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关于内存分配和值分配的基本C问题

memory-management c

VHristov · 技术社区 · 16 年前

我最近开始用C写硕士论文,我已经很久没用了。由于习惯了Java,我现在一直面临着各种各样的问题。我希望有人能帮我做下一件事,因为我已经挣扎了两天了。

typedef struct attribute
{
    int type;
    char * name;
    void * value;
} attribute;

typedef struct tuple
{
    int tuple_id;
    int attribute_count;
    attribute * attributes;
} tuple;

typedef struct table
{
    char * name;
    int row_count;
    tuple * tuples;
} table;

数据来自一个带有插入的文件(为威斯康辛州基准测试生成),我正在解析这个文件。我只有整数值或字符串值。示例行如下所示:

insert into table values (9205, 541, 1, 1, 5, 5, 5, 5, 0, 1, 9205, 10, 11, 'HHHHHHH', 'HHHHHHH', 'HHHHHHH');

我已经“设法”加载和解析数据,并分配它。但是,赋值位有问题,因为所有的值都指向相同的内存位置,也就是说,在我加载数据之后,所有的行看起来都一样。我是这样做的:

char value[10]; // assuming no value is longer than 10 chars
int i, j, k;

table * data = (table*) malloc(sizeof(data));
data->name = "table";
data->row_count = number_of_lines;
data->tuples = (tuple*) malloc(number_of_lines*sizeof(tuple));

tuple* current_tuple;

for(i=0; i<number_of_lines; i++)
{
    current_tuple = &data->tuples[i];
    current_tuple->tuple_id = i;
    current_tuple->attribute_count = 16; // static in our system
    current_tuple->attributes = (attribute*) malloc(16*sizeof(attribute));

    for(k = 0; k < 16; k++)
    {
        current_tuple->attributes[k].name = attribute_names[k];

        // for int values:
        current_tuple->attributes[k].type = DB_ATT_TYPE_INT;
        // write data into value-field
        int v = atoi(value);
        current_tuple->attributes[k].value = &v;

        // for string values:
        current_tuple->attributes[k].type = DB_ATT_TYPE_STRING;
        current_tuple->attributes[k].value = value;
    }

    // ...
}

 memcpy(current_tuple->attributes[k].value, &v, sizeof(int));

这将导致错误的访问错误。以下代码也是如此(因为我不确定哪一个是正确的用法):

 memcpy(current_tuple->attributes[k].value, &v, 1);

我都不知道我需要的是不是memcpy。。。

我也尝试过分配内存,比如:

 current_tuple->attributes[k].value = (int *) malloc(sizeof(int));

只得到“malloc:**error for object 0x100108e98:incorrect checksum for freed object-object maybuly was modified after been freed.”据我所知,这个错误已经为这个对象分配了内存,但是我不知道发生了什么。malloc(sizeof(attribute))是否只分配了存储一个整数和两个指针所需的内存(即,不是指针指向的内存)?

任何帮助都将不胜感激!

瓦西里

3 回复 | 直到 16 年前

BobbyShaftoe 16 年前

所以,你有几件事要做。首先,不要强制转换 malloc 用C语言编程。接下来,这会引起麻烦:

int v = atoi(value);
current_tuple->attributes[k].value = &v;

您在堆栈上分配了指向内存的值。如果在当前“v”超出范围后访问它,那就太糟糕了。

另外,您没有使用任何分支条件来确定值应该是string还是int。因此,如果您像前面提到的那样正确地分配了内容,那么最终将导致内存泄漏。我怀疑这部分是因为这只是一个例子。

检查返回值 马洛克 . 您可以创建一个包装函数来完成这项工作。另外,您可能需要做一些更好的日志记录。

马洛克

“和” /"

C Programming Language

char value[10]; /* assuming no value is longer than 10 chars */
int i, j, k;

table * data = malloc(sizeof(table));
     if(!data)
         exit(1); /* error */

data->name = "table";
data->row_count = number_of_lines;
data->tuples = malloc(number_of_lines*sizeof(tuple));
     if(!data->tuples)
         exit(1); /* error */

tuple* current_tuple;

for(i=0; i<number_of_lines; i++)
{
    current_tuple = &data->tuples[i];
    current_tuple->tuple_id = i;
    current_tuple->attribute_count = 16; /* static in our system */
    current_tuple->attributes = malloc(16*sizeof(attribute));

    for(k = 0; k < 16; k++)
    {
        current_tuple->attributes[k].name = attribute_names[k];

