freeing a file pointer without upsetting Valgrind

**Jesdisciple** · 03-16-2009

I thought I was finally clear of bugs when Valgrind printed a huge list of errors about a file I open and later free. They all come from "(below main)"; here are the first and last:

Code:

==32022== Invalid read of size 4
==32022==    at 0x40B11AD: (within /lib/tls/i686/cmov/libc-2.8.90.so)
==32022==    by 0x40B1E7F: (within /lib/tls/i686/cmov/libc-2.8.90.so)
==32022==    by 0x4072DE9: exit (in /lib/tls/i686/cmov/libc-2.8.90.so)
==32022==    by 0x405A68C: (below main) (in /lib/tls/i686/cmov/libc-2.8.90.so)
==32022==  Address 0x41a49f8 is 104 bytes inside a block of size 352 free'd
==32022==    at 0x4024E5A: free (vg_replace_malloc.c:323)
==32022==    by 0x804A3F6: outline_free (outline.c:324)
==32022==    by 0x8049369: main (main.c:234)

==32022== Invalid read of size 4
==32022==    at 0x416BD4A: (within /lib/tls/i686/cmov/libc-2.8.90.so)
==32022==    by 0x416BDD9: (within /lib/tls/i686/cmov/libc-2.8.90.so)
==32022==    by 0x401F422: _vgnU_freeres (vg_preloaded.c:60)
==32022==    by 0x40E12B3: _Exit (in /lib/tls/i686/cmov/libc-2.8.90.so)
==32022==    by 0x405A68C: (below main) (in /lib/tls/i686/cmov/libc-2.8.90.so)
==32022==  Address 0x41a49ec is 92 bytes inside a block of size 352 free'd
==32022==    at 0x4024E5A: free (vg_replace_malloc.c:323)
==32022==    by 0x804A3F6: outline_free (outline.c:324)
==32022==    by 0x8049369: main (main.c:234)

And here are the involved functions:

Code:

struct outline *outline_load(char *title){
    struct outline *this;
    if(title == (char *)NULL || strcmp(title, "") == 0)
        return (struct outline *)NULL;
    
    this = outline_create(title);
    char *path = malloc(strlen(outline_home) + 7 + strlen(title) + 1),
         *pattern = "%s" SLASH "files" SLASH "%s";
    CHECK(path);
    
    sprintf(path, pattern, outline_home, title);
    this->old = file_exists(path);
    if(this->old == 0)
        COMMAND("echo >", path); 
    this->file = fopen(path, "w+");
    
    ...
    
    free(path);
    return this;
}

void outline_free(void *o){
    struct outline *this = (struct outline *)o;
    outline_item_free(this->root);
    free(this->file);
    free(this);
}

I don't recall doing anything significant just before the errors appeared... Any ideas?

Thanks!
Chris

**matsp** · 03-16-2009

You should NOT free what you get back from fopen() - it is a pointer, but there is absolutely no certainty that it is dynamically allocated - and if it is, fclose() should free it as well - so you freeing this->file is not meaningful.

It is also very likely that fopen() is designed something along this way [certainly NOT the code you would find in the C library, but the concept is likely]:

Code:

FILE files[10000];
int nFiles; 

FILE *fopen(...)
{
   ....  a load of code to actually open a file. 
   nFiles++;
   return &files[nFiles];
}

--
Mats

**Jesdisciple** · 03-16-2009

Oh... That works; thanks!

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