I'm trying to write a small program to show me the internal representation of a directory in linux (debian, specifically). The idea was a small C program using open(".", O_RDONLY), but this seems to give no output. The program is the following:
#include <stdio.h>
#include <fcntl.h>
int main(int argc, char** argv)
{
int fd = open(argv[1],O_RDONLY,0 );
char buf;
printf("%i\n",fd);
while(read(fd, &buf, 1) > 0)
printf("%x ", buf);
putchar('\n');
}
When I run it on regular files it works as expected, but on a directory such as ".", it gives no output. The value of fd is 3 (as expected) but the call to read returns -1.
Why isn't this working, and how could I achieve to read the internal representation?
Thanks!
For handling directories, you need to use opendir/readdir/closedir. Read the corresponding man pages for more infos.
To check whether a filename corresponds to a directory, you first need to call stat for the filename and check whether it's a directory (S_ISDIR(myStatStruc.st_mode)).
Directories are a filesystem specific representation and are part of the file system. On extfs, they are a table of string/inode pairs, unlike files which have blocks of data(that you read using your code above).
To read directory-specific information in C, you need to use dirent.h .
Look at this page for more information
http://pubs.opengroup.org/onlinepubs/7908799/xsh/dirent.h.html
On POSIX systems, the system call "stat" would give you all the information about an inode on the filesystem(file/directory/etc.)
Related
I can examine a file's permission bits using the stat() system call, which returns a struct, which contains a field that in turn contains the file type and mode. Is there a way to do the same using nothing but the open and read syscalls? I.e. by analyzing each bit? For example the following code reads a file (the first four bytes) and determines whether it's an ELF file or not ..
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>
int main(int argc, char *argv[])
{
int fd = open("main", O_RDONLY);
char *buf = malloc(sizeof (char) * 4);
read (fd, buf, 4);
if (strcmp(buf, "ELF"))
printf("It is an ELF file.\n");
free(buf);
return 0;
}
Is there a similar way to read a file to extract the information bit-by-bit?
File permissions are not part of the file's contents but part of its directory entry, therefore you can't read the permissions using open or read on the file.
Using stat is the proper way to do this.
I saw that you mentioned in another comment that you're doing this for learning purposes only. Anyone else reading this for production work...DONT. It'll be non-portable! You probably just want to use stat on the containing directory.
You're going to want to take a look at your systems definition of the stat function. Here is one example of the stat function implementation. Its definitely not as easy as just calling stat. But if you study this source and follow links in it, you'll get an idea of how it works.
Unfortunately I'm sane enough to not study the source, and am unsure if it can be done with just combinations of open and read. My guess is no, though (just a guess)
I am trying to figure out the file type of a file, without using external libs or the "file" command.
I have viewed a number of posts and threads, and they point to using the stat() function (unix man stat) and playing with the "st_mode" from the stat struct.
But I have no idea how to do this, nor am I able to find a good example of doing it.
For example the program takes in a file F, I want to be able to read F similar to the program below and give similar output. And the filetype of F is a PDF, but it does not have the extension on it.
FURTHER EXAMPLE: If I have foo.pdf, but I changed the extension to *.png (foo.png) I can pass my program "foo.png" and say it is infact a .pdf file.
When a file is created, it makes a "magic number", example with a PDF, the magic number of PDF files start with "%PDF" (hex 25 50 44 46)."
How can I use the magic number to figure out the filetype.
I understand some type of table will need to be made at my end, to support files. And I am only doing a small handful <10.
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
void errorInput()
{
fprintf(stderr, "\nYou have received this message due to an error. \n");
fprintf(stderr, "Please type 'filetype <file>' to properly execute the program.\n");
fprintf(stderr, "Thank you and have a fine day! \n\n");
exit(0);
}
int main(int argc, char *argv[])
{
char command[128];
if (argc == 2)
{
strcpy(command, "file ");
strcat(command, argv[1]);
system(command);
}
else
{
errorInput();
}
return 0;
}
Thank You in advance!
