Thread: Valgrind complaining of unitialized variable with strstr

This is my first post on this forum so I apologize if I do something wrong.

As explained in my title, my code produces the desired result, but valgrind complains of using an uninitialized variable when running. Here's the code; I simplified it, changing some functionality and removing error handling for the sake of brevity and clarity, but still producing the same error:

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define INDEX "/tmp/index.html"
#define DURL "http://bay12games.com/dwarves/"
char *getDownloadURL (char *haystack) { 
   char *a = strstr(haystack,"\">Linux</a>");
   size_t len = a - haystack;
   char intermediary[len+1];
   memcpy(intermediary,haystack,len);
   a = strrchr(intermediary,'\"');
   len = strlen(a + 1) + 1; //Re-define.
   char *stringToReturn = malloc(strlen (DURL) + len);
   memcpy(stringToReturn,DURL,strlen(DURL)+1);
   strcat(stringToReturn,a+1);

   return stringToReturn;
}

int main() {
   FILE *f = fopen(INDEX, "r");
   fseek(f,0,SEEK_END);
   long len = ftell(f);
   fseek(f,0,SEEK_SET);
   char *page = malloc(len+1);
   fread(page,1,len,f);
   fclose(f);
   page[len] = 0;
   char *URL = getDownloadURL(page);
   printf("%s",URL);
   free(URL);
   free(page);
   return 0;

}

The full Valgrind error is:

[spoiler]

Code:

=115998== Memcheck, a memory error detector
==115998== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==115998== Using Valgrind-3.15.0 and LibVEX; rerun with -h for copyright info
==115998== Command: ./debug -s
==115998== 
fread success.
==115998== Conditional jump or move depends on uninitialised value(s)
==115998==    at 0x483B7C8: rindex (vg_replace_strmem.c:202)
==115998==    by 0x10932E: getDownloadURL (debug.c:23)
==115998==    by 0x1094C4: main (debug.c:51)
==115998==  Uninitialised value was created by a stack allocation
==115998==    at 0x109276: getDownloadURL (debug.c:7)
==115998== 
==115998== Conditional jump or move depends on uninitialised value(s)
==115998==    at 0x109338: getDownloadURL (debug.c:24)
==115998==    by 0x1094C4: main (debug.c:51)
==115998==  Uninitialised value was created by a stack allocation
==115998==    at 0x109276: getDownloadURL (debug.c:7)
==115998== 
==115998== Use of uninitialised value of size 8
==115998==    at 0x483BC82: strlen (vg_replace_strmem.c:461)
==115998==    by 0x109370: getDownloadURL (debug.c:28)
==115998==    by 0x1094C4: main (debug.c:51)
==115998==  Uninitialised value was created by a stack allocation
==115998==    at 0x109276: getDownloadURL (debug.c:7)
==115998== 
==115998== Use of uninitialised value of size 8
==115998==    at 0x483BC94: strlen (vg_replace_strmem.c:461)
==115998==    by 0x109370: getDownloadURL (debug.c:28)
==115998==    by 0x1094C4: main (debug.c:51)
==115998==  Uninitialised value was created by a stack allocation
==115998==    at 0x109276: getDownloadURL (debug.c:7)
==115998== 
==115998== Conditional jump or move depends on uninitialised value(s)
==115998==    at 0x483BC98: strlen (vg_replace_strmem.c:461)
==115998==    by 0x109370: getDownloadURL (debug.c:28)
==115998==    by 0x1094C4: main (debug.c:51)
==115998==  Uninitialised value was created by a stack allocation
==115998==    at 0x109276: getDownloadURL (debug.c:7)
==115998== 
==115998== Use of uninitialised value of size 8
==115998==    at 0x483B979: strcat (vg_replace_strmem.c:310)
==115998==    by 0x1093BB: getDownloadURL (debug.c:31)
==115998==    by 0x1094C4: main (debug.c:51)
==115998==  Uninitialised value was created by a stack allocation
==115998==    at 0x109276: getDownloadURL (debug.c:7)
==115998== 
==115998== Conditional jump or move depends on uninitialised value(s)
==115998==    at 0x483B997: strcat (vg_replace_strmem.c:310)
==115998==    by 0x1093BB: getDownloadURL (debug.c:31)
==115998==    by 0x1094C4: main (debug.c:51)
==115998==  Uninitialised value was created by a stack allocation
==115998==    at 0x109276: getDownloadURL (debug.c:7)
==115998== 
==115998== Conditional jump or move depends on uninitialised value(s)
==115998==    at 0x483B9B5: is_overlap (vg_replace_strmem.c:131)
==115998==    by 0x483B9B5: strcat (vg_replace_strmem.c:310)
==115998==    by 0x1093BB: getDownloadURL (debug.c:31)
==115998==    by 0x1094C4: main (debug.c:51)
==115998==  Uninitialised value was created by a stack allocation
==115998==    at 0x109276: getDownloadURL (debug.c:7)
==115998== 
==115998== Conditional jump or move depends on uninitialised value(s)
==115998==    at 0x483B9BA: is_overlap (vg_replace_strmem.c:140)
==115998==    by 0x483B9BA: is_overlap (vg_replace_strmem.c:127)
==115998==    by 0x483B9BA: strcat (vg_replace_strmem.c:310)
==115998==    by 0x1093BB: getDownloadURL (debug.c:31)
==115998==    by 0x1094C4: main (debug.c:51)
==115998==  Uninitialised value was created by a stack allocation
==115998==    at 0x109276: getDownloadURL (debug.c:7)
==115998== 
==115998== Conditional jump or move depends on uninitialised value(s)
==115998==    at 0x483B9BC: is_overlap (vg_replace_strmem.c:143)
==115998==    by 0x483B9BC: is_overlap (vg_replace_strmem.c:127)
==115998==    by 0x483B9BC: strcat (vg_replace_strmem.c:310)
==115998==    by 0x1093BB: getDownloadURL (debug.c:31)
==115998==    by 0x1094C4: main (debug.c:51)
==115998==  Uninitialised value was created by a stack allocation
==115998==    at 0x109276: getDownloadURL (debug.c:7)
==115998== 
==115998== Conditional jump or move depends on uninitialised value(s)
==115998==    at 0x483B9CB: strcat (vg_replace_strmem.c:310)
==115998==    by 0x1093BB: getDownloadURL (debug.c:31)
==115998==    by 0x1094C4: main (debug.c:51)
==115998==  Uninitialised value was created by a stack allocation
==115998==    at 0x109276: getDownloadURL (debug.c:7)
==115998== 
http://bay12games.com/dwarves/df_44_12_linux.tar.bz2==115998== 
==115998== HEAP SUMMARY:
==115998==     in use at exit: 0 bytes in 0 blocks
==115998==   total heap usage: 5 allocs, 5 frees, 440,956 bytes allocated
==115998== 
==115998== All heap blocks were freed -- no leaks are possible
==115998== 
==115998== For lists of detected and suppressed errors, rerun with: -s

[/spoiler]

Line 7, which it seems to complain about, is:

Code:

char *a = strstr(haystack,"\">Linux</a>");

Any ideas? I'm a C noob, so I'm stumped.

Originally Posted by Salem

Code:

   char intermediary[len+1];
   memcpy(intermediary,haystack,len);
   a = strrchr(intermediary,'\"');

memcpy doesn't put \0 characters on the ends of strings.

My god, of course. Thank you so much.