c - 大小 8 valgrind 的无效写入

Question

我已经开发了一个可以在 c 编程语言中调整大小的动态字符串，但是我有一些无效的读取 valgrind 报告给我，我试图找出原因，但我失败了:

PS:程序运行正常。

字符串.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include "error.h"
#include "dstring.h"

#define DEFAULT_SIZE    512 

struct dstring {
    char *str;
    char *estr;
    size_t msize;
};

lgw_return lgw_init_dstring(struct dstring **dstr, const char *data)
{
    size_t len;

    if((*dstr = malloc(sizeof *dstr)) == NULL)
        return LGW_ERROR_MALLOC;        

    if(data != NULL) {
        // intiliaze dstring with value of data
        len = strlen(data);

        if(((*dstr)->str = malloc(len + DEFAULT_SIZE)) == NULL) {
            free(*dstr);
            return LGW_ERROR_MALLOC;
        }

        strcpy((*dstr)->str, data);
        (*dstr)->estr = (*dstr)->str + len;
        (*dstr)->msize = len + DEFAULT_SIZE;
    }
    else {
        // create empty dstring
        if(((*dstr)->str = malloc(DEFAULT_SIZE)) == NULL) {
            free(*dstr);
            return LGW_ERROR_MALLOC;
        }

        *((*dstr)->str) = '\0';
        (*dstr)->estr = (*dstr)->str; 
        (*dstr)->msize = DEFAULT_SIZE;
    }

    return LGW_SUCCESS;
}

lgw_return lgw_destroy_dstring(struct dstring **dstr)
{
    free((*dstr)->str);
    free(*dstr);
    *dstr = NULL;
    return LGW_SUCCESS;
}

lgw_return lgw_write_dstring(struct dstring *dstr, const char *data, size_t off)
{
    size_t datalen = strlen(data);
    char *tmp_str;

    //if offset is beyond the '\0' char return error
    if(off > dstr->estr - dstr->str)
        return LGW_OUT_OF_RANGE;

    if(off + datalen >  dstr->msize) {

        if((tmp_str = realloc(dstr->str, dstr->msize +
            datalen + DEFAULT_SIZE)) == NULL) 

            return LGW_ERROR_MALLOC;    

        dstr->str = tmp_str;
        dstr->msize += datalen + DEFAULT_SIZE;
    }

    strcpy(dstr->str + off, data);
    dstr->estr = dstr->str + off + datalen;

    return LGW_SUCCESS;
}

lgw_return lgw_append_dstring(struct dstring *dstr, const char *data)
{
    return lgw_write_dstring(dstr, data, dstr->estr - dstr->str);
}
const char *lgw_get_dstring(struct dstring *dstr)
{
    return (const char *)(dstr->str);
}

字符串.h

#ifndef DSTRING_H_INCLUDED
#define DSTRING_H_INCLUDED

#include "error.h"

typedef struct dstring DynamicString;

lgw_return lgw_init_dstring(struct dstring **dstr, const char *data);
lgw_return lgw_destroy_dstring(struct dstring **dstr);
lgw_return lgw_write_dstring(struct dstring *dstr, const char *data, size_t off);
lgw_return lgw_append_dstring(struct dstring *dstr, const char *data);
const char *lgw_get_dstring(struct dstring *dstr);

#endif

主.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include "error.h"
#include "dstring.h"


int main()
{
    lgw_return rc;
    DynamicString *s;

    if((rc =lgw_init_dstring(&s, "test")) != LGW_SUCCESS) {
        fprintf(stderr, "ERROR : %s", lgw_strerror(rc));
        exit(EXIT_FAILURE);
    }

    lgw_append_dstring(s, "123");
    const char *str = lgw_get_dstring(s);

