用popen遭遇ENOMEM (Cannot allocate memory)的问题

遇到一个popen遭遇ENOMEM (Cannot allocate memory)的问题，记录一下

我需要在程序里获取标准输出的内容，于是在一个模块里使用了popen这个函数，本来一直运行着都没，但是最近这个模块老是出问题，最后定位到是popen调用出错。返回的errno是ENOMEM (Cannot allocate memory)，查看popen的文档并没有ENOMEM 相关的说明，到网上搜索，有人说popen无非是pipe+fork+execel的一个函数，可以跟一下看看是哪个函数的问题。
使用
strace -o app.starce ./app
跟进去看程序的调用信息，找到了对应的出错点
pipe2([17, 18], O_CLOEXEC)              = 0
clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7f084002aab0) = -1 ENOMEM (Cannot allocate memory)
close(18)                               = 0
close(17)  
可见，实际是clone函数出错了，man clone,确实有
ENOMEM Cannot  allocate  sufficient memory to allocate a task structure for the child, or to copy those parts of the caller's context that need to be copied.
写一个使用了fork的小程序，使用strace查看，
13097 getrlimit(RLIMIT_STACK, {rlim_cur=512*1024, rlim_max=512*1024}) = 0
13097 clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7feed3ccf9f0) = 13098
13097 fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 10), ...}) = 0
13097 mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0 <unfinished ...>
的确，fork函数调用了clone的。
我们都知道，linux使用fork创建子进程会copy父进程的堆，栈，静态存储区，文件描述符等等，那么就有可能父进程内存使用过多，导致子进程无法再从剩余的内存上分配内存。

同样使用system也有
rt_sigprocmask(SIG_BLOCK, [CHLD], [], 8) = 0
clone(child_stack=0, flags=CLONE_PARENT_SETTID|SIGCHLD, parent_tidptr=0x7fffed9206f8) = 32710

wait4(32710, [{WIFEXITED(s) && WEXITSTATUS(s) == 0}], 0, NULL) = 32710

那么使用vfork呢
vfork()                           = 13123
fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 10), ...}) = 0
fstat(1,  <unfinished ...>
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0 <unfinished ...>
可将，用vfork就并没有调用clone

于是自己写了一个函数来代替popen

<pre name="code" class="cpp" style="white-space: pre-wrap; word-wrap: break-word;">//#ifdef  OPEN_MAX//static long openmax = OPEN_MAX;//#elsestatic long openmax = 0;//#endif /* * If OPEN_MAX is indeterminate, we're not * guaranteed that this is adequate. */#define OPEN_MAX_GUESS 1024 long open_max(void){    if (openmax == 0) {      /* first time through */        errno = 0;        if ((openmax = sysconf(_SC_OPEN_MAX)) < 0) {           if (errno == 0)               openmax = OPEN_MAX_GUESS;    /* it's indeterminate */           else               printf("sysconf error for _SC_OPEN_MAX");        }    }     return(openmax);}static pid_t    *childpid = NULL;  /* ptr to array allocated at run-time */static int      maxfd;  /* from our open_max(), {Prog openmax} */FILE *vpopen(const char* cmdstring, const char *type){int pfd[2];FILE *fp;pid_tpid;if((type[0]!='r' && type[0]!='w')||type[1]!=0){errno = EINVAL;return(NULL);}    if (childpid == NULL) {     /* first time through */                  /* allocate zeroed out array for child pids */          maxfd = open_max();          if ( (childpid = (pid_t *)calloc(maxfd, sizeof(pid_t))) == NULL)              return(NULL);      }if(pipe(pfd)!=0){return NULL;}if((pid = vfork())<0){return(NULL);   /* errno set by fork() */  }else if (pid == 0) {/* child */if (*type == 'r'){close(pfd[0]);              if (pfd[1] != STDOUT_FILENO) {                  dup2(pfd[1], STDOUT_FILENO);                  close(pfd[1]);              } }else{            close(pfd[1]);              if (pfd[0] != STDIN_FILENO) {                  dup2(pfd[0], STDIN_FILENO);                  close(pfd[0]);              }  }/* close all descriptors in childpid[] */  for (int i = 0; i < maxfd; i++)  if (childpid[ i ] > 0)close(i);          execl("/bin/sh", "sh", "-c", cmdstring, (char *) 0);          _exit(127);}    if (*type == 'r') {          close(pfd[1]);          if ( (fp = fdopen(pfd[0], type)) == NULL)              return(NULL);      } else {          close(pfd[0]);          if ( (fp = fdopen(pfd[1], type)) == NULL)              return(NULL);      }    childpid[fileno(fp)] = pid; /* remember child pid for this fd */      return(fp);  }int vpclose(FILE *fp){    int     fd, stat;      pid_t   pid;        if (childpid == NULL)          return(-1);     /* popen() has never been called */        fd = fileno(fp);      if ( (pid = childpid[fd]) == 0)          return(-1);     /* fp wasn't opened by popen() */        childpid[fd] = 0;      if (fclose(fp) == EOF)          return(-1);        while (waitpid(pid, &stat, 0) < 0)          if (errno != EINTR)              return(-1); /* error other than EINTR from waitpid() */        return(stat);   /* return child's termination status */  }

</pre><pre name="code" class="cpp" style="white-space: pre-wrap; word-wrap: break-word;"><p>//我一度怀疑是程序使用的内存多，导致fork时无法给子进程分配内存</p><p>//但是文档上都说，fork时并非马上分配，只有实际使用到时才真正分配。//如果这样的话，那出现ENOMEM 的错误是为什么呢？不解</p>