为什么在传递给函数后访问指针会出现分段错误？

Question

我正在尝试为 2D FFT 编写 C 程序。这个想法是传递具有预分配内存的结构指针，其中函数将修改这些内存地址。

complex_num是一个支持复数运算的结构，如加减乘除、幂、复指数。我放置 printf("Hello\n"); 的地方语句是我的代码中发生分段错误的地方。感谢期待。

#include <stdio.h>
#include <math.h>
#include <stdlib.h>
typedef struct complex_num_struct {
    double real;
    double imag;
} complex_num;

complex_num add(complex_num num1, complex_num num2);
complex_num subtract(complex_num num1, complex_num num2);
complex_num multiply(complex_num num1, complex_num num2);
complex_num divide(complex_num num1, complex_num num2);
complex_num power(complex_num num, double n);
complex_num complex_exp(double theta);
double magnitude(complex_num num);
void display_complex_matrix(complex_num** matrix, int height, int width);
void fft2(complex_num** input, complex_num** output, int height, int width);
void fft_driver(complex_num input[], complex_num output[], int n, int step);
void fft(complex_num input[], complex_num output[], int n);
void display_complex_vec(complex_num * vec, int n);
complex_num conjugate(complex_num num);
complex_num ** matrix;
complex_num ** temp;

void main() {

    complex_num one;
    one.real = 1;
    one.imag = 0;

    complex_num height, width;

    height.real = 4;
    width.real = 4;
    height.imag = 0;
    width.imag = 0;

    complex_num zero;
    zero.real = 0;
    zero.imag = 0;

    complex_num input1[] = {one, one, one, one};
    complex_num input2[] = {one, one, one, one};
    complex_num input3[] = {zero, zero, zero, zero};
    complex_num input4[] = {zero, zero, zero, zero};

    complex_num* input[] = {input1, input2, input3, input4};
    complex_num* output[] = {input1, input2, input3, input4};
    complex_num* conj[] = {input1, input2, input3, input4};

    fft2(input, output, 4, 4);
    display_complex_matrix(output, 4, 4);

    for (int i = 0; i < 4; ++i) {
        for (int j = 0; j < 4; ++j) {
            conj[i][j] = conjugate(output[i][j]);
        }
    }

    fft2(conj, output, 4, 4);

    for (int i = 0; i < 4; ++i) {
        for (int j = 0; j < 4; ++j) {
            output[i][j] = conjugate(divide(conj[i][j], (multiply(height, width))));
        }
    }

    display_complex_matrix(output, 4, 4);
}
void fft2(complex_num** input, complex_num** output, int height, int width) {

    matrix = (complex_num **) malloc(height * sizeof(complex_num*));
    temp = (complex_num **) malloc(width * sizeof(complex_num*));

    for (int i = 0; i < height; ++i) {
        fft(input[i], temp[i], width);
    }

    for (int i = 0; i < height; ++i) {

        //matrix[i] = (complex_num*) malloc(height * sizeof(complex_num));

        for (int j = 0; j < width; ++j) {
            //matrix[i][j] = temp[j][i];
            printf("%lf", temp[j][i].real);

        }
    }

    for (int i = 0; i < height; ++i) {
        fft(matrix[i], temp[i], width);
    }

    for (int i = 0; i < height; ++i) {
        for (int j = 0; j < width; ++j) {
            output[i][j] = temp[j][i];
        }
    }
}

void display_complex_vec(complex_num vec[], int n) {

    printf("[");

    for (int i = 0; i < n; ++i) {
        printf("(%g, %g), ", vec[i].real, vec[i].imag);
    }

    printf("]\n");
}

void display_complex_matrix(complex_num** matrix, int height, int width) {


    for (int i = 0; i < height; ++i) {
        printf("[");

        for (int j = 0; j < width; ++j) {
            printf("(%g, %g), ", matrix[i][j].real, matrix[i][j].imag);
        }

        printf("]\n");
    }
}

void fft_driver(complex_num input[], complex_num output[], int n, int step) {
    complex_num diff, cexp;

    if (step < n) {

        fft_driver(output, input, n, step * 2);
        fft_driver(output + step, input + step, n, step * 2);

        for (int i = 0; i < n; i += 2 * step) {
            cexp = complex_exp(-M_PI * (double) i / (double) n);

            diff = multiply(complex_exp(-M_PI * (double) i / (double) n), output[i + step]);

