The GNU's clone() function got stuck - c

Question

I am writing a program using Code::Blocks 16.01 in CentOS 7.3. The program is contains the main function and a child process (or so-called thread) created by the clone() function. My purpose is to test whether the chdir() function affects the working directory in the main function by removing the CLONE_FS parameter. Hopefully it works, but new problem occurs. Please read my code first:
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <dirent.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <sched.h>
#include <fcntl.h>
#define STACK_SIZE 1024*1024*8
int thread_func(void *arg){
int i;
char *cdir;
for(i = 0; i < 100; i++){
switch(i%3){
case 0:
chdir("/home/centos/dirtest/dir000");
break;
case 1:
chdir("/home/centos/dirtest/dir001");
break;
case 2:
chdir("/home/centos/dirtest/dir002");
break;
}
cdir = getcwd(NULL,0);
fprintf(stderr,"Child Thread in # %d: %s\n",i,cdir);
}
free(cdir);
return 1;
}
int main(){
void *pstack = (void*)mmap(NULL, STACK_SIZE,
PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS | MAP_ANON,
-1, 0);
if(MAP_FAILED != pstack){
int ret,i;
char *cdir;
ret = clone(thread_func,
(void*)((unsigned char *)pstack + STACK_SIZE),
CLONE_VM | CLONE_FILES ,
NULL);
if(ret == -1){
fprintf(stderr,"Thread create failed\n");
return 0;
}
for(i = 0; i < 100; i++){
cdir = getcwd(NULL,0);
fprintf(stderr,"Main Function in # %d: %s\n",i,cdir);
}
free(cdir);
}
return 1;
}
However, when I run the generated exe file in the terminal by command line, it got stuck. Neither the main function nor the child process could finish its "for" loop, and I had to terminate the program by "Ctrl-C".
Could anybody find the problem ?
********Here are edits of new progress********
Thanks for the comments, I've made changes on the getcwd() function. Also I added the waitpid() function. However, it shows failure. The modified code is as follows:
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <dirent.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <sys/wait.h>
#include <sched.h>
#include <fcntl.h>
#define STACK_SIZE 1024*1024*8
int thread_func(void *arg){
int i;
char cdir[1024];
for(i = 0; i < 100; i++){
switch(i%3){
case 0:
chdir("/home/centos/dirtest/dir000");
break;
case 1:
chdir("/home/centos/dirtest/dir001");
break;
case 2:
chdir("/home/centos/dirtest/dir002");
break;
}
getcwd(cdir,sizeof(cdir));
fprintf(stderr,"Child Thread in # %d: %s\n",i,cdir);
}
return 1;
}
int main(){
/*void *pstack = (void*)mmap(NULL, STACK_SIZE,
PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS ,
-1, 0);
if(MAP_FAILED != pstack){*/
void *pstack = malloc(STACK_SIZE);
int ret,i;
char cdir[1024];
ret = clone(thread_func,
(void*)((char *)pstack + STACK_SIZE),
CLONE_VM | CLONE_FILES | CLONE_SIGHAND | CLONE_THREAD,
NULL);
if(ret == -1){
fprintf(stderr,"Thread create failed\n");
goto mem;
}
for(i = 0; i < 100; i++){
getcwd(cdir,sizeof(cdir));
fprintf(stderr,"Main Function in # %d: %s\n",i,cdir);
}
ret = waitpid(ret,0,0);
if(ret == -1){
fprintf(stderr,"waitpid failed\n");
}
mem:
//}
free(pstack);
return 1;
}

Answer 1

I'm quite certain that one of the problems is that you have multiple threads of execution touching libc without actually preparing libc for it. When threads are spawned normally, with pthreads, they run some code that prepares libc for multiple threads touching stdio (you print to stderr), enable locking for malloc (since you do mallocs inside getcwd), set up TLS and generally make sure that things don't run into each other.
You've done none of that. You're of course free to spawn your own threads with clone manually, but then it's your responsibility to make sure that all the code you call is thread safe. You can't just call into libc and hope for the best, that won't work. Libc expects that either there is just one thread executing or that the threads were created with pthreads.