二维char数组的malloc函数的分段错误

Question

Here is my code snippet to create the 2D array that holds char array. It would be great if someone could find out what could be the reason. I have tried using both malloc() and calloc() to allocate memory to the 2D array, yet no positive signs.

Code Snippet:

char** attrNames = (char **)malloc(3*sizeof(char*))
for (m = 0; m < 3; m++) {
        attrNames[m] = (char *)malloc(2 * sizeof(char*));
        strcpy(schema->attrNames[m], temp_buff2[m]);
    }

I am trying to allocate the memory and then going on a loop and again allocating memory and copy the data from a variable called temp_buff2 (has character data) into the char array.

Answer 1

Try the code below. Even though memory allocation error in your project might be unlikely, now is a good time to develop a good error handling reflex - it will save your bacon when you move on to more serious projects.

Note that char* pointer needs a buffer that is equal to the length of the string plus one extra byte. sizeof(char*) is a small value, only 8 on a 64-bit architecture - it just stores the value of the memory address where the string starts. Note that we need +1 on top of strlen() because strcpy() will store one extra byte (\\0) as a string terminator.

char** attrNames = (char **)malloc(3*sizeof(char*));

if (!attrName)
{
  // handle memory error
}

for (m = 0; m < 3; m++) {
        attrNames[m] = (char *)malloc(strlen(temp_buff2[m])+1);
        if (!attrNames[m]) 
        {
           // handle memory error
        }

        strcpy(schema->attrNames[m], temp_buff2[m]);
    }

Memory error can be handled by returning an error code from your function or via a fatal exit like this:

fprintf(stderr, "Out of memory\n");
exit(1);

You will need to #include <stdlib.h> for the prototype of exit().

Answer 2

You need to reserve enough space for whatever you have inside "temp_buff2". For example:

char** attrNames = (char **)malloc(3*sizeof(char*))
for (m = 0; m < 3; m++) {
    attrNames[m] = (char *)malloc( strlen(temp_buff2[m]) + 1 );
    strcpy(schema->attrNames[m], temp_buff2[m]);
}

Notice that I am adding 1 to the strlen result, this is because we need to reserve an additional byte for the null char terminator.