使用Strtok对C中的二维数组进行分段错误

问题描述

在尝试将诸如“1,2,3;4,5,6;7,8,9”之类的字符串拆分为二维数组时，我不断收到分段错误，为什么有人可以请教此代码给我解释一下？

int main(void){
    char string[100];
    char *token;
    char *end;
    int num;
    int row_counter;
    int column_counter;
    int counter_position;
    char *rows[100];
    int square_ints[40][40];

    row_counter = 0;
    column_counter = 0;
    string[] = "1,9";
    token = strtok(string,";");
    rows[row_counter] = token;
    row_counter++;
    while (token != NULL){
        token = strtok(NULL,";");
        rows[row_counter] = token;
        row_counter++;
    }

    counter_position = row_counter;

    for (row_counter = 0; row_counter < counter_position; row_counter++) {
        token = strtok(rows[row_counter],",");
        num = strtol(token,&end,10);
        square_ints[row_counter][column_counter] = num;
        printf("%d\n",square_ints[row_counter][column_counter]);
        column_counter++;
        while (token != NULL) {
            token = strtok(NULL,");
            num = strtol(token,10);
            square_ints[row_counter][column_counter] = num;
            printf("%d\n",square_ints[row_counter][column_counter]);
            column_counter++;
        }
    }
}

它应该打印：1 2 3 4 5 6 7 8 9 而我得到 1 2 3 Segmentation Fault

解决方法

扩展 MikeCAT 提到的内容：

作为 for 循环的开始，您需要：

token = strtok(rows[row_counter],",");
num = strtol(token,&end,10);

稍后你做：

token = strtok(NULL,10);

问题在于，在这两种情况下，strtok 都会返回 NULL，因此 token 的值是 NULL。当您将 token 传递给 strtol 时，它会尝试使用/取消引用它，并且会产生段错误。

因此，要修复，在每次 strtok 调用后，您需要添加：

if (token == NULL)
    break;

这是更正后的代码。我添加了一些调试 printf，以便您可以查看问题。

我使用了 cpp 条件来区分新旧代码：

#if 0
// old code
#else
// new code
#endif

#if 1
// new code
#endif

无论如何，这里是：

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

#ifdef DEBUG
#define dbgprt(_fmt...) \
    do { \
        printf(_fmt); \
    } while (0)
#else
#define dbgprt(_fmt...) \
    do { \
    } while (0)
#endif

int
main(void)
{
#if 0
    char string[100];
#else
    char string[] = "1,2,3;4,5,6;7,8,9";
#endif
    char *token;
    char *end;
    int num;
    int row_counter;
    int column_counter;
    int counter_position;
    char *rows[100];
    int square_ints[40][40];

    row_counter = 0;
    column_counter = 0;
#if 0
    string[] = "1,9";
#endif
    token = strtok(string,";");
    rows[row_counter] = token;
    row_counter++;
    while (token != NULL) {
        token = strtok(NULL,";");
        rows[row_counter] = token;
        row_counter++;
    }

    counter_position = row_counter;

    for (row_counter = 0; row_counter < counter_position; row_counter++) {
        dbgprt("DEBUG: rows[%d]='%s'\n",row_counter,rows[row_counter]);
        token = strtok(rows[row_counter],");
        dbgprt("DEBUG: token='%s'\n",token);
#if 1
        if (token == NULL)
            break;
#endif
        num = strtol(token,10);
        square_ints[row_counter][column_counter] = num;
        printf("%d\n",square_ints[row_counter][column_counter]);
        column_counter++;
        while (token != NULL) {
            token = strtok(NULL,");
            dbgprt("DEBUG2: token='%s'\n",token);
#if 1
            if (token == NULL)
                break;
#endif
            num = strtol(token,10);
            square_ints[row_counter][column_counter] = num;
            printf("%d\n",square_ints[row_counter][column_counter]);
            column_counter++;
        }
    }

    return 0;
}

请注意，虽然上述代码有效，但它是在给定循环之前和循环内部复制代码。如果我们添加一个额外的指针变量（例如 bp），我们可以稍微简化一下代码：

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

int
main(void)
{
    char string[] = "1,9";
    char *token;
    char *end;
    int num;
    int row_counter;
    int column_counter;
    int counter_position;
    char *bp;
    char *rows[100];
    int square_ints[40][40];

    row_counter = 0;
    column_counter = 0;

    bp = string;
    while (1) {
        token = strtok(bp,";");
        bp = NULL;
        if (token == NULL)
            break;
        rows[row_counter] = token;
        row_counter++;
    }

    counter_position = row_counter;

    for (row_counter = 0; row_counter < counter_position; row_counter++) {
        bp = rows[row_counter];
        if (bp == NULL)
            break;

        while (1) {
            token = strtok(bp,");
            bp = NULL;
            if (token == NULL)
                break;

            num = strtol(token,square_ints[row_counter][column_counter]);
            column_counter++;
        }
    }

    return 0;
}

还有另一个类似的函数 strsep 被一些人认为是“strtok done right”。这是使用该功能的版本：

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

int
main(void)
{
    char string[] = "1,9";
    char *token;
    char *end;
    int num;
    int row_counter;
    int column_counter;
    int counter_position;
    char *bp;
    char *rows[100];
    int square_ints[40][40];

    row_counter = 0;
    column_counter = 0;

    bp = string;
    while (1) {
        token = strsep(&bp,";");
        if (token == NULL)
            break;
        rows[row_counter] = token;
        row_counter++;
    }

    counter_position = row_counter;

    for (row_counter = 0; row_counter < counter_position; row_counter++) {
        bp = rows[row_counter];
        if (bp == NULL)
            break;

        while (1) {
            token = strsep(&bp,");
            if (token == NULL)
                break;

            num = strtol(token,square_ints[row_counter][column_counter]);
            column_counter++;
        }
    }

    return 0;
}

c strtok