qsort() 导致分段错误

Question

I've been tasked with implementing a type safe dynamic vector structure in C; however, I seem to have a problem: Every time I use the qsort() and then try casting the variables to an int* (in both erase_value() and print_vector_int() ) I get a segmentation fault.

Example input:

4
p 3
i 0 10
e 0 20
p 4

This works as intended but, as soon as I use qsort() , either the erase_value() or print_vector_int() break.

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

#define MAX_STR_LEN 64

typedef struct Vector {
    void **data;
    size_t element_size;
    size_t size;
    size_t capacity;
} Vector;

// Allocate vector to initial capacity (block_size elements),
// Set element_size, size (to 0), capacity
void init_vector(Vector *vector, size_t block_size, size_t element_size) {
    vector->data = (void **)malloc(sizeof(void *) * block_size);
    vector->element_size = element_size;
    vector->capacity = block_size;
    vector->size = 0;
}

// If new_capacity is greater than the current capacity,
// new storage is allocated, otherwise the function does nothing.
void reserve(Vector *vector, size_t new_capacity) {
    if (vector->capacity < new_capacity) {
        vector->capacity = new_capacity;
        vector->data = (void **)realloc(vector->data, sizeof(void *) * (vector->capacity));
    }
}

// Resizes the vector to contain new_size elements.
// If the current size is greater than new_size, the container is
// reduced to its first new_size elements.

// If the current size is less than new_size,
// additional zero-initialized elements are appended
void resize(Vector *vector, size_t new_size) {
    if (new_size <= vector->size) {
        for (size_t i = vector->size; i > new_size; --i) {
            free(vector->data[i]);
        }
        vector->size = new_size;
    } else {
        if (new_size > vector->capacity) {
            vector->capacity = (vector->capacity) * 2;
            vector->data = (void **)realloc(vector->data, sizeof(void *) * vector->capacity);
        }
        for (size_t i = vector->size; i < new_size; ++i) {
            vector->data[i] = (void *)calloc(1, vector->element_size);
        }
        vector->size = new_size;
    }
}

// Insert new element at index (0 <= index <= size) position
void insert(Vector *vector, int index, void *value) {
    if (index >= vector->capacity) {
        vector->capacity = vector->capacity * 2;
        vector-> data = (void **)realloc(vector->data, sizeof(void *) * (vector->capacity));
    }
    
    if (index >= 0 && index <= vector->size) {
        vector->data[vector->size] = (void *)malloc(vector->element_size);
       for (size_t i = vector->size; i > index; --i) {
           memcpy(vector->data[i], vector->data[i - 1], vector->element_size);
       }
       memcpy(vector->data[index], value, vector->element_size);
       ++(vector->size);
    }
}

// Add element to the end of the vector
void push_back(Vector *vector, void *value) {
    if (vector->size == 0)
        insert(vector, 0, value);
    else
        insert(vector, vector->size, value);
}

// Remove all elements from the vector
void clear(Vector *vector) {
    for (int i = (vector->size) - 1; i >= 0; --i) {
        free(vector->data[i]);
    }
    vector->size = 0;
}

// Erase element at position index
void erase(Vector *vector, int index) {
    for (size_t i = index; i < vector->size - 1; ++i) {
        memcpy(vector->data[i], vector->data[i + 1], vector->element_size);
    }
    free(vector->data[vector->size - 1]);
    --(vector->size);
}

// Erase all elements that compare equal to value from the container
void erase_value(Vector *vector, void *value, int(*cmp)(const void *, const void *)) {
    for(size_t i = 0; i < vector->size; ++i) {
        if (cmp(vector->data[i], value) == 0)
            erase(vector, i);
    }
}

// Erase all elements that satisfy the predicate from the vector
void erase_if(Vector *vector, int (*predicate)(void *)) {
    for (size_t i = 0; i < vector->size; ++i) {
        if (predicate(vector->data[i]))
            erase(vector, i);
    }
}

// Request the removal of unused capacity
void shrink_to_fit(Vector *vector) {
    vector->capacity = vector->size;
    vector->data = (void **)realloc(vector->data, sizeof(void *) * (vector->capacity));
}