            input[i / 2] = add(output[i], diff);
            input[(i + n) / 2] = subtract(output[i], diff);
            //printf("(%g, %g) * (%g, %g)\n", input[i / 2].real, input[i / 2].imag, input[(i + n) / 2].real, input[(i + n) / 2].imag);
        }
    }
}

void fft(complex_num* input, complex_num* output, int n) {

    complex_num* dummy = (complex_num *)malloc(n * sizeof(complex_num));
    complex_num* inp = (complex_num *)malloc(n * sizeof(complex_num));

    for (int i = 0; i < n; ++i) {
        dummy[i].real = 0;
        dummy[i].imag = 0;
        inp[i].real = 0;
        inp[i].imag = 0;
    }

    printf("%lf", inp[1].real);
    for (int i = 0; i < n; ++i) {
        dummy[i] = input[i];
        inp[i] = input[i];
    }

    fft_driver(inp, dummy, n, 1);

    printf("Hello\n");

    for (int i = 0; i < n; ++i) {
        output[i] = inp[i];
    }
}

complex_num complex_exp(double theta) {
    complex_num result;
    result.real = cos(theta);
    result.imag = sin(theta);

    return result;
}

double magnitude(complex_num num) {
    return sqrt(pow(num.real, 2) + pow(num.imag, 2));
}

complex_num add(complex_num num1, complex_num num2) {
    complex_num result;
    result.real = num1.real + num2.real;
    result.imag = num1.imag + num2.imag;

    return result;
}

complex_num conjugate(complex_num num) {
    complex_num result;
    result.real = num.real;
    result.imag = -num.imag;

    return result;
}

complex_num subtract(complex_num num1, complex_num num2) {
    complex_num result;
    result.real = num1.real - num2.real;
    result.imag = num1.imag - num2.imag;

    return result;
}

complex_num multiply(complex_num num1, complex_num num2) {
    complex_num result;
    result.real = num1.real * num2.real - num1.imag * num2.imag;
    result.imag = num1.real * num2.imag + num1.imag * num2.real;

    return result;
}

complex_num divide(complex_num num1, complex_num num2) {
    complex_num result;

    double magnitude_square = pow(num2.real, 2) + pow(num2.imag, 2);
    num2.imag = -num2.imag;

    result = multiply(num1, num2);
    result.real = result.real / magnitude_square;
    result.imag = result.imag / magnitude_square;

    return result;
}

complex_num power(complex_num num, double n) {
    complex_num result;

    double magnitude = sqrt(pow(num.real, 2) + pow(num.imag, 2));

    result.real = pow(magnitude, n) * cos(n * atan(num.imag / num.real));
    result.imag = pow(magnitude, n) * sin(n * atan(num.imag / num.real));

    return result;
}

以上代码产生以下输出：

0.000000Hello
Segmentation fault (core dumped)

在调试时，我发现段错误发生在我的代码在 printf 语句之后的循环中的以下函数中：

void fft(complex_num* input, complex_num* output, int n) {

    complex_num* dummy = (complex_num *)malloc(n * sizeof(complex_num));
    complex_num* inp = (complex_num *)malloc(n * sizeof(complex_num));

    for (int i = 0; i < n; ++i) {
        dummy[i].real = 0;
        dummy[i].imag = 0;
        inp[i].real = 0;
        inp[i].imag = 0;
    }

    printf("%lf", inp[1].real);
    for (int i = 0; i < n; ++i) {
        dummy[i] = input[i];
        inp[i] = input[i];
    }

    fft_driver(inp, dummy, n, 1);

    printf("Hello\n");

    for (int i = 0; i < n; ++i) {
        output[i] = inp[i];
    }
}

所需的输出应如下所示：-

0.000000Hello
0.000000Hello
0.000000Hello
0.000000Hello
4.0000004.0000000.0000000.0000000.0000000.0000000.0000000.0000000.0000000.0000000.0000000.0000000.0000000.0000000.0000000.0000000.000000Hello
0.000000Hello
0.000000Hello
0.000000Hello
[(8, 0), (0, 0), (0, 0), (0, 0), ]
[(4, -4), (0, 0), (0, 0), (0, 0), ]
[(0, 0), (0, 0), (0, 0), (0, 0), ]
[(4, 4), (0, 0), (0, 0), (0, 0), ]
0.000000Hello
0.000000Hello
0.000000Hello
0.000000Hello
4.0000004.0000004.0000004.0000004.0000004.0000004.0000004.0000004.0000004.0000004.0000004.0000004.0000004.0000004.0000004.0000000.000000Hello
0.000000Hello
0.000000Hello
0.000000Hello
[(1, 1), (1, 1), (1, 1), (1, 1), ]
[(0.0625, -0.0625), (0.0625, -0.0625), (0.0625, -0.0625), (0.0625, -0.0625), ]
[(0.0625, -0.0625), (0.0625, -0.0625), (0.0625, -0.0625), (0.0625, -0.0625), ]
[(0.0625, -0.0625), (0.0625, -0.0625), (0.0625, -0.0625), (0.0625, -0.0625), ]