                        if(k % 2)
                        {
               /* for int values:*/
               current_tuple->attributes[k].type = DB_ATT_TYPE_INT;
               /* write data into value-field */
                               current_tuple->attributes[k].value = malloc(sizeof(int));
                               if(!current_tuple->attributes[k].value)
                               {
                                    exit(1); /* error */
                               }    
               *current_tuple->attributes[k].value = atoi(value);

                        }
                        else
                        {
               /* for string values: */
               current_tuple->attributes[k].type = DB_ATT_TYPE_STRING;
                               current_tuple->attributes[k].value = malloc(strlen(value) +1);
                               if(!current_tuple->attributes[k].value)
                               {
                                    exit(1); /* error */
                               }
               strcpy(current_tuple->attributes[k].value, value);
                        }
    }


}

Carl Norum 16 年前

因此,对于字符串,您将在 attribute ,但对于整数,是否要将整数本身放入值字段?那你太努力了。只需使用:

current_tuple->attributes[k].value = (void *)v;

如果要保存指向整数的指针,则需要为其分配空间,因为存储指向局部作用域变量的指针将以失败告终。像这样:

int *v = malloc(sizeof(int));
*v = atoi(value);
current_tuple->attributes[k].value = v;

同样,对于字符串,您总是存储相同的局部变量指针 value

renick 15 年前

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

#define STR_TYPE 1
#define INT_TYPE 2

typedef struct tagAttribute
{
    int type;
    char *name; 
    // anonymous union here
    union {
        char *value;
        int  ivalue;
    };
} attribute, *PATTRIBUTE;


typedef struct tagTuple
{
    int tuple_id;
    int attribute_count;
    attribute *attributes;
} tuple, *PTUPLE;


typedef struct tagTable
{
    char *name;
    int row_count;
    tuple *tuples;
} table, *PTABLE;

    // allocator for table 
PTABLE allocTable(char* name, int row_count) {
    PTABLE mytable = calloc(sizeof(table),1);
    mytable->row_count = row_count;
    mytable->name = strdup(name);
    mytable->tuples = (PTUPLE)calloc(row_count,sizeof(tuple));
    for(int i=0; i< row_count; i++) {
        mytable->tuples[i].tuple_id= i; // ?
    }
    return(mytable);
}

    // allocator for attributes
void allocAttributes( PTUPLE mytuple,  int attr_count) {
    mytuple->attributes =   (PATTRIBUTE) calloc(attr_count, sizeof(attribute));
    mytuple->attribute_count = attr_count;
}

void setAttributeStr(PATTRIBUTE pa, char *name, char *value) {
    pa->type = STR_TYPE;
    pa->name = strdup(name);
    pa->value = strdup(value);
}

void setAttributeInt(PATTRIBUTE pa, char *name, int value) {
    pa->type = INT_TYPE;
    pa->name = strdup(name);
    pa->ivalue = value;

}

int main (int argc, const char * argv[]) {
    // insert code here...
    printf("Test!\n");

        // allocate a table with two tuples
    PTABLE mytable = allocTable("my_table", 2);
        // allocate two attributes in the first tuple

    PTUPLE t0 = & mytable->tuples[0];
    PTUPLE t1 = & mytable->tuples[1];

    allocAttributes( t0, 2);
    allocAttributes( t1, 2);

    setAttributeStr(  &t0->attributes[0], "my_strng_field", "value0");
    setAttributeInt( &t0->attributes[1], "my_int_field", 0xBEEF);
        // now assign 
    setAttributeStr(  &t1->attributes[0], "my_second_strng_field", "value0");
    setAttributeInt( &t1->attributes[1], "my__second_int_field", 0xDEAD);


    return 0;
}

你需要小心你的分配
或者对字符串使用strdup(),以便它们为您处理此问题

在我的MAC上工作。