    printf("value is : %s\n", str);
    lgw_destroy_dstring(&s);
}

valgrind 输出

==6919== Memcheck, a memory error detector
==6919== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==6919== Using Valgrind-3.10.0 and LibVEX; rerun with -h for copyright info
==6919== Command: ./a.out
==6919== 
==6919== Invalid write of size 8
==6919==    at 0x40084A: lgw_init_dstring (dstring.c:33)
==6919==    by 0x400AD7: main (test.c:14)
==6919==  Address 0x51de048 is 0 bytes after a block of size 8 alloc'd
==6919==    at 0x4C28C20: malloc (vg_replace_malloc.c:296)
==6919==    by 0x4007A0: lgw_init_dstring (dstring.c:20)
==6919==    by 0x400AD7: main (test.c:14)
==6919== 
==6919== Invalid write of size 8
==6919==    at 0x400860: lgw_init_dstring (dstring.c:34)
==6919==    by 0x400AD7: main (test.c:14)
==6919==  Address 0x51de050 is 8 bytes after a block of size 8 alloc'd
==6919==    at 0x4C28C20: malloc (vg_replace_malloc.c:296)
==6919==    by 0x4007A0: lgw_init_dstring (dstring.c:20)
==6919==    by 0x400AD7: main (test.c:14)
==6919== 
==6919== Invalid read of size 8
==6919==    at 0x400A30: lgw_append_dstring (dstring.c:87)
==6919==    by 0x400B21: main (test.c:19)
==6919==  Address 0x51de048 is 0 bytes after a block of size 8 alloc'd
==6919==    at 0x4C28C20: malloc (vg_replace_malloc.c:296)
==6919==    by 0x4007A0: lgw_init_dstring (dstring.c:20)
==6919==    by 0x400AD7: main (test.c:14)
==6919== 
==6919== Invalid read of size 8
==6919==    at 0x40093C: lgw_write_dstring (dstring.c:65)
==6919==    by 0x400A59: lgw_append_dstring (dstring.c:87)
==6919==    by 0x400B21: main (test.c:19)
==6919==  Address 0x51de048 is 0 bytes after a block of size 8 alloc'd
==6919==    at 0x4C28C20: malloc (vg_replace_malloc.c:296)
==6919==    by 0x4007A0: lgw_init_dstring (dstring.c:20)
==6919==    by 0x400AD7: main (test.c:14)
==6919== 
==6919== Invalid read of size 8
==6919==    at 0x40096F: lgw_write_dstring (dstring.c:68)
==6919==    by 0x400A59: lgw_append_dstring (dstring.c:87)
==6919==    by 0x400B21: main (test.c:19)
==6919==  Address 0x51de050 is 8 bytes after a block of size 8 alloc'd
==6919==    at 0x4C28C20: malloc (vg_replace_malloc.c:296)
==6919==    by 0x4007A0: lgw_init_dstring (dstring.c:20)
==6919==    by 0x400AD7: main (test.c:14)
==6919== 
==6919== Invalid write of size 8
==6919==    at 0x400A11: lgw_write_dstring (dstring.c:80)
==6919==    by 0x400A59: lgw_append_dstring (dstring.c:87)
==6919==    by 0x400B21: main (test.c:19)
==6919==  Address 0x51de048 is 0 bytes after a block of size 8 alloc'd
==6919==    at 0x4C28C20: malloc (vg_replace_malloc.c:296)
==6919==    by 0x4007A0: lgw_init_dstring (dstring.c:20)
==6919==    by 0x400AD7: main (test.c:14)
==6919== 
value is : test123
==6919== 
==6919== HEAP SUMMARY:
==6919==     in use at exit: 0 bytes in 0 blocks
==6919==   total heap usage: 2 allocs, 2 frees, 524 bytes allocated
==6919== 
==6919== All heap blocks were freed -- no leaks are possible
==6919== 
==6919== For counts of detected and suppressed errors, rerun with: -v
==6919== ERROR SUMMARY: 6 errors from 6 contexts (suppressed: 0 from 0)

Answer 1

您为分配结构指定了错误的内存量:

if((*dstr = malloc(sizeof *dstr)) == NULL)

既然dstr是(struct dstr **)，那么sizeof(*dstr)就是指针的大小，而不是结构体的大小。要解决这个问题，您可能需要这样写:

if((*dstr = malloc(sizeof **dstr)) == NULL)