C free invalid pointer

Question

I am teaching myself C  My goal is to make a C function that just walks a query string and splits on the ampersand and the equals sign  I am getting stuck on this error from Valgrind     5411   Invalid free     delete   delete     realloc     5411      at 0x402AC38  free  in  usr lib valgrind vgpreload memcheck-x86-linux so    5411      by 0x804857C  main  leak c 28    5411    Address 0x420a02a is 2 bytes inside a block of size 8 free d   5411      at 0x402AC38  free  in  usr lib valgrind vgpreload memcheck-x86-linux so    5411      by 0x804857C  main  leak c 28    5411      5411      5411   HEAP SUMMARY    5411       in use at exit  0 bytes in 0 blocks   5411     total heap usage  1 allocs  2 frees  8 bytes allocated   5411      5411   All heap blocks were freed -- no leaks are possible   5411      5411   For counts of detected and suppressed errors  rerun with  -v   5411   ERROR SUMMARY  20 errors from 9 contexts  suppressed  0 from 0    and the backtrace        Error in    leak   free    invalid pointer  0x08c1d00a             Backtrace             lib i386-linux-gnu libc so 6  0x767c2  0xb75f17c2   lib i386-linux-gnu libc so 6  0x77510  0xb75f2510    leak 0x804857d   lib i386-linux-gnu libc so 6   libc start main 0xf5  0xb7594905    leak 0x8048421          Memory map           08048000-08049000 r-xp 00000000 08 05 262764      home danny dev c-qs-parser leak 08049000-0804a000 r--p 00000000 08 05 262764      home danny dev c-qs-parser leak 0804a000-0804b000 rw-p 00001000 08 05 262764      home danny dev c-qs-parser leak 08c1d000-08c3e000 rw-p 00000000 00 00 0           heap  b757a000-b757b000 rw-p 00000000 00 00 0  b757b000-b7729000 r-xp 00000000 08 05 1312132     lib i386-linux-gnu libc-2 17 so b7729000-b772b000 r--p 001ae000 08 05 1312132     lib i386-linux-gnu libc-2 17 so b772b000-b772c000 rw-p 001b0000 08 05 1312132     lib i386-linux-gnu libc-2 17 so b772c000-b772f000 rw-p 00000000 00 00 0  b772f000-b774a000 r-xp 00000000 08 05 1312589     lib i386-linux-gnu libgcc s so 1 b774a000-b774b000 r--p 0001a000 08 05 1312589     lib i386-linux-gnu libgcc s so 1 b774b000-b774c000 rw-p 0001b000 08 05 1312589     lib i386-linux-gnu libgcc s so 1 b774c000-b7750000 rw-p 00000000 00 00 0  b7750000-b7751000 r-xp 00000000 00 00 0           vdso  b7751000-b7771000 r-xp 00000000 08 05 1312116     lib i386-linux-gnu ld-2 17 so b7771000-b7772000 r--p 0001f000 08 05 1312116     lib i386-linux-gnu ld-2 17 so b7772000-b7773000 rw-p 00020000 08 05 1312116     lib i386-linux-gnu ld-2 17 so bfe93000-bfeb4000 rw-p 00000000 00 00 0           stack  Aborted  core dumped    finally here is the code    include  lt stdio h gt   include  lt string h gt   include  lt stdlib h gt   int main            char p      t quote amp k id amp v 10       char p      t quote        char  token      char  tk       char  s      unsigned short int found       s   strdup p        if  s    NULL            while   token   strsep  amp s    amp        NULL                 found   0               printf  TOKEN   s n n   token                while   tk   strsep  amp token           NULL                     printf  TK   s n n   tk                    free tk                                 free token                              free s        return 0      Thanks

User · Answer

You can t call free on the pointers returned from strsep  Those are not individually allocated strings  but just pointers into the string s  that you ve already allocated  When you re done with s altogether  you should free it  but you do not have to do that with the return values of strsep

User · Answer

You re attempting to free something that isn t a pointer to a  freeable  memory address  Just because something is an address doesn t mean that you need to or should free it   There are two main types of memory you seem to be confusing - stack memory and heap memory    Stack memory lives in the live span of the function  It s temporary space for things that shouldn t grow too big  When you call the function main  it sets aside some memory for your variables you ve declared  p token  and so on   Heap memory lives from when you malloc it to when you free it  You can use much more heap memory than you can stack memory  You also need to keep track of it - it s not easy like stack memory    You have a few errors    You re trying to free memory that s not heap memory  Don t do that  You re trying to free the inside of a block of memory  When you have in fact allocated a block of memory  you can only free it from the pointer returned by malloc  That is to say  only from the beginning of the block  You can t free a portion of the block from the inside    For your bit of code here  you probably want to find a way to copy relevant portion of memory to somewhere else   say another block of memory you ve set aside  Or you can modify the original string if you want  hint  char value 0 is the null terminator and tells functions like printf to stop reading the string    EDIT  The malloc function does allocate heap memory     9 9 1 The malloc and free Functions  The C standard library provides an explicit allocator known as the malloc package  Programs allocate blocks from the heap by calling the malloc function     Computer Systems   A Programmer s Perspective  2nd Edition  Bryant  amp  O Hallaron  2011  EDIT 2    The C standard does not  in fact  specify anything about the heap or the stack  However  for anyone learning on a relevant desktop laptop machine  the distinction is probably unnecessary and confusing if anything  especially if you re learning about how your program is stored and executed  When you find yourself working on something like an AVR microcontroller as H2CO3 has  it is definitely worthwhile to note all the differences  which from my own experience with embedded systems  extend well past memory allocation

User · Answer

From where did you get the idea that you need to free token  and free tk   You don t  strsep   doesn t allocate memory  it only returns pointers inside the original string  Of course  those are not pointers allocated by malloc    or similar   so free  ing them is undefined behavior  You only need to free s  when you are done with the entire string   Also note that you don t need dynamic memory allocation at all in your example  You can avoid strdup   and free   altogether by simply writing char  s   p

[c] C free(): invalid pointer

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