pthread_t declarations need to be global? - pthreads

Question

[Edited and added reprex] When I put the declarations below in the beginning of the main() function, the behavior of the threads is erratic and incorrect. When I make these declarations global, or not the first declarations in main(), everything works fine. I'm using mingw-w64 on Windows 10.
pthread_t thr1, thr2, thr3;
Below is the program, it spawns three threads to read integers from three different text files and add them all to a single global variable. The call to pthread_join() for thr3 always fails with error code 3, and the final result of sum is different in different runs. But it all works fine if the pthread_t declarations are in a different location. I hope this is short enough it's less than 70 lines including whitespace.
#include <stdlib.h>
#include <pthread.h>
#include <stdio.h>
long sum = 0;
pthread_mutex_t sumLock = PTHREAD_MUTEX_INITIALIZER;
void* thrFunc(void* arg)
{
char curLine[50];
FILE *inFile = fopen((char*)arg, "r");
long curNum;
if (!inFile) {
printf("Error opening input file %s\n", arg);
pthread_exit((void*)1);
}
while (fgets(curLine, sizeof(curLine), inFile)) {
curNum = strtol(curLine, 0, 0);
pthread_mutex_lock(&sumLock);
sum += curNum;
pthread_mutex_unlock(&sumLock);
}
fclose(inFile);
pthread_exit((void*)0);
}
int main(void)
{
pthread_t thr1, thr2, thr3;
long threadResult;
int err;
if (pthread_create(&thr1, 0, thrFunc, (void*)"C:\\Users\\paulc\\long1.txt")) {
printf("Failed to create thread thr1\n");
exit(1);
}
if (pthread_create(&thr2, 0, thrFunc, (void*)"C:\\Users\\paulc\\long2.txt")) {
printf("Failed to create thread thr2\n");
exit(1);
}
if (pthread_create(&thr3, 0, thrFunc, (void*)"C:\\Users\\paulc\\long3.txt")) {
printf("Failed to create thread thr3\n");
exit(1);
}
if (err = pthread_join(thr1, (void**)&threadResult)) {
printf("failed to join thr1, error code = %d\n", err);
}
if (err = pthread_join(thr2, (void**)&threadResult)) {
printf("failed to join thr2, error code = %d\n", err);
}
if (err = pthread_join(thr3, (void**)&threadResult)) {
printf("failed to join thr3, error code = %d\n", err);
}
printf("Total: %ld\n", sum);
}