Memory leak in basic stack

问题:

I'm working on a real simple stack implementation, and I can't seem to figure out why I have a memory leak. What I expect from the code is that 5 nodes are allocated in push() and 5 nodes are freed in displayAndDestroy(). But Valgrind says I've allocated 6 nodes worth of data and only freed 5. I've been staring at this for a while and I'm not exactly sure where I went wrong.

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#include <stdio.h> #include <stdlib.h> #include <string.h>  typedef struct STACK{     char data[100];     struct STACK *next; } stack;  stack *push( stack *oldTop, char *data ) {     stack *newTop = malloc(sizeof(stack));     newTop->next = oldTop;      if(!data){         strcpy(newTop->data, newTop->next->data);     } else{         strcpy(newTop->data, data);     }      return( newTop ); }  void displayAndDestroy( stack *top ) {     stack *currentTop = top;     stack *temp;      int i=0;     while(currentTop){          printf("stack%d: %s\n", i, currentTop->data );          temp = currentTop->next;         free(currentTop);         currentTop = temp;          i++;     } }  stack *initializer( stack *top, char *fileName ) {     char word[100];     char ch;      FILE *fr = fopen(fileName, "r");      int i=0;     while( (ch=fgetc(fr)) != EOF ){         if( ch == '>' ){             fscanf(fr, "%s\n", word);             top = push( top, word );             i++;         }     }      return top; }  int main() {     stack *top = NULL;      top = initializer( top, "testData.txt" );      displayAndDestroy( top );      return 0; } 

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testData.txt

garbage  >stringone >2nd string >s3  moregarbage >THE 4FOURTH4 STRING  >5 finalgarbage 

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Valgrind says:

==19446== Memcheck, a memory error detector ==19446== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al. ==19446== Using Valgrind-3.10.0.SVN and LibVEX; rerun with -h for copyright info ==19446== Command: ./test ==19446==  stack0: 5 stack1: THE stack2: s3 stack3: 2nd stack4: stringone ==19446==  ==19446== HEAP SUMMARY: ==19446==     in use at exit: 568 bytes in 1 blocks ==19446==   total heap usage: 6 allocs, 5 frees, 1,128 bytes allocated ==19446==  ==19446== 568 bytes in 1 blocks are still reachable in loss record 1 of 1 ==19446==    at 0x4C2AB80: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so) ==19446==    by 0x4EA544C: __fopen_internal (iofopen.c:73) ==19446==    by 0x400880: initializer (test.c:47) ==19446==    by 0x400921: main (test.c:65) ==19446==  ==19446== LEAK SUMMARY: ==19446==    definitely lost: 0 bytes in 0 blocks ==19446==    indirectly lost: 0 bytes in 0 blocks ==19446==      possibly lost: 0 bytes in 0 blocks ==19446==    still reachable: 568 bytes in 1 blocks ==19446==         suppressed: 0 bytes in 0 blocks ==19446==  ==19446== For counts of detected and suppressed errors, rerun with: -v ==19446== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0) 

回答1:

Your memory leak is from the fopen call, as you can see from the stack trace of the leaking allocation -- it comes from the fact that you call fopen to open a file (which allocates a buffer struct on the heap to manage the file) and then never call fclose.

Add a call to fclose after you're done reading the file, and it will go away.

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