Existence of a file given its path - c

Given the path, is there a way to find out whether the file exists without opening the file?
Thanks

The most efficient way is access with the F_OK flag.
stat also works but it's much heavier weight since it has to read the inode contents, not just the directory.

You can use the stat system call. Make sure though that you check errno for the correct error because stat may return -1 for a number of other reasons/Failures.
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
main()
{
struct stat BUF;
if(stat("/Filepath/FileName",&BUF)==0)
{
printf("File exists\n");
}
}
Another way is by using the access function.
#include <unistd.h>
main()
{
if(access("/Filepath/FileName", F_OK) != -1 )
{
printf("File exists\n");
}
else
{
printf("File does not exist\n");
}
}

#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <errno.h>
int rc;
struct stat mystat;
rc = stat(path, &mystat);
Now check rc and (maybe) errno.
EDIT 2011-09-18 addendum:
Both access() and stat() return 0 if the path points to a non-file (directory, fifo,symlink, whatever)
In the stat() case, this can be tested with "((st_mode & S_IFREG) == S_IFREG)".
Best way still is to just try to open the file with open() or fopen().

Try to remove it (unlink()). If successful, it doesn't exist anymore. If unsuccessful,
interpret errno to see if it exists :)

Related

lstat returning <0

Basically, this should be a simple piece of code that opens a directory stream and looks for symbolic links. Whenever a symbolic link is found, it should print ("Symbolic link found");
However, the lstat(dirp->d_name,&bufcall always returns a value < 0, and I don't know why.
I created the two symbolic link opening the file folder, opening a terminal window inside the folder and running
ln -s ciao.txt link1 and
ln -s ciao2.txt link2
I know I should call closedir() later in my code, please don't care about this.
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <dirent.h>
#include <unistd.h>
void main (int argc, char* argv[])
{
char buffer[100],dir[100];
struct stat buf;
int x;
DIR *dp;
struct dirent *dirp;
if((dp=opendir(argv[1]))==NULL)
{
printf("\nError opening directory stream, now exiting...\n");
exit(-1);
}
while((dirp=readdir(dp))!=NULL)
{
lstat(dirp->d_name,&buf);
if(S_ISLNK(buf.st_mode))
printf("\n%s Is a symbolic link\n",dirp->d_name);
else
printf("\n%s Is not a symbolic link\n",dirp->d_name);
}
}
Some help would be appreciated. Thanks.
d_name is the file name in the directory, not a full path name. You must eather chdir into the directory you are looking at, or construct full path names for the files.
The simplest solution is to add this line just before your while loop:
chdir(argv[1]);

Setting Immutable Flag using ioctl() in C

I have attempted to make a script that creates a file and then sets it as immutable similar to the chattr +i command for linux. The script compiles (with gcc), runs and the file is created. However the file itself is not immutable and can be removed with a simple rm -f. I have attempted to stacktrace where chattr is called and I found a function called ioctl. I then used what little information I could gather and came up with what I have below. I narrowed it down from ext2_fs.h but it just doesn't seem to work. I've clearly overlooked something.
Updates to previous entry: Compiles but returns -1 on ioctl() function. Bad address shown with perror().
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/ioctl.h>
#include <linux/fs.h>
int main()
{
FILE *fp;
char shovel[16] = "I have a shovel!";
fp = fopen("/shovel.txt", "w+");
fwrite(shovel, sizeof(shovel[0]), sizeof(shovel)/sizeof(shovel[0]), fp);
ioctl(fileno(fp), FS_IOC_SETFLAGS, 0x00000010);
fclose(fp);
}
Any help appreciated.
You are using the right ioctl command, but you're passing it the wrong arguments.
The manpage for ioctl_list(2) shows that FS_IOC_SETFLAGS expects to receive a pointer to int (an int *), yet you're passing it an integer literal (hence the Bad Address error).
The fact that you don't to any error checking whatsoever is also not helping.
The correct flag to pass to FS_IOC_SETFLAGS is a pointer holding the value EXT2_IMMUTABLE_FL, which is defined in ext2fs/ext2_fs.h (some older / different Linux distributions seem to have it under linux/ext2_fs.h), so you'll need to #include <ext2fs/etx2_fs.h>. Make sure to install e2fslibs-dev (and probably you'll need linux-headers too).
This code is working:
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/ioctl.h>
#include <linux/fs.h>
#include <ext2fs/ext2_fs.h>
int main()
{
FILE *fp;
char shovel[16] = "I have a shovel!";
if ((fp = fopen("shovel.txt", "w+")) == NULL) {
perror("fopen(3) error");
exit(EXIT_FAILURE);
}
fwrite(shovel, sizeof(shovel[0]), sizeof(shovel)/sizeof(shovel[0]), fp);
int val = EXT2_IMMUTABLE_FL;
if (ioctl(fileno(fp), FS_IOC_SETFLAGS, &val) < 0)
perror("ioctl(2) error");
fclose(fp);
return 0;
}
Remember to run this as root.
UPDATE:
As Giuseppe Guerrini suggests in his answer, you might want to use FS_IMMUTABLE_FL instead, and you won't need to include ext2_fs.h:
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/ioctl.h>
#include <linux/fs.h>
int main()
{
FILE *fp;
char shovel[16] = "I have a shovel!";
if ((fp = fopen("shovel.txt", "w+")) == NULL) {
perror("fopen(3) error");
exit(EXIT_FAILURE);
}
fwrite(shovel, sizeof(shovel[0]), sizeof(shovel)/sizeof(shovel[0]), fp);
int val = FS_IMMUTABLE_FL;
if (ioctl(fileno(fp), FS_IOC_SETFLAGS, &val) < 0)
perror("ioctl(2) error");
fclose(fp);
return 0;
}
The main problem is that the ioctl wants a pointer to the mask, not a direct constant. You have to define a int variable, store the mask (0x10) in it and pass its address as third argument of ioctl.
Also, I'd add some hints:
other programs to change attributes are used to use low-level I/O directly (open, close...). Also, the file is usually opened with O_RDONLY.
Use FS_IMMUTABLE_FL istead the raw constant.
Get the current attribute mask first (FS_IOC_SETFLAGS) and mask it with the new flag, so other settings are not lost by the service.