Like Jonathon Reinhart Pointed, don't try to reinvent the wheel use libmagic:
#include <stdio.h>
#include <magic.h>
int main(void) {
struct magic_set *magic = magic_open(MAGIC_MIME|MAGIC_CHECK);
magic_load(magic,NULL);
printf("Output1: '%s'\n",magic_file(magic,"ValgrindOut.xml"));
printf("Output2: '%s'\n",magic_file(magic,"program"));
printf("Output3: '%s'\n",magic_file(magic,"Chapter9.pdf"));
printf("Output4: '%s'\n",magic_file(magic,"test.txt"));
printf("Output5: '%s'\n",magic_file(magic,"linux-3.17.tar.xz"));
printf("Output6: '%s'\n",magic_file(magic,"gcc-5.2.0.tar.gz"));
printf("Output7: '%s'\n",magic_file(magic,"/home/michi"));
return 0;
}
Compile:
gcc -o program program.c -lmagic
Output:
Output1: 'application/xml; charset=us-ascii'
Output2: 'application/x-executable; charset=binary'
Output3: 'application/pdf; charset=binary'
Output4: 'text/plain; charset=utf-8'
Output5: 'application/x-xz; charset=binary'
Output6: 'application/gzip; charset=binary'
Output7: 'inode/directory; charset=binary'
First, you need to include sys/stat.h
Next, you need to declare a struct stat in your code:
struct stat s
Next, you pass a pointer to your stat structure along with the file/object name:
returnval = stat("filename", &s);
Check the return value, you'll get < 0 on error. If no error the object/file exists, we can use a macro function to determine the file type:
if (S_ISREG(s.st_mode))
/* Regular text file... */
else if (S_ISDIR(s.st_mode))
/* Is a directory.... */
I suggest you have a look at the man page (man 3 stat) and it will give you all of the types that st_type may potentially be (it can be used to identify files, directories, block devices, sym links, etc)
Another very useful member of the stat struct is st_size which gives you a files size in bytes.
ETA - the stat() system call won't tell you if a file is a PDF or anything like that - normally we'd use the extension, if there is no extension and you're trying to identify specific file formats then stat() won't be of much use to you.
Most files usually will have a portion called as Header/MetaData. It is in this portion/segments of the file which will contain details about the file it self.Also, these Headers/MetaData Segments will also contain the Signature to identify the file type. But be aware most of these Signatures will be in an Hex Signature format
Example
PDF Signature - 25 50 44 46(In Hex) or %PDF
JPEG Signature - Start FF D8 and end of file FF D9
So, Basically you need to open the file in a binary format and parse the file structure and compare it to see if it matches with any one of the file types you define in your program.Like suppose you wanna check if it's pdf file then you need to first open the file in binary mode then scan the file till you get the bytcode/hex code which matches the bytcode/hex code of a pdf file. Use the C fopen() function in binary mode i.e "rb".
Or you can open the file normally without binary mode like this,
unsigned int data;
data=fgetc(pfile);
You might want to look into this for further details,
Magic Number
File Signatures
I have a program where I need to set the permissions of a file (say /home/hello.t) using chmod and I have to read the permissions to be set from a file. For this I first read the permissions into a character array and then try to modify the permissions of the file. But I see that permissions are set in a weird manner.
A sample program I have written:
main()
{
char mode[4]="0777";
char buf[100]="/home/hello.t";
int i;
i = atoi(mode);
if (chmod (buf,i) < 0)
printf("error in chmod");
}
I see that the permissions of the file are not set to 777. Can you please help me out on how to set the permissions of the file after reading the same from a character array.
The atoi() function only translates decimal, not octal.