Answer 1

您有段错误，因为您正在 void fft(complex_num* input, complex_num* output, int n) 中的未初始化地址处写入 :

output[i] = inp[i];

输出来自fft2行

fft(input[i], temp[i], width);

其中 temp 由

设置

temp = (complex_num **) malloc(width * sizeof(complex_num*));

但是temp[i]没有初始化

也许 fft(input[i], temp[i], width); 应该是 fft(input[i], &temp[i], width); 而 fft 变成：

void fft(complex_num* input, complex_num** output, int n) {

  complex_num* dummy = (complex_num *)malloc(n * sizeof(complex_num));
  complex_num* inp = (complex_num *)malloc(n * sizeof(complex_num));

  for (int i = 0; i < n; ++i) {
    dummy[i].real = 0;
    dummy[i].imag = 0;
    inp[i].real = 0;
    inp[i].imag = 0;
  }

  printf("%lf", inp[1].real);
  for (int i = 0; i < n; ++i) {
    dummy[i] = input[i];
    inp[i] = input[i];
  }

  fft_driver(inp, dummy, n, 1);

  printf("Hello\n");

  *output = inp;
}

无论如何，调用中的fft2还有一个问题：

fft(matrix[i], &temp[i], width);

因为 matrix[i] 的初始化被注释掉了，matrix[i] 没有被初始化并且在 fft 中你正在访问那些行中的未知地址

dummy[i] = input[i];
inp[i] = input[i];

行 //matrix[i] = (complex_num*) malloc(height * sizeof(complex_num)); 不得在评论中，但也将分配的大小更改为：

matrix[i] = (complex_num*) malloc(width * sizeof(complex_num));

及matrix[i][j] = temp[j][i];行以下不得在评论中

执行所有更改（我为 M_PI 使用 3.1415927）

/tmp % ./a.out
0.000000Hello
0.000000Hello
0.000000Hello
0.000000Hello
4.0000004.0000000.0000000.0000000.0000000.0000000.0000000.0000000.0000000.0000000.0000000.0000000.0000000.0000000.0000000.0000000.000000Hello
0.000000Hello
0.000000Hello
0.000000Hello
[(8, 0), (0, 0), (0, 0), (0, 0), ]
[(4, -4), (0, 0), (0, 0), (0, 0), ]
[(0, 0), (0, 0), (0, 0), (0, 0), ]
[(4, 4), (0, 0), (0, 0), (0, 0), ]
0.000000Hello
0.000000Hello
0.000000Hello
0.000000Hello
8.0000004.0000000.0000004.0000008.0000004.0000000.0000004.0000008.0000004.0000000.0000004.0000008.0000004.0000000.0000004.0000000.000000Hello
0.000000Hello
0.000000Hello
0.000000Hello
[(1, -0), (1, -0), (1, -0), (1, -0), ]
[(1, -2.8606e-18), (1, -2.8606e-18), (1, -2.8606e-18), (1, -2.8606e-18), ]
[(0, 0), (0, -0), (0, -0), (0, -0), ]
[(0, 2.8606e-18), (0, 2.8606e-18), (0, 2.8606e-18), (0, 2.8606e-18), ]

我不知道这是否是预期的输出，但至少 valgrind 没有检测到任何非法内存访问或未初始化的值。

你有内存泄漏，要删除它们:

在fft2中替换

for (int i = 0; i < height; ++i) {
  fft(matrix[i], &temp[i], width);
}

来自

for (int i = 0; i < height; ++i) {
  free(temp[i]);
  fft(matrix[i], &temp[i], width);
  free(matrix[i]);
}

free(matrix);

并在末尾添加

for (int i = 0; i < height; ++i)
  free(temp[i]);
free(temp);

在fft末尾前加free(dummy);

