閱讀命令結果錯誤（fgets）

我用一個按鈕寫了一個簡單的可可應用程序。閱讀命令結果錯誤（fgets）

當單擊該按鈕時，計時器被觸發：

這裏是定時器：

-(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"); 
} 
}

當我運行應用程序時，我會得到「與fgets錯誤」有時，它似乎是「行== fgets（行，4096，fp））」失敗，但我不是爲什麼？

請問有人可以向我解釋爲什麼我在這裏得到一個「fgets錯誤」有時是隨機的？

PS：我試圖改變comamnd（從「ls」到「ps」）; 在終端，我得到的結果是：

PID TTY   TIME CMD 
    744 ttys000 0:00.02 -bash

而在計時器的新代碼：

-(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"); 
    } 
    }

而且我得到的日誌：

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

我想跑應用程序幾次和「fgets錯誤」隨機出來 ...

來源

2011-02-17 supersurabbit

您需要將您的日誌調用放在while循環之外。我們知道while循環沒有輸入問題，否則`isFgetsOK`會被修改。我想知道當fgets沒有返回傳遞給它的指針時你得到了什麼輸出。爲了安全起見，你也應該初始化行「」...... – jswolf19 2011-02-17 13:11:57

另外，`errno`將是一個很好的檢查... – jswolf19 2011-02-17 13:14:22

的手冊頁（http://pubs.opengroup.org/onlinepubs/007908799/xsh/fgets.html）：

成功完成後，與fgets（）回報秒。如果數據流處於文件結尾，則設置流的文件結束指示符，並且fgets（）返回空指針。如果發生讀取錯誤，則設置錯誤指示器，則fgets（）返回空指針，並將errno設置爲指示錯誤。

~~在文件末尾，則返回null，如果該文件是小於4096個字符，這將打擊EOF在第一次循環，所以 isFgetsOK = YES;將無法達成。~~

對不起，曲解...如果流是在 EOF fgets被調用，而不是如果它在呼叫期間到達EOF，它返回空^^;

來源

2011-02-17 08:36:00 jswolf19

閱讀命令結果錯誤（fgets）

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