When using pthread for multithreading, most threads call pthread_exit() implicitly on return from the thread start routine. Besides, pthread_exit() also can be used to terminate the initial process thread in main(), leaving other threads to continue operation. The process will go away automatically when the last thread terminates. If you don’t care about the process exit status, or if is difficult to know the created thread IDs(e.g. created by third party APIs), you can call
pthread_detach(pthread_self());
pthread_exit(NULL);
at the end of the main() function.
However, you must be carefull when using pthread_exit() in the main thread. Because after calling pthread_exit() and before the process really terminate, the process becomes “zombie” - it still exists even though it is “dead”, just like a Unix/Linux process that’s terminated but hasn’t yet been “reaped” by a wait operation. The zombie process may retain most or all of the system resources that it used when running, so it is not a good idea to leave threads in this state for longer than necessary. And obviously, zombie process cannot save you resources! So also don’t try pthread_exit() for saving CPU and memory.
I also noticed a undocumented problem caused by pthread_exit() - it may lead to failure of open procfs(/proc/) files! If one of your threads would open /proc/mounts(currently I only find this file will go wrong, and other procfs files like /proc/cpuinfo or /proc/uptime can be successfully opened) during its life, and pthread_exit() is called after creating these threads in the main thread, you will meet the “Invalid argument” error because of functions like open("/proc/mounts",'r').
Following program demonstrates this problem:
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#define NUM_THREADS 5
void *PrintHello(void *threadid)
{
long tid;
tid = (long)threadid;
printf("Hello World! It's me, thread #%ld!\n", tid);
sleep(10); // sleep sometime
/* Read file */
FILE* fp=fopen("/proc/mounts","r"); // would fail
//FILE* fp=fopen("/proc/cpuinfo","r"); // would succeed
if(fp==NULL) {
fprintf(stderr,"Failed to open file!\n");
} else {
char line[80];
if(fgets(line,80,fp)==NULL)
fprintf(stderr,"Failed to read file!\n");
else
fprintf(stdout,"%s\n",line);
fclose(fp);
}
sleep(1200);
pthread_exit(NULL);
}
int main(int argc, char *argv[])
{
pthread_t threads[NUM_THREADS];
int rc;
long t;
for(t=0;t<NUM_THREADS;t++){
printf("In main: creating thread %ld\n", t);
rc = pthread_create(&threads[t], NULL, PrintHello, (void *)t);
if (rc){
printf("ERROR; return code from pthread_create() is %d\n", rc);
exit(-1);
}
}
/* Last thing that main() should do */
pthread_detach(pthread_self());
pthread_exit(NULL);
}
After building above code, say pthread_exit_test, run this program and find the PID of it. Then cat /proc/<PID>/status, you will find the process status like
Name: pthread_exit_te
State: Z (zombie)
Tgid: 3091
Pid: 3091
PPid: 2849
TracerPid: 0
Uid: 370845 370845 370845 370845
Gid: 25 25 25 25
Utrace: 0
FDSize: 0
Groups: 25 1000000312
Threads: 6
SigQ: 0/78966
SigPnd: 0000000000000000
ShdPnd: 0000000000000000
SigBlk: 0000000000000000
SigIgn: 0000000000000004
SigCgt: 0000000180000000
CapInh: 0000000000000000
CapPrm: 0000000000000000
CapEff: 0000000000000000
CapBnd: ffffffffffffffff
Cpus_allowed: 3f
Cpus_allowed_list: 0-5
Mems_allowed: 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000001
Mems_allowed_list: 0
voluntary_ctxt_switches: 14
nonvoluntary_ctxt_switches: 1
For the above program, if replace pthread_exit() with pthread_join() or while(1) {sleep(120); }, it would work well. The while loop is especially usefull when you don’t know the thread id to be joined.
Until now I am still don’t know why openning /proc/mounts would fail and why openning /proc/cpuinfo could succeed in above code. I also tried other system file or link, and found all of them could be successively readed.