这些更改消除了所有内存泄漏：

/tmp % valgrind --leak-check=full ./a.out
==12924== Memcheck, a memory error detector
==12924== Copyright (C) 2002-2012, and GNU GPL'd, by Julian Seward et al.
==12924== Using Valgrind-3.8.1 and LibVEX; rerun with -h for copyright info
==12924== Command: ./a.out
==12924== 
0.000000Hello
0.000000Hello
0.000000Hello
0.000000Hello
4.0000004.0000000.0000000.0000000.0000000.0000000.0000000.0000000.0000000.0000000.0000000.0000000.0000000.0000000.0000000.0000000.000000Hello
0.000000Hello
0.000000Hello
0.000000Hello
[(8, 0), (0, 0), (0, 0), (0, 0), ]
[(4, -4), (0, 0), (0, 0), (0, 0), ]
[(0, 0), (0, 0), (0, 0), (0, 0), ]
[(4, 4), (0, 0), (0, 0), (0, 0), ]
0.000000Hello
0.000000Hello
0.000000Hello
0.000000Hello
8.0000004.0000000.0000004.0000008.0000004.0000000.0000004.0000008.0000004.0000000.0000004.0000008.0000004.0000000.0000004.0000000.000000Hello
0.000000Hello
0.000000Hello
0.000000Hello
[(1, -0), (1, -0), (1, -0), (1, -0), ]
[(1, -2.8606e-18), (1, -2.8606e-18), (1, -2.8606e-18), (1, -2.8606e-18), ]
[(0, 0), (0, -0), (0, -0), (0, -0), ]
[(0, 2.8606e-18), (0, 2.8606e-18), (0, 2.8606e-18), (0, 2.8606e-18), ]
==12924== 
==12924== HEAP SUMMARY:
==12924==     in use at exit: 0 bytes in 0 blocks
==12924==   total heap usage: 44 allocs, 44 frees, 2,688 bytes allocated
==12924== 
==12924== All heap blocks were freed -- no leaks are possible
==12924== 
==12924== For counts of detected and suppressed errors, rerun with: -v
==12924== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 6 from 6)

另请注意 main 必须是 int main() 而不是 void main()

如果我把所有的代码都帮你，因为有很多变化:

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

#ifndef M_PI
#define M_PI 3.1415927
#endif

typedef struct complex_num_struct {
    double real;
    double imag;
} complex_num;

complex_num add(complex_num num1, complex_num num2);
complex_num subtract(complex_num num1, complex_num num2);
complex_num multiply(complex_num num1, complex_num num2);
complex_num divide(complex_num num1, complex_num num2);
complex_num power(complex_num num, double n);
complex_num complex_exp(double theta);
double magnitude(complex_num num);
void display_complex_matrix(complex_num** matrix, int height, int width);
void fft2(complex_num** input, complex_num** output, int height, int width);
void fft_driver(complex_num input[], complex_num output[], int n, int step);
void fft(complex_num input[], complex_num * output[], int n);
void display_complex_vec(complex_num * vec, int n);
complex_num conjugate(complex_num num);
complex_num ** matrix;
complex_num ** temp;

int main() {

    complex_num one;
    one.real = 1;
    one.imag = 0;

    complex_num height, width;

    height.real = 4;
    width.real = 4;
    height.imag = 0;
    width.imag = 0;

    complex_num zero;
    zero.real = 0;
    zero.imag = 0;

    complex_num input1[] = {one, one, one, one};
    complex_num input2[] = {one, one, one, one};
    complex_num input3[] = {zero, zero, zero, zero};
    complex_num input4[] = {zero, zero, zero, zero};

    complex_num* input[] = {input1, input2, input3, input4};
    complex_num* output[] = {input1, input2, input3, input4};
    complex_num* conj[] = {input1, input2, input3, input4};

    fft2(input, output, 4, 4);
    display_complex_matrix(output, 4, 4);

    for (int i = 0; i < 4; ++i) {
        for (int j = 0; j < 4; ++j) {
            conj[i][j] = conjugate(output[i][j]);
        }
    }

    fft2(conj, output, 4, 4);

    for (int i = 0; i < 4; ++i) {
        for (int j = 0; j < 4; ++j) {
            output[i][j] = conjugate(divide(conj[i][j], (multiply(height, width))));
        }
    }

    display_complex_matrix(output, 4, 4);
}
void fft2(complex_num** input, complex_num** output, int height, int width) {

    matrix = (complex_num **) malloc(height * sizeof(complex_num*));
    temp = (complex_num **) malloc(width * sizeof(complex_num*));

    for (int i = 0; i < height; ++i) {
        fft(input[i], &temp[i], width);
    }

    for (int i = 0; i < height; ++i) {

        matrix[i] = (complex_num*) malloc(width * sizeof(complex_num));

        for (int j = 0; j < width; ++j) {
            matrix[i][j] = temp[j][i];
            printf("%lf", temp[j][i].real);

