我有以下C程序。它询问用户的坐标数量。然后使用malloc分配内存,将坐标(整数)存储在已分配的内存中,然后释放内存。
#include<stdio.h>
#include<stdlib.h>
int main(int argc, char *argv[]) /* Arguments to main() not necessary but used to keep with convention.*/
{
int num_of_coordinates;
printf("How many co-ordinates: ");
scanf("%d", &num_of_coordinates);
int *coordinate_array = malloc(num_of_coordinates * 2);
int i, j;
/* Below for loop takes the x and y coordinate of all points. */
/* These coordinates are stored in coordinate_aaray[]. */
for (i=0; i < num_of_coordinates*2; i++)
{
j = (i / 2) + 1 ;
if (i % 2 != 0)
{
printf("Enter x coordinate of point %d: ",j);
scanf("%d",&coordinate_array[i]);
}
else
{
printf("Enter y-coordinate of point %d: ",j);
scanf("%d",&coordinate_array[i]);
}
}
for (i=0; i < num_of_coordinates*2; i++)
{
printf("%d ",coordinate_array[i]);
}
printf("\n");
/* Free the allocated memory. */
free (coordinate_array);
return 0;
}
当我运行这些程序时,在number_of_coordinates
等于或小于3之前我没有遇到任何问题。
-bash-4.1$ ./a.out
How many co-ordinates: 3
Enter y-coordinate of point 1: 1
Enter x coordinate of point 1: 2
Enter y-coordinate of point 2: 3
Enter x coordinate of point 2: 4
Enter y-coordinate of point 3: 5
Enter x coordinate of point 3: 6
1 2 3 4 5 6
-bash-4.1$
但是当我给num_of_coordinates
一个4或更多的值时,我得到一个运行时错误(很可能是因为free(coordinate_array)
)。
-bash-4.1$ ./a.out
How many co-ordinates: 4
Enter y-coordinate of point 1: 1
Enter x coordinate of point 1: 2
Enter y-coordinate of point 2: 3
Enter x coordinate of point 2: 4
Enter y-coordinate of point 3: 5
Enter x coordinate of point 3: 6
Enter y-coordinate of point 4: 7
Enter x coordinate of point 4: 8
1 2 3 4 5 6 7 8
*** glibc detected *** ./a.out: free(): invalid next size (fast): 0x000000000185b010 ***
实际上运行时错误消息很长,所以我只是显示了该错误的第一行。
为什么在num_of_coordinates
大于或等于4的情况下会发生此错误?
感谢。
答案 0 :(得分:1)
我犯了一个愚蠢的错误。
我使用以下声明分配了不足的内存量。
int *coordinate_array = malloc(num_of_coordinates * 2);
但是,由于每个数字都是int
,我必须使用以下声明。
int *coordinate_array = malloc(num_of_coordinates * 2 * sizeof(int));
进行此更改后,程序无需运行时错误。
-bash-4.1$ ./a.out
How many co-ordinates: 4
Enter y-coordinate of point 1: 1
Enter x coordinate of point 1: 2
Enter y-coordinate of point 2: 3
Enter x coordinate of point 2: 4
Enter y-coordinate of point 3: 5
Enter x coordinate of point 3: 6
Enter y-coordinate of point 4: 7
Enter x coordinate of point 4: 8
1 2 3 4 5 6 7 8
-bash-4.1$
答案 1 :(得分:1)
这一行存在许多问题:
int *coordinate_array = malloc(num_of_coordinates * 2);
1) the amount of allocated memory is 1byte * num_of_coordinates * 2
That is not large enough to hold num_of_coordinates*2 integers
use:
int *coordinate_array = malloc(num_of_coordinates * 2 * sizeof int);
2) always check the returned value from malloc (and family)
to assure the operation was successful
int *coordinate_array = NULL;
if( NULL == (coordinate_array = malloc(num_of_coordinates * 2 * sizeof int) ) )
{ // then, malloc failed
perror( "malloc for coordinate array failed" );
exit( EXIT_FAILURE );
}
// implied else, malloc successful
答案 2 :(得分:0)
确保分配所需的确切字节数。正如上面提到的另一个用户,你应该使用:
malloc(num_of_coordinates * sizeof(int)* 2)
另外,只需要仔细检查一下,如何记录值以测试代码。例如,记录num_of_coordinates以确保您扫描了正确的值。
要进行调试,您可以尝试以下方法:http://dmalloc.com/