，因此isFgetsOK = YES;不会到达。

抱歉，误解了...如果在调用 fgets 时流处于EOF，则返回 null，而不是在调用期间达到 EOF... ^^;

于 2011-02-17T08:36:00.290 回答