        }
    }

    for (int i = 0; i < height; ++i) {
        free(temp[i]);
        fft(matrix[i], &temp[i], width);
        free(matrix[i]);
    }

    free(matrix);

    for (int i = 0; i < height; ++i) {
        for (int j = 0; j < width; ++j) {
            output[i][j] = temp[j][i];
        }
    }
    for (int i = 0; i < height; ++i)
      free(temp[i]);
    free(temp);
}

void display_complex_vec(complex_num vec[], int n) {

    printf("[");

    for (int i = 0; i < n; ++i) {
        printf("(%g, %g), ", vec[i].real, vec[i].imag);
    }

    printf("]\n");
}

void display_complex_matrix(complex_num** matrix, int height, int width) {


    for (int i = 0; i < height; ++i) {
        printf("[");

        for (int j = 0; j < width; ++j) {
            printf("(%g, %g), ", matrix[i][j].real, matrix[i][j].imag);
        }

        printf("]\n");
    }
}

void fft_driver(complex_num input[], complex_num output[], int n, int step) {
    complex_num diff, cexp;

    if (step < n) {

        fft_driver(output, input, n, step * 2);
        fft_driver(output + step, input + step, n, step * 2);

        for (int i = 0; i < n; i += 2 * step) {
            cexp = complex_exp(-M_PI * (double) i / (double) n);

            diff = multiply(complex_exp(-M_PI * (double) i / (double) n), output[i + step]);

            input[i / 2] = add(output[i], diff);
            input[(i + n) / 2] = subtract(output[i], diff);
            //printf("(%g, %g) * (%g, %g)\n", input[i / 2].real, input[i / 2].imag, input[(i + n) / 2].real, input[(i + n) / 2].imag);
        }
    }
}

void fft(complex_num* input, complex_num** output, int n) {

  complex_num* dummy = (complex_num *)malloc(n * sizeof(complex_num));
  complex_num* inp = (complex_num *)malloc(n * sizeof(complex_num));

  for (int i = 0; i < n; ++i) {
    dummy[i].real = 0;
    dummy[i].imag = 0;
    inp[i].real = 0;
    inp[i].imag = 0;
  }

  printf("%lf", inp[1].real);
  for (int i = 0; i < n; ++i) {
    dummy[i] = input[i];
    inp[i] = input[i];
  }

  fft_driver(inp, dummy, n, 1);

  free(dummy);
  printf("Hello\n");

  *output = inp;
}

complex_num complex_exp(double theta) {
    complex_num result;
    result.real = cos(theta);
    result.imag = sin(theta);

    return result;
}

double magnitude(complex_num num) {
    return sqrt(pow(num.real, 2) + pow(num.imag, 2));
}

complex_num add(complex_num num1, complex_num num2) {
    complex_num result;
    result.real = num1.real + num2.real;
    result.imag = num1.imag + num2.imag;

    return result;
}

complex_num conjugate(complex_num num) {
    complex_num result;
    result.real = num.real;
    result.imag = -num.imag;

    return result;
}

complex_num subtract(complex_num num1, complex_num num2) {
    complex_num result;
    result.real = num1.real - num2.real;
    result.imag = num1.imag - num2.imag;

    return result;
}

complex_num multiply(complex_num num1, complex_num num2) {
    complex_num result;
    result.real = num1.real * num2.real - num1.imag * num2.imag;
    result.imag = num1.real * num2.imag + num1.imag * num2.real;

    return result;
}

complex_num divide(complex_num num1, complex_num num2) {
    complex_num result;

    double magnitude_square = pow(num2.real, 2) + pow(num2.imag, 2);
    num2.imag = -num2.imag;

    result = multiply(num1, num2);
    result.real = result.real / magnitude_square;
    result.imag = result.imag / magnitude_square;

    return result;
}

complex_num power(complex_num num, double n) {
    complex_num result;

    double magnitude = sqrt(pow(num.real, 2) + pow(num.imag, 2));

    result.real = pow(magnitude, n) * cos(n * atan(num.imag / num.real));
    result.imag = pow(magnitude, n) * sin(n * atan(num.imag / num.real));

    return result;
}

为什么在传递给函数后访问指针会出现分段错误？

Why does accessing pointers after passing to a function give segmentation fault?

c

pointers

structure

fft