c - How to free an allocated memory without losing its value

Question

Hello I wrote this function in C that takes a string of words and returns a two dimensional char array with each case initialized to a word in the right order, I compiled and it did the assigned task

char **decompose_string(char *string)
{
int i,j=0;
char* temp=malloc(sizeof(char*));
char **words_array=malloc((string_words_number(string)+1)*sizeof(char*)); //string_words_number return the number of words in string string
for(i=0;i<string_words_number(string);i++)
{   

    temp=NULL;
    int l=0;
    while(string[j]!=' ' && *string)
    {   
        temp=realloc(temp,(l+1)*sizeof(char));
        temp[l]=string[j];
        j++;l++;
    }
    j++;
    temp[l]='\0';
    tab_mots=realloc(words_array,(string_words_number(string)+1)*sizeof(char)*(j-1));
    words_array[i]=temp;

}
words_array[i]=NULL;
return words_array;}

And my main:

int main()
{
char* string1= "word1 and word2 and word3";
printf("Our initial string: %s\n",string1);
char** words_array1;
printf("After decomposition:\n");
words_array1=decompose_string(string1);
display_words_array(words_array1); //displays each element of the array in a line
words_array1=destroy_array(words_array1);
return 0;}

But as I executed the valgrind command to see weither there are any memory leaks this was the result:

==4648== Memcheck, a memory error detector
==4648== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==4648== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
==4648== Command: ./test_csvl
==4648== 

Our initial array: word1 and word2 and word3
After decomposition:
==4648== Invalid write of size 1
==4648==    at 0x4009D9: decompose_string (in /home/omaima/2I001/TME3/test_csvl)
==4648==    by 0x40078F: main (in /home/omaima/2I001/TME3/test_csvl)
==4648==  Address 0x5204634 is 0 bytes after a block of size 4 alloc'd
==4648==    at 0x4C2FD5F: realloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==4648==    by 0x40097D: decompose_string (in /home/omaima/2I001/TME3/test_csvl)
==4648==    by 0x40078F: main (in /home/omaima/2I001/TME3/test_csvl)
==4648== 
==4648== Invalid read of size 1
==4648==    at 0x4C30F74: strlen (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==4648==    by 0x4EA969B: puts (ioputs.c:35)
==4648==    by 0x400888: display_words_array (in /home/omaima/2I001/TME3/test_csvl)
==4648==    by 0x40079F: main (in /home/omaima/2I001/TME3/test_csvl)
==4648==  Address 0x5204634 is 0 bytes after a block of size 4 alloc'd
==4648==    at 0x4C2FD5F: realloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==4648==    by 0x40097D: decompose_string (in /home/omaima/2I001/TME3/test_csvl)
==4648==    by 0x40078F: main (in /home/omaima/2I001/TME3/test_csvl)
==4648== 
word1
and
word2
and
word3
==4648== 
==4648== HEAP SUMMARY:
==4648==     in use at exit: 8 bytes in 1 blocks
==4648==   total heap usage: 30 allocs, 29 frees, 1,545 bytes allocated
==4648== 
==4648== LEAK SUMMARY:
==4648==    definitely lost: 8 bytes in 1 blocks
==4648==    indirectly lost: 0 bytes in 0 blocks
==4648==      possibly lost: 0 bytes in 0 blocks
==4648==    still reachable: 0 bytes in 0 blocks
==4648==         suppressed: 0 bytes in 0 blocks
==4648== Rerun with --leak-check=full to see details of leaked memory
==4648== 
==4648== For counts of detected and suppressed errors, rerun with: -v
==4648== ERROR SUMMARY: 9 errors from 2 contexts (suppressed: 0 from 0)

I know that the missing free is the one for temp in the function but if I do free it I'll end up losing its value which I need for my array. So my question would be can I free it without losing its value ? or any other solution to guarantee both the functionality of my function and the erase of any memory leaks. Thank you for your time.

Sorry I forgot here's the function I used to free the allocated space:

char **destroy_words_array( char** words_array)
{
    int i;
    for(i=0;i<count_words(words_array);i++) free(words_array[i]); //wount_words return the number of elements of the array
    free(words_array);
    return words_array;
}

标签： cvalgrindfree

Answer 1

内存泄漏不是你想的那样。它发生在您的功能开始时：

char* temp=malloc(sizeof(char*));
char **words_array=malloc((string_words_number(string)+1)*sizeof(char*)); 
for(i=0;i<string_words_number(string);i++)
{   

    temp=NULL;

您为指向 的指针分配空间，然后在进入循环temp时覆盖该指针。for删除该malloc呼叫，泄漏就消失了。

但是，您有一个更大的问题，那就是在分配的缓冲区末尾进行读写。这发生在这里：

temp=NULL;
int l=0;
while(string[j]!=' ' && *string)
{   
    temp=realloc(temp,(l+1)*sizeof(char));
    temp[l]=string[j];
    j++;l++;
}
j++;
temp[l]='\0';

假设有问题的字符串有两个字符要读取。在循环的第一次迭代中，l为 0，因此您将l+1 == 1字节分配给temp. 然后写入temp[l] == temp[0]，这很好，然后递增l到 1。在下一次迭代中，为 分配l+1 == 2字节temp，写入temp[l] == temp[1]并递增l到 2。到目前为止仍然很好。

问题是当你temp[l]='\0';在循环之外做的时候。 l现在是 2，分配的内存大小是 2，所以你在数组末尾写了一个元素。

您需要在这里再分配一个字节：

j++;
temp=realloc(temp,(l+1)*sizeof(char));
temp[l]='\0';

另请注意，您应该检查整个代码的返回值，malloc以防realloc万一失败。

此外，您没有正确检查string. 你需要做：

while(string[j]!=' ' && string[j]!=0)