Strange behavior with errno...

[Wodarczyk Christopher-CWODARC1]

I am running a program which uses popen followed by fgets.  popen is used to 
redirect output from the 'bin/ps -a' command to a file pointer which is parsed 
by fgets.  I am seeing some strange behavior with the methods though....  here 
is the code as we've had it before....

    const int lineSize=80;
    char *cmd="/bin/ps -a"; // Run ps for all processes
    FILE *ptr;
    char buf[lineSize];
    char *unixPidStr, *junk, *process_name;
    int fileCnt = 0;
    char  *p = NULL;
    char  **pp = &p;
    std::ostringstream oss;

    if((ptr=popen(cmd,"r"))==NULL)
    {
      return(OM_FAIL);
    }
    // Read until list of PIDs is empty or until 5 are read, the max
    // Make sure to read and discard the first line, which is the header

    if(fgets(buf, lineSize, ptr) == NULL )
    {
      oss << "Any errors? " << strerror(errno) << endl;
      logInfo((oss.str()).c_str(), LogOrDisplay_t_LOG_ONLY, FN_ADMIN);
      pclose(ptr);
      return(OM_FAIL);
    }
    else
      logInfo(buf, LogOrDisplay_t_LOG_ONLY, FN_ADMIN);

-----------------------------------------------------------------------------

Now, if I change the code to this....

    const int lineSize=80;
    char *cmd="/bin/ps -a"; // Run ps for all processes
    FILE *ptr;
    char buf[lineSize];
    char *unixPidStr, *junk, *process_name;
    int fileCnt = 0;
    char  *p = NULL;
    char  **pp = &p;
    std::ostringstream oss;

    errno = 0; // Reset errno to 0
    if((ptr=popen(cmd,"r"))==NULL)
    {
      return(OM_FAIL);
    }
    // Read until list of PIDs is empty or until 5 are read, the max
    // Make sure to read and discard the first line, which is the header

    if(fgets(buf, lineSize, ptr) == NULL )
    {
      oss << "Any errors? " << strerror(errno) << endl;
      logInfo((oss.str()).c_str(), LogOrDisplay_t_LOG_ONLY, FN_ADMIN);
      pclose(ptr);
      return(OM_FAIL);
    }
    else
      logInfo(buf, LogOrDisplay_t_LOG_ONLY, FN_ADMIN);

    I end up in the else case and fgets parses my file just fine... is there 
some weird thing going on with errno?  I looked up the manpages which indicates 
errno is thread-safe (my program is multi-threaded) and I don't specifically 
set errno in any of my functions that I use anywhere.  It just seems necessary 
to zero out errno before I do the popen and fgets system calls....

BTW... without resetting errno the fault doesn't occur all the time.  
Basically, this code is timed and gets executed about every 10 seconds or so... 
out 10 times, the fault occurs about 6 times... so it passes 4 out of 10 times 
without resetting errno to 0.

If I do decide to go with a fix with resetting errno to 0, is there any 
possible impact it could have on something...?

Thanks for any input!

Christopher Wodarczyk