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Reputation: 229274

glibc, possible race condition between closing FILE* while exiting?

I've distilled a larger program down to the code shown at the bottom. Running this program in valgrind will eventually report this error:

==7234== Invalid read of size 4
==7234==    at 0x34A7275FC8: _IO_file_write@@GLIBC_2.2.5 (in /usr/lib64/libc-2.15.so)
==7234==    by 0x34A7275EA1: new_do_write (in /usr/lib64/libc-2.15.so)
==7234==    by 0x34A7276D44: _IO_do_write@@GLIBC_2.2.5 (in /usr/lib64/libc-2.15.so)
==7234==    by 0x34A7278DB6: _IO_flush_all_lockp (in /usr/lib64/libc-2.15.so)
==7234==    by 0x34A7278F07: _IO_cleanup (in /usr/lib64/libc-2.15.so)
==7234==    by 0x34A7238BBF: __run_exit_handlers (in /usr/lib64/libc-2.15.so)
==7234==    by 0x34A7238BF4: exit (in /usr/lib64/libc-2.15.so)
==7234==    by 0x34A722173B: (below main) (in /usr/lib64/libc-2.15.so)
==7234==  Address 0x542f2e0 is 0 bytes inside a block of size 568 free'd
==7234==    at 0x4A079AE: free (vg_replace_malloc.c:427)
==7234==    by 0x34A726B11C: fclose@@GLIBC_2.2.5 (in /usr/lib64/libc-2.15.so)
==7234==    by 0x40087C: writer (t.c:22)
==7234==    by 0x34A7607D13: start_thread (in /usr/lib64/libpthread-2.15.so)
==7234==    by 0x34A72F167C: clone (in /usr/lib64/libc-2.15.so)

From the above output, this seems to be what's happening:

As far as I can tell, the test program doesn't do anything undefined, but I'd be happy to be wrong on that.

Valgrind hooks into various functions, so it is possible it is a valgrind bug and not glibc.

t.c:

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>

void *test(void *arg)
{
    return NULL;
}
void *writer(void *arg)
{
    for(;;) {
        char a[100];
        FILE *f = fopen("out", "w");

        if(f == NULL)
           abort();

        fputs("Test", f);

        if(fgets(a, 100, stdin))
            fputs(a, f);
        fclose(f);  //line 22
    }

    return NULL;
}

int main(int argc, char *argv[])
{
    pthread_t tid1,tid2;


    pthread_create(&tid1, NULL, writer, NULL);
    pthread_create(&tid2, NULL, test, NULL);
    pthread_join(tid2, NULL);
    //pthread_join(tid1, NULL); //no bug if we wait for writer()
    return 0;
}
//compile: gcc t.c -g -pthread

May take several minutes to trigger an error from valgrind, with:

while [ true ]; do 
  echo test |valgrind --error-exitcode=2 ./a.out  || break  
done

Environment: Fedora 17, glibc-2.15, gcc-4.7.0-5, kernel 3.5.3-1.fc17.x86_64 , valgrind-3.7.0-4

Upvotes: 2

Views: 1568

Answers (1)

David Schwartz
David Schwartz

Reputation: 182837

You have a race condition. You have a thread that calls exit, which is documented to close all open stdio streams. You then have another thread that, potentially after exit has closed it, accesses such a stream. You cannot access a FILE* after it's closed -- it is permitted to point to garbage.

If a thread does something that makes calling exit unsafe, you must ensure you don't call exit. It's really that simple.

Upvotes: 4

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