For octal conversion, use strtol() (or, as Chris Jester-Young points out, strtoul() - though the valid sizes of file permission modes for Unix all fit within 16 bits, and so will never produce a negative long anyway) with either 0 or 8 as the base. Actually, in this context, specifying 8 is best. It allows people to write 777 and get the correct octal value. With a base of 0 specified, the string 777 is decimal (again).
Additionally:
Do not use 'implicit int' return type for main(); be explicit as required by C99 and use int main(void) or int main(int argc, char **argv).
Do not play with chopping trailing nulls off your string.
char mode[4] = "0777";
This prevents C from storing a terminal null - bad! Use:
char mode[] = "0777";
This allocates the 5 bytes needed to store the string with a null terminator.
Report errors on stderr, not stdout.
Report errors with a newline at the end.
It is good practice to include the program name and file name in the error message, and also (as CJY pointed out) to include the system error number and the corresponding string in the output. That requires the <string.h> header (for strerror()) and <errno.h> for errno. Additionally, the exit status of the program should indicate failure when the chmod() operation fails.
Putting all the changes together yields:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <sys/stat.h>
int main(int argc, char **argv)
{
char mode[] = "0777";
char buf[100] = "/home/hello.t";
int i;
i = strtol(mode, 0, 8);
if (chmod (buf,i) < 0)
{
fprintf(stderr, "%s: error in chmod(%s, %s) - %d (%s)\n",
argv[0], buf, mode, errno, strerror(errno));
exit(1);
}
return(0);
}
Be careful with errno; it can change when functions are called. It is safe enough here, but in many scenarios, it is a good idea to capture errno into a local variable and use the local variable in printing operations, etc.
Note too that the code does no error checking on the result of strtol(). In this context, it is safe enough; if the user supplied the value, it would be a bad idea to trust them to get it right.
One last comment: generally, you should not use 777 permission on files (or directories). For files, it means that you don't mind who gets to modify your executable program, or how. This is usually not the case; you do care (or should care) who modifies your programs. Generally, don't make data files executable at all; when files are executable, do not give public write access and look askance at group write access. For directories, public write permission means you do not mind who removes any of the files in the directory (or adds files). Again, occasionally, this may be the correct permission setting to use, but it is very seldom correct. (For directories, it is usually a good idea to use the 'sticky bit' too: 1777 permission is what is typically used on /tmp, for example - but not on MacOS X.)
here's another question about splice(). I'm hoping to use it to copy files, and am trying to use two splice calls joined by a pipe like the example on splice's Wikipedia page. I wrote a simple test case which only tries to read the first 32K bytes from one file and write them to another:
#define _GNU_SOURCE
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
int main(int argc, char **argv) {
int pipefd[2];
int result;
FILE *in_file;
FILE *out_file;
result = pipe(pipefd);
in_file = fopen(argv[1], "rb");
out_file = fopen(argv[2], "wb");
result = splice(fileno(in_file), 0, pipefd[1], NULL, 32768, SPLICE_F_MORE | SPLICE_F_MOVE);
printf("%d\n", result);
result = splice(pipefd[0], NULL, fileno(out_file), 0, 32768, SPLICE_F_MORE | SPLICE_F_MOVE);
printf("%d\n", result);
if (result == -1)
printf("%d - %s\n", errno, strerror(errno));
close(pipefd[0]);
close(pipefd[1]);
fclose(in_file);
fclose(out_file);
return 0;
}
When I run this, the input file seems to be read properly, but the second splice call fails with EINVAL. Anybody know what I'm doing wrong here?
Thanks!
From the splice manpage:
EINVAL Target file system doesn't support splicing; target file is
opened in append mode; neither of the descriptors refers to a
pipe; or offset given for non-seekable device.
We know one of the descriptors is a pipe, and the file's not open in append mode. We also know no offset is given (0 is equivalent to NULL - did you mean to pass in a pointer to a zero offset?), so that's not the problem. Therefore, the filesystem you're using doesn't support splicing to files.
What kind of file system(s) are you copying to/from?
