返回一个指向结构体的指针，该结构体中有一个 char ** 指针到指针

Question

I just cant understand, how am I supposed to return a struct which contains a double pointer in it.... when ever i try it prints correct values in same function... but prints garbage values when i print after i return it to main...

struct arguments{
char **argv;
int argc;
};

this is the struct.

struct arguments * shell_make_argu(char *line)
{
   struct arguments *data=malloc(sizeof(struct arguments));
   int bufsize = word_BUFSIZE , i;
   data->argc = 0;
   data->argv = malloc(bufsize * sizeof(char*));
   char *word;

  if (!data->argv) {
    fprintf(stderr, "shell memory allocation error\n");
    exit(EXIT_FAILURE);
 }

word = strtok(line, word_separater);
while (word != NULL) {
    data->argv[data->argc] = word;
    data->argc++;

    if (data->argc >= bufsize) {
      bufsize += word_BUFSIZE;
      data->argv = realloc(data->argv, bufsize * sizeof(char*));
      if (!data->argv) {
        fprintf(stderr, "shell memory allocation error\n");
        exit(EXIT_FAILURE);
      }
    }

    word = strtok(NULL, word_separater);
}
data->argv[data->argc] = NULL;
for(i=0;i<data->argc;i++)
    printf("Command was: %s\n",data->argv[i]);
return data;
}

and this is how i am receiving the struct in main....

int main(int argc,char **argv)
{
struct arguments *data = malloc(sizeof(struct arguments));
char *line;
int status = 1 , i;
do {
    printf("T_Shell>> ");                           
    line = shell_take_input();                     
    if(line != '\0' && strcmp(line,"found ctrl_D") != 0){
        data = shell_make_argu(line);
        free(line);
        for(i=0;i<data->argc;i++)
            printf("Command was: %s\n",data->argv[i]);
        shell_execute_comand(data->argc,data->argv);
    }
    else if(line == '\0')
        printf("\n");
    else
        break;
 } while (status);
 system("clear");
 return 0;
}

Answer 1

You did free to line before printing data->argv in main function, which is bad because each elements of data->argv are pointing to the buffer pointed by line .

Do the free after calling shell_execute_comand , or allocate new memory and copy the strings returned by strtok instead of assigning strtok 's return value directly to the elements of data->argv .

Answer 2

shell_make_argu() function:

struct arguments* shell_make_argu(char *line)
{
   struct arguments *data = malloc(sizeof(struct arguments));
   int bufsize = word_BUFSIZE , i;

   data->argc = 0;
   data->argv = malloc(bufsize * sizeof(char*));

   char *word;

   if (!data->argv) 
   {
       fprintf(stderr, "shell memory allocation error\n");
       exit(EXIT_FAILURE);
   }

   word = strtok(line, word_separater);
   while (word != NULL) 
   {
       if (data->argc == bufsize)  <-- Moved realloc at beginning of loop
       {
           bufsize += word_BUFSIZE;
           data->argv = realloc(data->argv, bufsize * sizeof(char*));
           if (!data->argv) 
           {
               fprintf(stderr, "shell memory allocation error\n");
               exit(EXIT_FAILURE);
           }
       }

       data->argv[data->argc] = malloc(strlen(word)+1);  <-- Added allocation of argv[argc] char pointer
       strcpy(data->argv[data->argc], word);   <-- Added storing token

       data->argc++;

       word = strtok(NULL, word_separater);
   }

   //data->argv[data->argc] = NULL;   <-- Not required, can be removed
   for(i=0;i<data->argc;i++)
       printf("Command was: %s\n",data->argv[i]);

    return data;
}

main() function:

int main(int argc, char **argv)
{
    struct arguments *data;  <-- Removed malloc, make_shell_argu() returns allocated pointer
    char *line;
    int status = 1 , i;
    do 
    {
        printf("T_Shell>> ");                           
        line = shell_take_input();                     
        if(line != NULL && strcmp(line,"found ctrl_D") != 0)  <-- Changed condition check with (line != NULL)
        {
            data = shell_make_argu(line);
            free(line);

            for(i=0;i<data->argc;i++)
                printf("Command was: %s\n",data->argv[i]);

            shell_execute_comand(data->argc,data->argv);

            for(i=0;i<data->argc;i++)  <-- Added memory releasing code
            {
                free(data->argv[i]);
            }
            free(data->argv);
            free(data);                 <-- Freeing 'data' also, make_shell_argu() will return new allocated pointer next time
        }
        else if(line == NULL)  <-- Changed condition check to (line == NULL)
        {
            printf("\n");
        }
        else
        {
            status = 0;
        }
    } while (status);

    system("clear");

    return 0;
}