Call to fdopendir() corrupts file descriptor

I stumbled upon a problem in a program I was working on. The following reproduces my issue:
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <sys/types.h>
#include <dirent.h>
#include <fcntl.h>
#include <sys/stat.h>
int main(int argc, char *argv[])
{
int fd, ret_fd;
DIR *dirp;
fd = open("./", O_RDONLY);
#if 1
if ((dirp = fdopendir(fd)) == NULL) {
perror("dirp");
return 1;
}
closedir(dirp);
#endif
ret_fd = openat(fd, "Makefile", O_RDONLY);
if (ret_fd == -1) {
perror("ret_fd");
return 1;
}
return 0;
}
Basically, the call to openat(), which has been preceeded by fdopendir(), fails with: Bad file descriptor. However, this does not happen if fdopendir() is omitted.
I know that fdopendir() makes internal use of the file descriptor, but shouldn't it revert any changes to it after calling closedir()?
What can I do to prevent openat() from failing in this case?
The POSIX description of fdopendir() says:
Upon calling closedir() the file descriptor shall be closed.
So the descriptor is likely to be closed by the time you call openat().
And this is from a typical Linux man page for fdopendir():
After a successful call to fdopendir(), fd is used internally by the
implementation, and should not otherwise be used by the application.

How to read a file permission and store it in a variable in C

In Linux platform(Ubuntu system).
As describe in title.
I try to get a number like '0644' and store it in a variable for later use.
the stat, fstat, lstat system calls can be used to retrieve the permission of a file.
The field st_mode of the stat structure contains the permission of the file specified as argument of the system call. Then a variable of type mode_t can be used as local storage in your application.
This is an example :
#include <sys/types.h>
#include <sys/stat.h>
#include <time.h>
#include <stdio.h>
#include <stdlib.h>
#define FILE_NAME "test.c"
int main(int argc, char *argv[])
{
struct stat sb;
mode_t file_permision;
if (stat(FILE_NAME, &sb) == -1) {
perror("stat");
exit(EXIT_FAILURE);
}
file_permision = sb.st_mode;
printf(" File permission : %o (octal)\n",
(unsigned int) file_permision);
return 0;
}

Distinguishing a pipe from a file in Unix

Given a FILE*, is it possible to determine the underlying type? That is, is there a function that will tell me if the FILE* is a pipe or a socket or a regular on-disk file?
There's a fstat(2) function.
NAME
stat, fstat, lstat - get file status
SYNOPSIS
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
int fstat(int fd, struct stat *buf);
You can get the fd by calling fileno(3).
Then you can call S_ISFIFO(buf) to figure it out.
Use the fstat() function. However, you'll need to use the fileno() macro to get the file descriptor from file FILE struct.
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
FILE *fp = fopen(path, "r");
int fd = fileno(fp);
struct stat statbuf;
fstat(fd, &statbuf);
/* a decoding case statement would be good here */
printf("%s is file type %08o\n", path, (statbuf.st_mode & 0777000);

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