使用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) = 0clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7f084002aab0) = -1 ENOMEM (Cannot allocate memory)close(18) = 0close(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}) = 013097 clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7feed3ccf9f0) = 1309813097 fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 10), ...}) = 013097 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) = 0clone(child_stack=0, flags=CLONE_PARENT_SETTID|SIGCHLD, parent_tidptr=0x7fffed9206f8) = 32710wait4(32710, [{WIFEXITED(s) && WEXITSTATUS(s) == 0}], 0, NULL) = 32710那么使用vfork呢vfork() = 13123fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 10), ...}) = 0fstat(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

//#ifdef  OPEN_MAX
//static long openmax = OPEN_MAX;
//#else
static 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_t	pid;

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 */

}

//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//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