FUSE Preprocessing Filesystem

Question

EDIT: Updated code, does everything I need it to for now, still maybe a little unclean, but much better than it was.
A while ago I was thinking up ways to pre-process files such as .Xdefaults when they were read, to allow per machine configs and other such things (I think it turns out that Xdefaults actually supports this anyway), and I thought the coolest way to do this would be to use a FUSE filesystem.
Originally I was going to use the standard cpp program to handle the processing, but I've changed this to use gpp for the time being since it seems to work better for this kind of thing. You can have files with stuff like
#if 0
Not going to be printed
#else
This is printed
#exec date
#endif
Compile with
gcc -o prepfs prepfs.c -lfuse -D_FILE_OFFSET_BITS=64 -DFUSE_USE_VERSION=26
Run with
./prepfs mountpoint -b basedir -pp preprocessor (eg, gpp -x "%s")
Your mountpoint will simply mirror whatever basedir is set to, but run all files through the preprocessor before outputting them.
prepfs.c
PrepFS, by HashBox
Modified from "Hello World" example, original notice below:
FUSE: Filesystem in Userspace
Copyright (C) 2001-2005 Miklos Szeredi <[email protected]>
This program can be distributed under the terms of the GNU GPL.
#include <fuse.h>
#include <fuse/fuse_opt.h>
#include <stdlib.h>
#include <stdio.h>
#include <stddef.h>
#include <string.h>
#include <dirent.h>
struct options {
char *base_dir;
char *pp_path;
} options;
static struct fuse_opt prepfs_opts[] =
{ "-b %s", offsetof(struct options, base_dir), 0},
{ "-pp %s", offsetof(struct options, pp_path), 0}
static char *get_extended_path(const char *path)
char *extended_path = malloc(sizeof(char) * PATH_MAX + 1);
extended_path[0] = '\0';
strncat(extended_path, options.base_dir, PATH_MAX);
strncat(extended_path, path, PATH_MAX);
return extended_path;
static int prepfs_getattr(const char *path, struct stat *stbuf)
char *expath = get_extended_path(path);
stat(expath, stbuf); /* Return unmodified stat, although filesize will be wrong */
free(expath);
return 0;
static int prepfs_readdir(const char *path, void *buf, fuse_fill_dir_t filler, off_t offset, struct fuse_file_info *fi)
char *expath = get_extended_path(path);
DIR *dirp = opendir(expath);
free(expath);
struct dirent *ent;
while ((ent = readdir(dirp)) != NULL) {
filler(buf, ent->d_name, NULL, 0);
closedir(dirp);
return 0;
static int prepfs_read(const char *path, char *buf, size_t size, off_t offset, struct fuse_file_info *fi)
char *expath = get_extended_path(path);
char pp_temp[PATH_MAX + strlen(options.pp_path)];
sprintf(pp_temp, options.pp_path, expath);
free(expath);
size = 0;
FILE *fp = popen(pp_temp, "r");
while (!feof(fp)) {
fread(buf + size, sizeof(char), 1, fp);
size += 1;
pclose(fp);
size -= 1;
return size;
static struct fuse_operations prepfs_oper = {
.getattr = prepfs_getattr,
.readdir = prepfs_readdir,
.read = prepfs_read,
int main(int argc, char *argv[])
int ret;
struct fuse_args args = FUSE_ARGS_INIT(argc, argv);
memset(&options, 0, sizeof(struct options));
if (fuse_opt_parse(&args, &options, prepfs_opts, 0) == -1) {
return -1;
if (!options.base_dir || !options.pp_path) {
fprintf(stderr, "Usage: %s mountpoint -b basedir -pp preprocessor (eg, gpp -x \"%%s\")\n", argv[0]);
return -1;
ret = fuse_main(args.argc, args.argv, &prepfs_oper, NULL);
fuse_opt_free_args(&args);
return ret;
Last edited by HashBox (2009-06-29 09:18:57)