// Print integer vector
void print_vector_int(Vector *vector) {
    printf("%ld\n", vector->capacity);
    for (int i = 0;i < vector->size; ++i) {
        int item = *(int *)vector->data[i];
        printf("%d ", item);
    }
}

int int_cmp(const void *v1, const void *v2) {
    const int aa = *(const int *)v1;
    const int bb = *(const int *)v2;
    return aa - bb;
}

int is_even(void *value) {
    const int aa = *(int *)value;
    return aa % 2 == 0;
}

void read_int(void *value) {
    scanf("%d", (int *)value);
}

void vector_test(Vector *vector, int n, void (*read)(void *),
         int (*cmp)(const void *, const void *), int (*predicate)(void *)) {
    char op[2];
    int index;
    size_t size;
    void *v = malloc(vector->element_size);
    for (int i = 0; i < n; ++i) {
        scanf("%s", op);
        switch (op[0]) {
          case 'p': // push_back
            read(v);
            push_back(vector, v);
            break;
          case 'i': // insert
            scanf("%d", &index);
            read(v);
            insert(vector, index, v);
            break;
          case 'e': // erase
            scanf("%d", &index);
            read(v);
            erase(vector, index);
            erase_value(vector, v, cmp);
            break;
          case 'd': // erase (predicate)
            erase_if(vector, predicate);
            break;
          case 'r': // resize
            scanf("%zu", &size);
            resize(vector, size);
            break;
          case 'c': // clear
            clear(vector);
            break;
          case 'f': // shrink
            shrink_to_fit(vector);
            break;
          case 's': // sort
            qsort(vector->data, vector->size,
                  vector->element_size, cmp);
            break;
          default:
            printf("No such operation: %s\n", op);
            break;
        }
    }
    free(v);
}

int main(void) {
    int n;
    Vector vector_int;
    scanf("%d", &n);
    init_vector(&vector_int, 4, sizeof(int));
    vector_test(&vector_int, n, read_int, int_cmp, is_even);
    print_vector_int(&vector_int);
    free(vector_int.data);
    return 0;
}

I suspect it might be a problem with the int_cmp() , since q-sorting a sorted list does not result in a seg-fault.

Answer 1

There are multiple problems in your code:

• you use qsort on an array of pointers to allocated blocks, not an array of elements. The geometry of your vector is inappropriate for qsort with the comparison function as coded.

• in function resize , the loop to free the elements starts at vector->size and accesses the element at this offset, which was not allocated. The correct way to write a downward loop is:

   for (size_t i = vector->size; i-- > new_size;) { free(vector->data[i]); }

• in erase_value and erase_if you should not increment i when the matching element is deleted.

• int_cmp should not rely on subtracting the values: this subtraction can overflow for large values of opposite signs.

You should change the implementation of you vector to allocate an array of elements instead of an array of pointers to allocated arrays of bytes.

Here is a modified version:

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

typedef struct Vector {
    void *data;
    size_t element_size;
    size_t size;
    size_t capacity;
} Vector;

#define VP(v, i)  ((void *)((unsigned char *)(vector)->data + (i) * (vector)->element_size))

// Allocate vector to initial capacity (block_size elements),
// Set element_size, size (to 0), capacity
void init_vector(Vector *vector, size_t capacity, size_t element_size) {
    vector->data = calloc(capacity, element_size);
    vector->element_size = element_size;
    vector->capacity = capacity;
    vector->size = 0;
}

// Free vector data
void free_vector(Vector *vector) {
    free(vector->data);
    vector->data = NULL;
    vector->size = vector->capacity = 0;
}

// If new_capacity is greater than the current capacity,
// new storage is allocated, otherwise the function does nothing.
void reserve(Vector *vector, size_t new_capacity) {
    if (vector->capacity < new_capacity) {
        vector->data = realloc(vector->data, new_capacity * vector->element_size);
        vector->capacity = new_capacity;
    }
}

// Resizes the vector to contain new_size elements.
// If the current size is greater than new_size, the container is
// reduced to its first new_size elements.
// If the current size is less than new_size,
// additional zero-initialized elements are appended
void resize(Vector *vector, size_t new_size) {
    if (new_size > vector->size) {
        if (new_size > vector->capacity) {
            size_t new_capacity = vector->capacity * 2;
            while (new_size > new_capacity) {
                new_capacity = new_capacity * 2;
            }
            vector->data = realloc(vector->data, new_capacity * vector->element_size);
            vector->capacity = new_capacity;
        }
        memset(VP(vector, vector->size), 0, (new_size - vector->size) * vector->element_size);
    }
    vector->size = new_size;
}

