在C中传递指向struct的指针？

Question

I am working on a C program to traverse a file system in Linux, record the memory of each file, and spit out a histogram at the end. I am having issues with passing the pointer to structs, and am not too familiar with how these are passed in C.

I am trying to pass my head pointer into my readDirectory function, but the way its behaving, its passing in an empty linked list head in every time the function is called. Within the function, it adds on nodes as expected, but every time it calls itself recursively, it seems the list is wiped out and the head is back to being NULL.I assume I am passing them wrong, so can someone please tell me the correct way to pass them, or point me to a good resource that explains it well?

The issue is also happening when I pass it to the printHistrogram function, but if I can fix it elsewhere, I'll know how to fix it here, too.

Thanks in advance. -Chris

#include <stdio.h>
#include <stdlib.h>
#include <dirent.h>
#include <unistd.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <fcntl.h>

struct memList
{
    int mem;
    struct memList* next;
};

void readDirectory(const char*, struct memList*);
void printHistogram(struct memList*, int binSize);



int main(int argc, char* argv[])
{
    struct memList* head = NULL;

    if (argc != 3)
    {
        perror("Not enough parameters\n");
    }

    int binSize = strtol(argv[2], NULL, 10);

    readDirectory(argv[1], head);

    printHistogram(head, binSize);

    return 0;
}

void readDirectory(const char * passedDir, struct memList* head)
{
    DIR * directory = opendir(passedDir);

    if (directory == NULL)
            printf("Unable to open directory\n");

    while(1)
    {
        struct dirent * current;

        current = readdir(directory);
        if (!current)
            break;

        if ((current->d_type == 4) && (strcmp(current->d_name, ".") != 0) && (strcmp(current->d_name, "..") != 0))  //current path is directory but not the current or parent
        {
            char * path = malloc(sizeof(char) * 300);

            strcpy(path, passedDir);

            if (path[strlen(path) - 1] != '/')
                strcat(path, "/");
            strcat(path, current->d_name);

            readDirectory(path, head);

            free(path);
        }

        else
        {   
            char * path = malloc(sizeof(char) * 300);

            strcpy(path, passedDir);

            if (path[strlen(path) - 1] != '/')
                strcat(path, "/");
            strcat(path, current->d_name);

            struct stat tempStat;
            stat(path, &tempStat);

            free(path);

            int temp = tempStat.st_size;

            if (head == NULL)
            {   
                head = (struct memList*)malloc(sizeof(struct memList));
                head->next = NULL;
                head->mem = temp;
            }
            else
            {
                struct memList * tempStruct = (struct memList*)malloc(sizeof(struct memList));
                tempStruct->next = head;
                tempStruct->mem = temp;
                head = tempStruct;
                //printf("mem is %d\n", head->mem);   //debugging
            }
        }

    }
    closedir(directory);
}

void printHistogram(struct memList* head, int binSize)
{
    int numElements = 10;

    int * array = (int*)malloc(sizeof(int) * numElements);
    for (int i = 0; i < numElements; i++)
        array[i] = 0;

    while(head != NULL)
    {
        //printf("mem is %d\n", head->mem);   //debugging
        int temp = head->mem;
        int temp2 = ((temp - (temp % binSize)) / binSize);

        if ((temp2 + 1) > numElements)
        {
            int * new = realloc(array, (sizeof(int) * (temp2 + 1)));
            array = new;
            for (int i = numElements; i < (temp2 + 1); i++)
                array[i] = 0;
            numElements = (temp2 + 1);
        }

        array[temp2] += 1;      

        head = head->next;
    }

    for (int i = 0; i < numElements; i++)
    {
        printf("%d\n", array[i]);
        printf("Block %d:  ", i + 1);
        for (int j = 0; j < array[i]; j++)
            printf("*");
        printf("\n");
    }
}

Answer 1

Study "pass by reference" and "pass by value". C is pass by value.

If you want head to be modified such that it keeps it's value outside readDirectory then you need to pass a pointer to a pointer.