c++ - call system() return 256

Question

I want to system() to print pstack info in my application, but it failed and return 256.But I pick up these codes as below to one new main function,that's ok. why?

char cmd[256] ;
string pstackCmd;
struct timeval tv;
struct tm lt;(&tv, NULL);

localtime_r(&tv.tv_sec, &lt);
sprintf(cmd, "pstack `pgrep app` > pstack_%04d%02d%02d%02d%02d%02d%03d",
                            lt.tm_year+1900,
                            lt.tm_mon+1,
                            lt.tm_mday,
                            lt.tm_hour,
                            lt.tm_min,
                            lt.tm_sec,
                            (int)(tv.tv_usec/1000));
pstackCmd = string(cmd);
int retValue = system(pstackCmd.c_str());
if ( retValue != 0)
{
    printf("pstack `pgrep app`,retValue:%d",retValue);
}

Answer 1

Instead of this:

if (retValue != 0)

You need this:

if (retValue == -1 || WEXITSTATUS(retValue) != 0)

That is, you check for failure first by seeing if system() returned -1; if it did not then the exit status of the command (with zero meaning success, traditionally).