File system in userspace (FUSE) on Linux : C implementation

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  • BBFS is a good starting point to develop a file system in C
  • Application should keep in mind file offset before issuing a request. A write should be followed with a seek to offset zero, before issuing read.
  • FUSE 2.7 reads 8K data by default, in two 4K chunks
  • Read happens in last 4K and the first 4K data block

Example:

I had set the file size as 4MB in getattr () implementation.

int bb_getattr(const char *path, struct stat *statbuf)
{
    int retstat = 0;
    char fpath[PATH_MAX];
    bb_fullpath(fpath, path);

    memset(statbuf, 0, sizeof(struct stat));
    if (strcmp(path, "/") == 0) {
        statbuf->st_mode = S_IFDIR | 0755;
        statbuf->st_nlink = 2;
    } else {
        statbuf->st_mode = S_IFREG | 0444;
        statbuf->st_nlink = 1;
        statbuf->st_size = 4 * 1024* 1024;
    }   
    return retstat;
}

The sequence of calls and their arguments is as follows:

bb_getattr(path="/abcd.txt", statbuf=0xc5387960)
    bb_fullpath:  rootdir = "/tmp", path = "/abcd.txt", fpath = "/tmp/abcd.txt"
bb_open(path"/abcd.txt", fi=0xc5daaa50)
    bb_fullpath:  rootdir = "/tmp", path = "/abcd.txt", fpath = "/tmp/abcd.txt"
    fi:
    flags = 0x00008002
    fh_old = 0x00000000
    writepage = 0
    direct_io = 0
    keep_cache = 0
    fh = 0x0000000000000001
    lock_owner = 0x0000000000000000
bb_write(path="/abcd.txt", buf=0xc4966050, size=10, offset=0, fi=0xc5387a50)
    fi:
    flags = 0x00000000
    fh_old = 0x00000001
    writepage = 0
    direct_io = 0
    keep_cache = 0
    fh = 0x0000000000000001
    lock_owner = 0x0000000000000000
bb_read(path="/abcd.txt", buf=0x06ccbd90, size=12288, offset=4096, fi=0xc5daaa50)  <- Here
    fi:
    flags = 0x00000000
    fh_old = 0x00000001
    writepage = 0
    direct_io = 0
    keep_cache = 0
    fh = 0x0000000000000001
    lock_owner = 0x0000000000000000

bb_read(path="/abcd.txt", buf=0x06ccbd90, size=4096, offset=0, fi=0xc5daaa50)
    fi:
    flags = 0x00000000
    fh_old = 0x00000001
    writepage = 0
    direct_io = 0
    keep_cache = 0
    fh = 0x0000000000000001
    lock_owner = 0x0000000000000000
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