字符串数组C的分段错误

Question

我编写了一个程序，它将一个字符串分解为由空格分隔的标记，然后将每个单独的字符串复制到一个字符串数组中。

程序运行良好，直到它到达 for 循环，并且在成功将第一个字符串添加到数组并打印后，程序崩溃。我调试了一下，发现

args[i] = malloc((strlen(comm_str) + 1) * sizeof(char));

返回SEGFAULT 然后第二次执行循环。调用堆栈也打印出以下内容：

地址：75F943F9，函数：strlen()，文件：C:\Windows\syswow64\msvcrt.dll。`

我尝试自己更正程序，但没有结果。起初我以为循环试图访问超出绑定区域，但我认为我已经正确地 malloc'd 一切。

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

int main(){

    char **args = NULL;
    char input[] = "cmdline -s 20 -r -t parameter -p 20 filename";

    int num = 0;
    char *comm_str = strtok(input, " ");                            /*Tokens command line*/

    while(comm_str != NULL){                                        /*Counts number of tokens*/
        num++;
        printf("%s %d\n", comm_str, strlen(comm_str));
        comm_str = strtok(NULL, " ");
    }
    printf("\n");

    args = malloc(num * sizeof(char*));                          /*Allocates memory for the first string entry*/
    if(!args){
        return 0;
    }

    comm_str = strtok(input, " ");                                  /*Starts tokening from beginning*/
    for(int i = 0; i < num; i++){
        args[i] = malloc((strlen(comm_str) + 1) * sizeof(char));    /*Allocates memory for strings*/
        if(!args[i]){
            for(int b = 0; b < i; b++){
                free(args[b]);
            }
            free(args);
            return 0;
        }
        strcpy(args[i], comm_str);
        printf("%s\n", args[i]);
        comm_str = strtok(NULL, " ");
    }

    printf("%d\n", num);

}

Answer 1

strtok，如您所知，正在更改字符串。

计算单词数后，字符串将包含一个单词。因此下一个strtok 将返回NULL。

以非破坏性方式计算参数的数量，或制作字符串的副本。

Answer 2

根据 strtok 的手册页： “使用这些功能时要小心。如果您确实使用它们，请注意： 这些函数修改它们的第一个参数。 这些函数不能用于常量字符串。”

所以，第一次解析找到 num 时，修改了输入字符串，因此第二次解析垃圾，这会导致 SEGFAULT。另外，你不应该使用常量字符串：

char input[] = "cmdline -s 20 -r -t parameter -p 20 filename";

Answer 3

第一个问题是你没有包含相关的函数原型。这将使它假设一切都是整数。见下文：

$ gcc -Wall x.c -o x
x.c: In function ‘main’:
x.c:9:5: warning: implicit declaration of function ‘strtok’ [-Wimplicit-function-declaration]
x.c:9:22: warning: initialization makes pointer from integer without a cast [enabled by default]
x.c:13:9: warning: implicit declaration of function ‘strlen’ [-Wimplicit-function-declaration]
x.c:13:37: warning: incompatible implicit declaration of built-in function ‘strlen’ [enabled by default]
x.c:13:9: warning: format ‘%d’ expects argument of type ‘int’, but argument 3 has type ‘long unsigned int’ [-Wformat]
x.c:14:18: warning: assignment makes pointer from integer without a cast [enabled by default]
x.c:18:5: warning: implicit declaration of function ‘malloc’ [-Wimplicit-function-declaration]
x.c:18:12: warning: incompatible implicit declaration of built-in function ‘malloc’ [enabled by default]
x.c:23:14: warning: assignment makes pointer from integer without a cast [enabled by default]
x.c:24:5: error: ‘for’ loop initial declarations are only allowed in C99 mode
x.c:24:5: note: use option -std=c99 or -std=gnu99 to compile your code
x.c:25:27: warning: incompatible implicit declaration of built-in function ‘strlen’ [enabled by default]
x.c:27:13: error: ‘for’ loop initial declarations are only allowed in C99 mode
x.c:28:17: warning: implicit declaration of function ‘free’ [-Wimplicit-function-declaration]
x.c:28:17: warning: incompatible implicit declaration of built-in function ‘free’ [enabled by default]
x.c:30:13: warning: incompatible implicit declaration of built-in function ‘free’ [enabled by default]
x.c:33:9: warning: implicit declaration of function ‘strcpy’ [-Wimplicit-function-declaration]
x.c:33:9: warning: incompatible implicit declaration of built-in function ‘strcpy’ [enabled by default]
x.c:35:18: warning: assignment makes pointer from integer without a cast [enabled by default]
x.c:40:1: warning: control reaches end of non-void function [-Wreturn-type]

我建议你在继续之前解决这个问题。

Answer 4

strtok 在您的情况下将每个 ' ' 替换为 NULL。考虑以下几点：

char input[] = "cmdline -s 20 -r -t parameter -p 20 filename";
size_t inputSize = strlen(input);

for(int i = 0; i < inputSize; i++)
    printf("%02X ", input[i]);
printf("\n\n");

char *comm_str = strtok(input, " "); 
while(comm_str != NULL)
    comm_str = strtok(NULL, " ");

for(int i = 0; i < inputSize; i++)
    printf("%02X ", input[i]);
printf("\n\n");

输出：

63 6D 64 6C 69 6E 65 20 2D 73 20 32 30 20 2D 72 20 2D 74 20 70 61 72 61 6D 65 74 65 72 20 2D 70 20 32 30 20 66 69 6C 65 6E 61 63 6D 64 6C 69 6E 65 00 2D 73 00 32 30 00 2D 72 00 2D 74 00 70 61 72 61 6D 65 74 65 72 00 2D 70 00 32 30 00 66 69 6C 65 6E 61