c++ - popen ("ps -fu $USER","r") 没有给出限制为 81 个字符长的所有原始输出结果

Question

我使用的是 linux redhat，在我的代码中我有这个简单的流程应该读取 ps -fu $USER 输出的每一行它确实读取了但不是所有行只有一半。

const char* szPs      = "ps -fu $USER";      // The PS command.
bool bSubProcessFound = false;
char szBuf[102400];  //big buffer
FILE* pPsOut = popen(szPs, "r");
  if (pPsOut == NULL) {

     bFailure = true;
     return (0);
  } 
// Go thru all commands.
  std::string sPsLine;

  bool bFound = false;
  int sizeofbug = sizeof(szBuf); //just checking size
  while (fgets(szBuf, sizeof(szBuf), pPsOut) != NULL) {

    LogWrite("befor   process: %s in ps", szBuf);
    // Erase new line at the end of ps string.
    szBuf[strlen(szBuf) - 1] = '\0';
    // Extract command line.
    sPsLine = szBuf;
    printf("after   process: %s in ps", sPsLine.c_str());
  } 

例如，如果我在我得到的 shell 中运行 ps -fu $USER 命令:

fooy    28407 28401  0 09:59 ?        00:00:04 java -Dappsubprocessname=catalina_gui -classpath .:/home/fooy/simple_java_server TCPServer
fooy    26256 26242  2 Oct20 pts/13   00:23:57 /usr/bin/java -Xms128m -Xmx512m -Dorg.eclipse.equinox.p2.reconciler.dropins.directory=/usr/share/eclipse/dropins -XX:CompileCommand=exclude,

但是当我运行我得到的代码时:

fooy    28407 28401  0 09:59 ?        00:00:03 java -Dappsubprocessname=catalin
fooy    26256 26242  1 Oct20 pts/13   00:23:20 /usr/bin/java -Xms128m -Xmx512m

为什么它会切断看起来有 81 个字符限制的行？甚至像这样的简单命令:

char* in = NULL;
  size_t len =0;
  FILE * psAux = popen("/bin/ps ax", "r");
  while(getline(&in,&sizeofbug,psAux)!=-1)
  {
      fputs(in,stdout);
  }

并且每行长度的输出是 MAX 81:

3377 ?        Ss     0:00 httpd -f /home/wwwww02/DevDev/Apache/conf/httpd.conf
 3378 ?        S      0:00 httpd -f /home/wwwww02/DevDev/Apache/conf/httpd.conf
 3380 ?        S      0:00 httpd -f /home/wwwww02/DevDev/Apache/conf/httpd.conf
 3381 ?        S      0:00 httpd -f /home/wwwww02/DevDev/Apache/conf/httpd.conf
 3382 ?        S      0:00 httpd -f /home/wwwww02/DevDev/Apache/conf/httpd.conf
 3383 ?        S      0:00 httpd -f /home/wwwww02/DevDev/Apache/conf/httpd.conf
 3384 ?        S      0:00 httpd -f /home/wwwww02/DevDev/Apache/conf/httpd.conf
 3521 ?        S      0:15 [flush-8:32]
 3616 ?        Ss     0:00 sshd: foo123 [priv]
 3624 ?        S      0:00 sshd: foo123@notty
 3625 ?        Ss     0:00 tcsh -c /usr/libexec/openssh/sftp-server
 3639 ?        S      0:00 /usr/libexec/openssh/sftp-server
 3640 ?        Ss     0:00 tcsh -c /usr/libexec/openssh/sftp-server
 3655 ?        S      0:00 /usr/libexec/openssh/sftp-server
 4150 ?        Sl     0:25 SSSBatchCCC.exe -name ValueTxImporter -ORBInitRef Nam
 4154 ?        S      0:29 [flush-8:48]
 4323 ?        S      0:00 httpd -f /home/wwwww02/DevDev/Apache/conf/httpd.conf
 4515 ?        Ss     0:00 sshd: wwwww02 [priv]
 4526 ?        S      0:00 sshd: wwwww02@notty
 4527 ?        Ss     0:00 tcsh -c /usr/libexec/openssh/sftp-server
 4547 ?        S      0:00 /usr/libexec/openssh/sftp-server
 4657 ?        S      0:01 httpd -f /home/wwwww02/DevDev/Apache/conf/httpd.conf
 4742 ?        Ss     0:00 sshd: foo123 [priv]
 4749 ?        Ss     0:00 sshd: foo123 [priv]
 4754 ?        S      0:00 sshd: foo123@pts/7
 4756 ?        S      0:00 sshd: foo123@notty
 4757 ?        Ss     0:00 tcsh -c /usr/libexec/openssh/sftp-server
 4773 ?        S      0:00 /usr/libexec/openssh/sftp-server
 4782 pts/7    Ss     0:00 -tcsh
 5183 ?        Ss    46:18 /usr/sbin/abrtd
 5228 ?        Ss     2:24 abrt-dump-oops -d /var/spool/abrt -rwx /var/log/messa
 5337 pts/13   S+     0:00 /bin/sh /usr/bin/eclipse
 5338 pts/13   S+     0:00 /usr/lib64/eclipse/eclipse
 5352 pts/13   Sl+    6:37 /usr/bin/java -Xms128m -Xmx512m -Dorg.eclipse.equinox
 5472 ?        S      0:00 httpd -f /home/wwwww02/DevDev/Apache/conf/httpd.conf
 5519 ?        Sl    11:14 libvirtd --daemon
 5864 ?        S      0:01 httpd -f /home/wwwww02/DevDev/Apache/conf/httpd.conf
 6363 ?        S<     0:00 /sbin/udevd -d

Answer 1

是 ps 本身削减了输出。

来自 man ps :

If ps can not determine display width, as when output is redirected (piped) into a file or another command, the output width is undefined. (it may be 80, unlimited, determined by the TERM variable, and so on)

因此，如果通过 popen() 执行 ps，则需要明确告诉 ps 它在打印输出时应采用多少“列”。

再次来自 man ps :

OUTPUT MODIFIERS
...
--cols n set screen width
--columns n set screen width