Your example runs on my system when both files are on ext3 but fails when I use an external drive (I forget offhand if it is DOS or NTFS). My guess is that one or both of your files are on a file system that splice does not support.
The splice(2) system call is for copying between files and pipes and not between files, so it can not be used to copy between files, as has been pointed out by the other answers.
As of Linux 4.5 however a new copy_file_range(2) system call is available that can copy between files. In the case of NFS it can even cause server side copying.
The linked man page contains a full example program.
I am working on TTCN-3 (Testing and Test Control Notation) scripting language. I wanted to prepare on guideline checker for this code files.
For that I want to read lines of TTCN-3 script file( some thing like file.ttcn ) one by one into a buffer. But for me fopen / sopen / open / fgetc / fscanf are not able to work properly and are not reading the file correctly. It is giving NULL. Is there any way I can read characters of it into a buffer. I think C cannot read files with more than three extension characters (like .ttcn). Forgive me if my assumption is wrong.
My Environment is Turbo C on windows.
Edit:
Yes I checked those errors also but they are giving unknown error for read()
and no such file or directory exists.
My code is as follows
#include <errno.h>
#include <io.h>
#include <fcntl.h>
#include <sys\stat.h>
#include <process.h>
#include <share.h>
#include <stdio.h>
int main(void)
{
int handle;
int status;
int i=0;
char ch;
FILE *fp;
char *buffer;
char *buf;
clrscr();
handle = sopen("c:\\tc\\bin\\hi.ttcn", O_BINARY, SH_DENYNONE, S_IREAD);
/here even I used O_TEXT and others/
if (!handle)
{
printf("sopen failed\n");
// exit(1);
}
printf("\nObtained string %s #",buf);
close(handle);
fp=fopen("c:\\tc\\bin\\hi.ttcn","r"); \\sorry for the old version of one slash
if(fp==NULL) \\I was doing it with argv[1] for opening
{ \\user given file name
printf("\nCannot open file");
}
ch=fgetc(fp);
i=0;
while(i<10)
{
printf("\ncharacter is %c %d",ch,ch);
i++; //Here I wanted to take characters into
ch=fgetc(fp); //buffer
}
getch();
return 0;
}
The most likely culprit is your Turbo C, an ancient compiler. It's techincally a DOS compiler, not Windows. That would limit it's RunTme Library to 8.3 filenames. Upgrade to something newer - Turbo C++ seems like a logical successor, but Microsoft's VC++ Express would work as well.
Your assumption is wrong about extensions. If fopen is returning NULL, you should output the result of strerror(errno) or use the perror() function to see why it failed.
Edit: The problem is probably because you have "c:\tc\bin\hi.ttcn". in C, "\t" is interpreted as tab, for example.
You could do
"c:\\tc\\bin\\hi.ttcn"
But this is extremely ugly, and your system should accept:
"c:/tc/bin/hi.ttcn"
MS-DOS does not know about long file names, thos including files with extensions longer than 3 characters. Therefore, the CRT provided by Turbo C most probably does not look for the name you are providing, but a truncated one - or something else.
Windows conveniently provides a short (i.e. matching the 8.3 format, most of the time ending in ~1 unless you play with files having the same 8-character prefix) file name for those; one way to discover it is to open a console window and to run "dir /x" in the folder your file is stored.
Find the short name associated to your file and patch it into your C source file.
Edit: Darn, I'll read the comments next time. All credits to j_random_hacker.
Now that you've posted the code, another problem comes to light.
The following line:
fp=fopen("c:\tc\bin\hi.ttcn","r");
Should instead read:
fp=fopen("c:\\tc\\bin\\hi.ttcn","r");
In C strings, the backslash (\) is an escape character that is used to encode special characters (e.g. \n represents a newline character, \t a tab character). To actually use a literal backslash, you need to double it. As it stands, the compiler is actually trying to open a file named "C:<tab>c<backspace>in\hi.ttcn" -- needless to say, no such file exists!