使用字符串數組的分段錯誤C

Question

我製作了一個程序，將字符串分解爲由空格分隔的令牌，然後將每個單獨的字符串複製到一個字符串數組中。

程序工作正常，直到達到for-loop，併成功將第一個字符串添加到數組中並打印出來後，程序崩潰。我調試它，發現

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

返回SEGFAULT然後執行第二次循環。同時調用堆棧打印出以下幾點：

地址：75F943F9，功能：strlen的（），文件：C：\ WINDOWS \ SysWow64資料\ msvcrt.dll.`

我試圖糾正自己的程序，但沒有結果。我首先想到的是，循環嘗試訪問出界區域，但我認爲我正確地使用了malloc。

#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正如您所知，正在改變字符串。

計算出字數後，字符串將包含一個單詞。因此下一個strtok將返回NULL。

以非破壞性的方式計算參數的數量，或者創建一個字符串的副本。

Answer 2

第一個問題是你沒有包含相關的函數原型。這將使它假設一切都是一個整數。見下：

$ 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] 

我建議你在繼續之前解決這個問題。

Answer 3

按照手冊頁的strtok： 「使用這些功能時一定要謹慎，如果你使用它們，請注意： 這些功能修改他們的第一個參數 這些功能不能在常量字符串中使用。」

因此，您第一次解析查找num時，您修改了您的輸入字符串，因此第二次解析導致SEGFAULT的垃圾。此外，你不應該使用常量字符串：

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

Answer 4

strtok替換每個' '在你的情況下，NULL。考慮以下幾點：

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"); 

輸出：

 
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 6D 65 

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 6D 65