问题
Code directory structure,
./Computing$ ls -LR
.:
list file.txt mergeSort.c program.exe type.h
./list:
arrayImpl.c linkedListImpl.c list.h
Compilation procedure:
$./Computing
gcc -Wall -Werror -DARRAY -I. mergeSort.c ./list/*.c -o program
Here is the complete code having files, mergeSort.c, list/*, type.h
With the given representation of List,
typedef struct List{
void **array;
/* Housekeeping - Array enhancement/shrink */
int lastItemPosition;
int size;
}List;
mergesort is performed below on list->array, where aux maintains the shallow copy of array
void mergeSort(List *list, size_t listSize, isLess less){
if(list != NULL){
void **aux = malloc(list->size * sizeof(void*)); //Auxillary shadow copy
if(aux != NULL){
printf("Size of list: %d\n", listSize);
mSort(list->array, aux, 0, listSize-1, less);
}else{
fprintf(stderr, "mergeSort() - Malloc failure");
exit(EXIT_FAILURE);
}
}else{
fprintf(stderr, "mergeSort() - List is NULL");
}
}
static void mSort(void **array, void **aux, int low, int high, isLess less){
if(high <= low) return;
int mid = (low + high)/2;
mSort(array, aux, low, mid, less);
mSort(array, aux, mid+1, high, less);
merge(array, aux, low, mid, high, less);
}
static void merge(void **array, void **aux, int low, int mid, int high, isLess less){
for(int index = 0; index <= high; index++){
aux[index] = array[index]; //Shallow copy
}
printf("Low-%d, Mid-%d, High-%d\n", low, mid, high);
int leftIndex = low; int rightIndex = mid+1;
printf("leftIndex-%d, rightIndex-%d\n", leftIndex, rightIndex);
for(int index = 0; index <= high; index++){
if(leftIndex > mid) /* right array exhausted */ array[index] = aux[rightIndex++];
else if(rightIndex > high) /*left array exhausted*/ array[index] = aux[leftIndex++];
else if( less(aux[rightIndex], aux[leftIndex]) ) array[index] = aux[rightIndex++];
else array[index] = aux[leftIndex++];
}
}
where the use code is,
bool less(const void *key, const void *item){
printf("\nIn less function\n");
printf("left-%d, Right-%d\n\n", ((Record *)key)->age, ((Record *)item)->age);
if( ((Record *)key)->age < ((Record *)item)->age ){
printf("Return true\n");
return true;
}else{
printf("Return false\n");
return false;
}
}
int main(){
FILE *pFile = fopen("file.txt", "r");
checkHandleFailure(pFile, FILE_HANDLE_FAILURE);
char buf[MAX_RECORD_SIZE];
DBCache *cache = initCache(pFile);
readHeader(pFile, cache, buf);
readData(pFile, cache, buf);
printRecords(cache);
printf("Before calling mergesort() \n");
mergeSort(cache->records, listGetSize(cache->records), less);
}
Actual output is:
$ ./program.exe
Age,LastName,FirstName
------------------------------
50,B,A
30,A,B
20,X,D
10,F,A
90,V,E
60,N,M
Records#: 6
Before calling mergesort()
Size of list: 6
Low-0, Mid-0, High-1
leftIndex-0, rightIndex-1
In less function
left-30, Right-50
Return true
Low-0, Mid-1, High-2
leftIndex-0, rightIndex-2
In less function
left-20, Right-30
Return true
Low-3, Mid-3, High-4
leftIndex-3, rightIndex-4
In less function
left-90, Right-10
Return false
Low-3, Mid-4, High-5
leftIndex-3, rightIndex-5
Segmentation fault (core dumped)
In less function
How to resolve this problem?
Cygwin is not supporting coredump trace format using gdb, trace is provided(above)
回答1:
The two for loops in merge should start at low, not at zero. The second for loop starting at 0 is probably causing the segmentation fault. The first for loop starting at 0 shouldn't cause a segmentation fault, but it consumes extra time.
static void merge(....)
/* ... */
for(int index = low; index <= high; index++){ // low not 0
aux[index] = array[index];
}
/* ... */
for(int index = low; index <= high; index++){ // low not 0
if(leftIndex > mid) /* ... */
/* ... */
回答2:
To make a generic merge sort you must not use void **, because you will force the user to use a nested pointer too. So your implementation is not generic here.
I propose you an implementation that use pointer arithmetic, it's not easy to understand. But I think it's better than use int. I change the function less in a compare function because it's more idiomatic in C.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
typedef int msort_cmp(void const *a, void const *b);
static void msort_merge(char *begin, char const *end, char *copy_begin,
char *copy_mid, char const *copy_end, size_t size,
msort_cmp *cmp) {
memcpy(copy_begin, begin, (size_t)(end - begin));
for (char *right = copy_mid; begin < end; begin += size) {
if (copy_begin >= copy_mid) {
memcpy(begin, right, size);
right += size;
} else if (right >= copy_end) {
memcpy(begin, copy_begin, size);
copy_begin += size;
} else if (cmp(right, copy_begin) > 0) {
memcpy(begin, right, size);
right += size;
} else {
memcpy(begin, copy_begin, size);
copy_begin += size;
}
}
}
static void msort_partition(char *begin, char const *end, char *copy,
size_t size, msort_cmp *cmp) {
size_t bytes = (size_t)(end - begin);
if (bytes > size) {
size_t tmp = bytes / 2;
size_t offset = tmp - tmp % size;
char *mid = begin + offset;
msort_partition(begin, mid, copy, size, cmp);
msort_partition(mid, end, copy, size, cmp);
msort_merge(begin, end, copy, copy + offset, copy + bytes, size, cmp);
}
}
static int msort_aux(char *begin, char const *end, size_t size,
msort_cmp *cmp) {
char *copy = malloc((size_t)(end - begin));
if (copy == NULL) {
return 1;
}
msort_partition(begin, end, copy, size, cmp);
return 0;
}
static int msort(void *begin, void const *end, size_t size, msort_cmp *cmp) {
return msort_aux(begin, end, size, cmp);
}
static int cmp_int_aux(int const *a, int const *b) {
if (*a < *b) {
return 1;
} else if (*a > *b) {
return -1;
} else {
return 0;
}
}
static int cmp_int(void const *a, void const *b) { return cmp_int_aux(a, b); }
static void print_int(int const *array, size_t size) {
printf("array: ");
for (size_t i = 0; i < size; i++) {
printf(" %d", array[i]);
}
printf("\n");
}
static int cmp_pint(void const *a, void const *b) {
return cmp_int(*(int *const *)a, *(int *const *)b);
}
static void print_pint(int *const *parray, size_t size) {
printf("parray: ");
for (size_t i = 0; i < size; i++) {
printf(" %d", *parray[i]);
}
printf("\n");
}
#define SIZE 42
int main(void) {
int array[SIZE];
srand((unsigned int)time(NULL));
for (size_t i = 0; i < SIZE; i++) {
array[i] = rand() % SIZE - SIZE / 2;
}
print_int(array, SIZE);
msort(array, array + SIZE, sizeof *array, &cmp_int);
print_int(array, SIZE);
int *parray[SIZE];
for (size_t i = 0; i < SIZE; i++) {
array[i] = rand() % SIZE - SIZE / 2;
parray[i] = array + i;
}
print_pint(parray, SIZE);
msort(parray, parray + SIZE, sizeof *parray, &cmp_pint);
print_pint(parray, SIZE);
}
来源：https://stackoverflow.com/questions/41434564/mergesort-segmentation-fault