mutil_thread append string cause core - c++

Question

i use multi thread to update each item(string) of global vector
each thread update item(string) with different index
i think is a good way to avoid updating same data
but i still get core, i do not know why
extern vector<string> gTestVec;
#define NUM 10
void * worker(void * args) {
thread_data * p = (thread_data *)args;
int i = p->thread_id;
for (int j=0; j<100; j++) {
gTestVec[i] += "a";
}
return NULL;
}
void do_complete_stage_test::excute() {
int i = 0;
pthread_t thd[NUM];
thread_data data[NUM];
for (i=0; i<NUM; i++) {
gTestVec.push_back(format("%d", i));
data[i].thread_id = i;
if (0 != pthread_create(&(thd[i]), NULL, &worker, (void *)&data[i])) {
printf("pthread_create failed");
}
}
for (int i=0; i<NUM; i++) {
if (0 != pthread_join(thd[i], NULL)) {
printf("pthread_join failed");
}
}
}
when i run the code,sometimes get coredump
Starting program: /data/settle_script/isp_tran_collect/bin/isp_tran_collect -p 2134234
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
[New Thread 0x7ffff2623700 (LWP 6316)]
[New Thread 0x7fffefb0e700 (LWP 6317)]
[New Thread 0x7fffef30d700 (LWP 6318)]
Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7fffef30d700 (LWP 6318)]
0x00007ffff6787d67 in ?? () from /lib64/libstdc++.so.6
(gdb) bt
#0 0x00007ffff6787d67 in ?? () from /lib64/libstdc++.so.6
#1 0x00007ffff678899b in std::string::reserve(unsigned long) () from /lib64/libstdc++.so.6
#2 0x00007ffff6788bbf in std::string::append(char const*, unsigned long) () from /lib64/libstdc++.so.6
#3 0x000000000044babe in append (__s=0x880492 "a", this=<optimized out>) at /usr/include/c++/4.8.2/bits/basic_string.h:1009
#4 operator+= (__s=0x880492 "a", this=<optimized out>) at /usr/include/c++/4.8.2/bits/basic_string.h:942
#5 worker (args=<optimized out>) at ../src/do_complete_stage_test.cpp:21
#6 0x00007ffff7bc6e25 in start_thread () from /lib64/libpthread.so.0
#7 0x00007ffff5ee635d in clone () from /lib64/libc.so.6
thanks for your help!!!

Answer 1

You are potentially changing the capacity of the vector after you already started some threads.
The easiest way to prevent the vector from re-allocating and moving its contents is to reserve the amount of space before you start the first worker thread.
So call
gTestVec.reserve(NUM);
Before your loop.