Allocation in function, unusable in main

问题:

I am trying to allocate space and put in file data in my function for my array of char arrays pointer in main. I get a segmentation fault when I run the program. Can anyone tell me why?

datawords[0] is printed correctly in the function.

This is my function:

void database_extract (char **data_words) {
FILE *f_data;
f_data = fopen("database.txt","r");     
struct stat st;
stat("database.txt", &st);
data_words = (char **)malloc(st.st_size * sizeof(char));
if (data_words == NULL) {
  printf("No room\n");
  exit(EXIT_FAILURE);
}
data_words[0] = "test";
printf("%s",data_words[0]);
}

This is my main:

int main () {       
char **data_words;
database_extract (data_words);
printf("%s",data_words[0]);
}

Any help would be greatly appreciated.

回答1:

You need to pass to the function a pointer to the data_words array in order to use the allocation in the main.

try this:

void database_extract (char ***data_words) {
FILE *f_data;
f_data = fopen("database.txt","r");     
struct stat st;
stat("database.txt", &st);
*data_words = (char **)malloc(st.st_size * sizeof(char));
if (data_words == NULL) {
    printf("No room\n");
    exit(EXIT_FAILURE);
}
data_words[0] = "test";
printf("%s",data_words[0]);

}

and in the main:

int main () {       
char **data_words;
database_extract (&data_words);
printf("%s",data_words[0]);

}

I am not sure if I got all the * right it gets me confused sometimes but the idea is to pass a pointer to the function.

回答2:

When you want something to be initialized by a function you need to use a & on the function call and one extra * in the function signature

void database_extract (char ***data_words) {

which is matched by

database_extract (&data_words);