// Insert new element at index (0 <= index <= size) position
void insert(Vector *vector, size_t index, void *value) {
    if (index <= vector->size) {
        if (vector->size == vector->capacity) {
            resize(vector, vector->size + 1);
        }
        memmove(VP(vector, index + 1), VP(vector, index),
                (vector->size - index) * vector->element_size);
        memmove(VP(vector, index), value, vector->element_size);
        vector->size += 1;
    }
}

// Add element to the end of the vector
void push_back(Vector *vector, void *value) {
    insert(vector, vector->size, value);
}

// Remove all elements from the vector
void clear(Vector *vector) {
    vector->size = 0;
}

// Erase element at position index
void erase(Vector *vector, size_t index) {
    if (index < vector->size) {
        memmove(VP(vector, index), VP(vector, index + 1),
                (vector->size - index - 1) * vector->element_size);
        vector->size -= 1;
    }
}

// Erase all elements that compare equal to value from the container
void erase_value(Vector *vector, void *value, int (*cmp)(const void *, const void *)) {
    for (size_t i = 0; i < vector->size;) {
        if (cmp(VP(vector, i), value) == 0) {
            erase(vector, i);
        } else {
            i++;
        }
    }
}

// Erase all elements that satisfy the predicate from the vector
void erase_if(Vector *vector, int (*predicate)(void *)) {
    for (size_t i = 0; i < vector->size;) {
        if (predicate(VP(vector, i))) {
            erase(vector, i);
        } else {
            i++;
        }
    }
}

// Print all elements of the vector
void print_vector(Vector *vector, void (*print)(void *)) {
    printf("%zu %zu {", vector->size, vector->capacity);
    for (size_t i = 0; i < vector->size; i++) {
        printf(" ");
        print(VP(vector, i));
    }
    printf(" }\n");
}

// Request the removal of unused capacity
void shrink_to_fit(Vector *vector) {
    vector->capacity = vector->size;
    vector->data = realloc(vector->data, vector->capacity * vector->element_size);
}

// Print integer vector
void int_print(void *p) {
    int item = *(int *)p;
    printf("%d", item);
}

int int_cmp(const void *v1, const void *v2) {
    const int aa = *(const int *)v1;
    const int bb = *(const int *)v2;
    return (aa > bb) - (aa < bb);
}

int is_even(void *value) {
    const int aa = *(const int *)value;
    return aa % 2 == 0;
}

void read_int(void *value) {
    int *vp = (int *)value;
    *vp = 0;
    scanf("%d", vp);
}

void vector_test(Vector *vector, int n,
                 void (*read)(void *),
                 int (*cmp)(const void *, const void *),
                 int (*predicate)(void *),
                 void (*print)(void *))
{
    char op[2];
    size_t index, size;
    void *v = malloc(vector->element_size);
    for (int i = 0; i < n; ++i) {
        if (scanf("%1s", op) != 1)
            break;
        switch (op[0]) {
        case 'p': // push_back
            read(v);
            push_back(vector, v);
            break;
        case 'i': // insert
            scanf("%zu", &index);
            read(v);
            insert(vector, index, v);
            break;
        case 'e': // erase
            scanf("%zu", &index);
            read(v);
            erase(vector, index);
            erase_value(vector, v, cmp);
            break;
        case 'd': // erase (predicate)
            erase_if(vector, predicate);
            break;
        case 'r': // resize
            scanf("%zu", &size);
            resize(vector, size);
            break;
        case 'c': // clear
            clear(vector);
            break;
        case 'f': // shrink
            shrink_to_fit(vector);
            break;
        case '=': // print
            print_vector(vector, print);
            break;
        case 's': // sort
            qsort(vector->data, vector->size,
                  vector->element_size, cmp);
            break;
        default:
            printf("No such operation: %s\n", op);
            break;
        }
    }
    free(v);
}

int main() {
    Vector vector_int[1];
    int n = 0;

    if (scanf("%d", &n) == 1) {
        init_vector(vector_int, 4, sizeof(int));
        vector_test(vector_int, n, read_int, int_cmp, is_even, int_print);
        print_vector(vector_int, int_print);
        free_vector(vector_int);
    }
    return 0;
}