函数内部c中的Malloc使用

Question

I am trying to pass a pointer to a function.In this function i use malloc to reserve space.The problem is that when i return in main the program doesn't respond.Here is my symplified code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int def(char **B){
    int i;
    B = malloc(3 * sizeof( char ));
    for(i = 0; i < 2 ; i++){
        B[i] = malloc(5 * sizeof(char));
    }
    for(i = 0; i < 2 ; i++){
        scanf("%s" , B[i]);
    }
    for(i = 0; i < 2 ; i++){
        printf("%s\n" , B[i]);
    }
    return 0;
}

int main(int argc, char *argv[]) {
    char **B;
    int i;
    def(B);
    for(i = 0; i < 2 ; i++){
        printf("%s\n" , B[i]);
    }
    return 0;
}

Answer 1

int def(char **B)

should have been

char** def(char **B) 

and its return value should have been

return B; 
 /* else the memory allocated inside the function will be freed at
  * the end and by accessing it later you have undefined behavior for the 
  * rest of the program
  */ 

B = malloc(3 * sizeof( char ));

should have been

B = malloc(3 * sizeof( char*)); // you have two levels of indirection. so char* first

for(i = 0; i < 2 ; i++) // similary with the other for loops

should have been

 for(i = 0; i < 3 ; i++) // you used 3 in the above step

def(B);

should have been

B=def(B);

It is a good practice to use free() to free the allocated memory though it will be automatically freed at the end of the program

Answer 2

It seems you are trying to malloc something that looks like a 2D array (or rather an array of strings).

However,

def(B);

is a call by value so B doesn't change when the function returns.

If you want to change B you need

def(&B);

and then you need to change the function signature accordingly - then you'll be a three star programmer.

If you want to do this correct then read:

Create 2D array by passing pointer to function in c

and read the answer given by @Lundin - that's the way to do it

Answer 3

You seem to be trying to allocate a list of strings, but you do not provide the ability to reference this outside the def function. Part 1 of your problem is that you need to be providing a pointer to your list:

int def(char ***B) {

Another option is to return the pointer you have created:

char** def() {

Secondly in your first malloc you allocate space for 3 chars but you need char pointers. sizeof char != sizeof char* .

B = malloc(3 * sizeof( char ));

Should be:

*B = malloc(3 * sizeof char*);

Also you should use const int or #define to define your size constants so you dont accidentally get the wrong size in different places, eg.

#define SIZE_OF_LIST 3
int def(char ***B){
    int i;
    B = malloc(SIZE_OF_LIST * sizeof( char ));
    for(i = 0; i < SIZE_OF_LIST ; i++){

Finally when you pass your pointer to the function def you need to do this (top accommodate the first change):

def(&B);

or

B = def();

Answer 4

Actually, it is worse than that. All C function arguments are 'by value', meaning that the variables inside the function are local copies of the variables passed in at the call. So when you assign a value to an argument (which you are allowed to do, unless it is declared 'const' [I assume C++ 'const' has been adopted in C by now] ), you are NOT assigning to the variable passed in at the call. So in this case the statement

B = malloc(3 * sizeof(char *));

does NOT alter the pointer B declared in main() ; it just leaks memory. Assuming you really need to return an int for this function, you need to add a level of indirection:

int def(char ***B){
    *B = malloc(3 * sizeof(char *));
    for(i=0; i<2; i++){
        (*B)[i] = malloc(5 * sizeof(char));
    }
    ...
}

...

char **B;
...
def( &B );   /* Note the 'address-of' operator! */

If the return doesn't have to be int : eliminate the argument to def() , change it to return ****char**, return the result of the first malloc() directly, and change the call to B = def();

Answer 5

You problem is that when you call def(B) , you are creating a char ** that is a copy of B . If you change this new B , the changes doesn't reflect on the main function, since they were made to the copy of B .

You should use a char *** and call def(&B) (i do not consider this a good approach, tho) or you could have B initialized at the main and call def(B) to alloc and read its char * or you could return the new B .

Also, check the problems pointed in other answers.