Fuse Filesystem Problem

Question

Hi, i'm developing a fuse filesystem that stores data in the RAM, but i'm having problems when i write something to a file. The file turns into a blank file.

Here's the code:

#define FUSE_USE_VERSION 26

#include <fuse/fuse.h>
#include <string.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>

char hello_data[200] = {'h', 'e', 'l', 'l', 'o', '\n'};
int hello_size = 6;

int homsifs_getattr(const char* path, struct stat* stbuf)
{
    memset(stbuf, 0, sizeof(struct stat));

    if (strcmp(path, "/") == 0)
     stbuf->st_mode = S_IFDIR | 0666;
    else if (strcmp(path, "/hello") == 0)
    {
     stbuf->st_mode = S_IFREG | 0666;
     stbuf->st_size = hello_size;
    }
    else
     return -ENOENT;

    return 0;
}

int homsifs_readdir(const char* path, void* buf, fuse_fill_dir_t filler,
off_t offset, struct fuse_file_info* fi)
{
    if (strcmp(path, "/") != 0)
     return -ENOENT;

    filler(buf, ".", NULL, 0);
    filler(buf, "..", NULL, 0);
    filler(buf, "hello", NULL, 0);

    return 0;
}

int homsifs_open(const char* path, struct fuse_file_info* fi)
{
    if (strcmp(path, "/hello") != 0)
     return -ENOENT;

    return 0;
}

int homsifs_read(const char* path, char* buf, size_t size, off_t offset,
struct fuse_file_info* fi)
{
    int offset_size = hello_size - offset;

    if (size > offset_size)
     size = offset_size;

    memcpy(buf, hello_data + offset, size);

    return size;
}

int homsifs_truncate(const char* path, off_t size)
{
    if (size > 200)
     return -ENOMEM;

    if (hello_size < size)
     memset(hello_data + size, 0, size - hello_size);

    hello_size = size;

    return 0;
}

int homsifs_write(const char* path, const char* buf, size_t size, off_t offset,
struct fuse_file_info* fi)
{
    if (strcmp(path, "/hello") != 0)
     return -ENOENT;

    memcpy(hello_data + offset, buf, size);

    return size;
}

struct fuse_operations homsifs_operations =
{
    .getattr = homsifs_getattr,
    .readdir = homsifs_readdir,
    .open = homsifs_open,
    .read = homsifs_read,
    .truncate = homsifs_truncate,
    .write = homsifs_write
};

int main(int argc, char** argv)
{
    return fuse_main(argc, argv, &homsifs_operations, NULL);
}

Anyone knows what's wrong?

Thanks.

Answer 1

When a program opens the file for writing, it gets truncated. After that, any data written is inaccessible because you're not correctly updating hello_size in homsifs_write.