Answer 1

EDIT: Updated code, does everything I need it to for now, still maybe a little unclean, but much better than it was.
A while ago I was thinking up ways to pre-process files such as .Xdefaults when they were read, to allow per machine configs and other such things (I think it turns out that Xdefaults actually supports this anyway), and I thought the coolest way to do this would be to use a FUSE filesystem.
Originally I was going to use the standard cpp program to handle the processing, but I've changed this to use gpp for the time being since it seems to work better for this kind of thing. You can have files with stuff like
#if 0
Not going to be printed
#else
This is printed
#exec date
#endif
Compile with
gcc -o prepfs prepfs.c -lfuse -D_FILE_OFFSET_BITS=64 -DFUSE_USE_VERSION=26
Run with
./prepfs mountpoint -b basedir -pp preprocessor (eg, gpp -x "%s")
Your mountpoint will simply mirror whatever basedir is set to, but run all files through the preprocessor before outputting them.
prepfs.c
PrepFS, by HashBox
Modified from "Hello World" example, original notice below:
FUSE: Filesystem in Userspace
Copyright (C) 2001-2005 Miklos Szeredi <[email protected]>
This program can be distributed under the terms of the GNU GPL.
#include <fuse.h>
#include <fuse/fuse_opt.h>
#include <stdlib.h>
#include <stdio.h>
#include <stddef.h>
#include <string.h>
#include <dirent.h>
struct options {
char *base_dir;
char *pp_path;
} options;
static struct fuse_opt prepfs_opts[] =
{ "-b %s", offsetof(struct options, base_dir), 0},
{ "-pp %s", offsetof(struct options, pp_path), 0}
static char *get_extended_path(const char *path)
char *extended_path = malloc(sizeof(char) * PATH_MAX + 1);
extended_path[0] = '\0';
strncat(extended_path, options.base_dir, PATH_MAX);
strncat(extended_path, path, PATH_MAX);
return extended_path;
static int prepfs_getattr(const char *path, struct stat *stbuf)
char *expath = get_extended_path(path);
stat(expath, stbuf); /* Return unmodified stat, although filesize will be wrong */
free(expath);
return 0;
static int prepfs_readdir(const char *path, void *buf, fuse_fill_dir_t filler, off_t offset, struct fuse_file_info *fi)
char *expath = get_extended_path(path);
DIR *dirp = opendir(expath);
free(expath);
struct dirent *ent;
while ((ent = readdir(dirp)) != NULL) {
filler(buf, ent->d_name, NULL, 0);
closedir(dirp);
return 0;
static int prepfs_read(const char *path, char *buf, size_t size, off_t offset, struct fuse_file_info *fi)
char *expath = get_extended_path(path);
char pp_temp[PATH_MAX + strlen(options.pp_path)];
sprintf(pp_temp, options.pp_path, expath);
free(expath);
size = 0;
FILE *fp = popen(pp_temp, "r");
while (!feof(fp)) {
fread(buf + size, sizeof(char), 1, fp);
size += 1;
pclose(fp);
size -= 1;
return size;
static struct fuse_operations prepfs_oper = {
.getattr = prepfs_getattr,
.readdir = prepfs_readdir,
.read = prepfs_read,
int main(int argc, char *argv[])
int ret;
struct fuse_args args = FUSE_ARGS_INIT(argc, argv);
memset(&options, 0, sizeof(struct options));
if (fuse_opt_parse(&args, &options, prepfs_opts, 0) == -1) {
return -1;
if (!options.base_dir || !options.pp_path) {
fprintf(stderr, "Usage: %s mountpoint -b basedir -pp preprocessor (eg, gpp -x \"%%s\")\n", argv[0]);
return -1;
ret = fuse_main(args.argc, args.argv, &prepfs_oper, NULL);
fuse_opt_free_args(&args);
return ret;
Last edited by HashBox (2009-06-29 09:18:57)