I am getting confusing output from valgrind when running the memory checker
on my pthread code. Specifically, it seems to not like the use of
the pthread_attr_setstack() function. I have reduced my problem to a small
example, pasted at the end of this message. What I'm trying to do is:
1. Allocate a 64KB buffer for use as the thread stack
2. Call pthread_attr_setstack() to set the stack attributes
3. Call pthread_create(), let the thread run to completion, and then
pthread_join()
4. Clear out the threads stack via memset() to ensure I don't leave any
interesting data in the heap.
5. free() the thread stack
This all seems to work fine, but valgrind complains that the memset() in
step 4 is an invalid write. An example of this error is here:
==26698== Invalid write of size 8
==26698== at 0x4C3665A: memset (in
/usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==26698== by 0x108A4E: main (in /home/cteague/develop/hello/pthread_test)
==26698== Address 0x545ae88 is 59,912 bytes inside a block of size 65,536
alloc'd
==26698== at 0x4C2FB0F: malloc (in
/usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==26698== by 0x1089B6: main (in /home/cteague/develop/hello/pthread_test)
I am very confused. How can it be an invalid write inside my own block of
allocated memory? It looks to me like a problem with valgrind - but my
experience has been that valgrind is incorrect far less frequently than I
am! My command line is this:
valgrind --leak-check=full ./pthread_test
And my test code is here below. I would really appreciate any insight into
this problem. A few notes:
- I thought the problem was the "stack guard" in pthread, but
using pthread_attr_setguardsize() to set it to 0 did not help.
- Yes, I do need to use pthread_attr_setstack(). I am on an embedded
platform where I need to specify from which memory the stack will live in.
- I have reproduced this on valgrind 3.10 and 3.13
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <pthread.h>
static void* thread_start(void *arg)
{
printf("thread running\n");
return NULL;
}
int main(int argc, char **argv)
{
pthread_t thd;
pthread_attr_t thd_attr;
size_t stack_size = 64*1024;
void* p_stack;
printf("start\n");
p_stack = malloc(stack_size);
if (NULL != p_stack) {
if (0 == pthread_attr_init(&thd_attr)) {
if (0 == pthread_attr_setstack(&thd_attr, p_stack, stack_size)) {
if (0 == pthread_create(&thd, &thd_attr, thread_start, NULL)) {
printf("thread created\n");
pthread_join(thd, NULL);
}
}
pthread_attr_destroy(&thd_attr);
}
memset(p_stack, 'A', stack_size);
free(p_stack);
}
printf("done\n");
return 0;
}
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