我在 C/Linux 上使用 libxml2 从 xml 文件中提取信息。我创建了一个函数来查找某个标记的第一次出现并返回该标记的副本。

例如给定以下 xml 文本:

<a><b>First occurrence of tag b<c>Child node</c></b><b>Second occurrence of tag b</b></a>

我想提取第一个标签，如果存在，则包含所有子标签。

这是我使用的代码的简化版本:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <libxml/parser.h>
#include <libxml/tree.h>

#ifdef LIBXML_TREE_ENABLED

static int
xml_extract_first_occurrence_by_name(xmlNode * start_node, xmlNode * first_occurrence, xmlChar * node_name)
{    
    xmlNode *cur_node = NULL;

    for (cur_node = start_node; cur_node; cur_node = cur_node->next) {
        if ( !xmlStrcmp(cur_node->name, node_name) ) {
            // use libxml2 function xmlCopyNode to recursively copy cur_node (i.e. copy node + children) to first_occurrence
            // (since cur_node will not be valid outside this function)
            *first_occurrence = *xmlCopyNode(cur_node, 1);
            return 1;
        }

        // if not found in parent search recursively in children
        if (cur_node->children)
            if ( xml_extract_first_occurrence_by_name(cur_node->children, first_occurrence, node_name) )
                return 1;
    }
}

int
main(int argc, char **argv)
{
    xmlDoc *doc = NULL;
    xmlNode *root_element = NULL;

    if (argc != 2)
        return(1);

    // initialize libxml2 and check for version mismatches
    LIBXML_TEST_VERSION

    /*parse the file and get the DOM */
    doc = xmlReadFile(argv[1], NULL, XML_PARSE_NOBLANKS);

    /*Get the root element node */
    root_element = xmlDocGetRootElement(doc);

    // allocate memory for the node to be extracted (including up to 10 children)
    xmlNode *first_occurrence = malloc(11 * sizeof(xmlNode));

    // search in tag root_element the first occurrence of tag "b"
    xml_extract_first_occurrence_by_name(root_element, first_occurrence, "b");

    printf("\nFirst occurrence of tag b -> Text content: %s\n", first_occurrence->children->content);
    printf("\nFirst occurrence of tag b -> Child tag: %s\n", first_occurrence->children->next->name);
    printf("\nFirst occurrence of tag b -> Child tag text content: %s\n", first_occurrence->children->next->children->content);

    free(first_occurrence);

    /*free the document */
    xmlFreeDoc(doc);

    /*
     *Free the global variables that may
     *have been allocated by the parser.
     */
    xmlCleanupParser();

    return 0;
}
#else
int main(void) {
    fprintf(stderr, "Tree support not compiled in\n");
    exit(1);
}
#endif

程序按预期运行，也没有编译器警告。如果我通过 valgrind 运行它(启用泄漏检查)，我会收到错误。

valgrind --leak-check=full ./first-occurrence-test.bin first-occurrence-test.xml
==18986== Memcheck, a memory error detector
==18986== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==18986== Using Valgrind-3.10.0.SVN and LibVEX; rerun with -h for copyright info
==18986== Command: ./first-occurrence-test.bin first-occurrence-test.xml
==18986== 

First occurrence of tag b -> Text content: First occurrence of tag b

First occurrence of tag b -> Child tag: c

First occurrence of tag b -> Child tag text content: Child node
==18986== 
==18986== HEAP SUMMARY:
==18986==     in use at exit: 281 bytes in 8 blocks
==18986==   total heap usage: 77 allocs, 69 frees, 47,465 bytes allocated
==18986== 
==18986== 281 (60 direct, 221 indirect) bytes in 1 blocks are definitely lost in loss record 7 of 7
==18986==    at 0x402A17C: malloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==18986==    by 0x409A01E: ??? (in /usr/lib/i386-linux-gnu/libxml2.so.2.9.1)
==18986==    by 0x80487C0: xml_extract_first_occurrence_by_name (first-occurrence-test.c:30)
==18986==    by 0x8048848: xml_extract_first_occurrence_by_name (first-occurrence-test.c:36)
==18986==    by 0x80488FD: main (first-occurrence-test.c:63)
==18986== 
==18986== LEAK SUMMARY:
==18986==    definitely lost: 60 bytes in 1 blocks
==18986==    indirectly lost: 221 bytes in 7 blocks
==18986==      possibly lost: 0 bytes in 0 blocks
==18986==    still reachable: 0 bytes in 0 blocks
==18986==         suppressed: 0 bytes in 0 blocks
==18986== 
==18986== For counts of detected and suppressed errors, rerun with: -v
==18986== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)

我尝试删除该函数并在 main 中执行所有操作。相同的结果。

我在监督什么吗？这是 xmlCopyNode 可能存在的问题(或我不知道的问题)吗？还是误报？

感谢任何帮助或评论。

编辑:我让它工作，关键是确认 xmlCopyNode() 已经分配内存(无需再次分配)并使用 xmlFreeNode() 释放 xmlCopyNode() 分配的内存。谢谢你们的回答。

最佳答案

given that C passes by value, not by reference,
to change where a pointer points to,
a sub function needs the address of the actual pointer
therefore:

this line: 
*first_occurrence = *xmlCopyNode(cur_node, 1);
has a few problems:
Below is what needs to be done to correct the problems
and eliminate the memory leak

-------------
note that:
the xmlCopyNode returns a pointer to a a copy of the original node.
I.E. it returns a pointer, that must be (eventually) passed to some form of free()
-------------

Note: the passed parameter: 'first_occurrence'
is already pointing to an allocated memory area via:
(this is where the leak starts)
xmlNode *first_occurrence = malloc(11 * sizeof(xmlNode));

-------------
suggest changing this line:
*first_occurrence = *xmlCopyNode(cur_node, 1);

to :
*first_occurrence = xmlCopyNode(cur_node, 1);

-------------

then remove the malloc so this line:
xmlNode *first_occurrence = malloc(11 * sizeof(xmlNode));

becomes:
xmlNode *first_occurrence = NULL;

-------------

finally change this line so the parameter contains the address of
rather than the contents of first_occurrence
xml_extract_first_occurrence_by_name(root_element, first_occurrence, "b");

to this:
xml_extract_first_occurrence_by_name(root_element, &first_occurrence, "b");

关于c - xmlCopyNode (libxml2) 内存泄漏？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/27900525/