Is there a C function to get permissions of a file?
Solution 1
The permission bits can be ascertained using the st_mode field of the struct returned by the stat function. The individual bits can be extracted using the constants S_IRUSR (User Read), S_IWUSR (User Write), S_IRGRP (Group Read) etc.
Example:
struct stat statRes;
if(stat(file, &statRes) < 0)return 1;
mode_t bits = statRes.st_mode;
if((bits & S_IRUSR) == 0){
//User doesn't have read privilages
}
In terms of passing this to chmod, mode_t is just a typedef of a uint_32, so that should be simple enough.
Solution 2
The permission can be checked using stat(2), extracting information from S_* flags. Here a function using stat(2):
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
int getChmod(const char *path){
struct stat ret;
if (stat(path, &ret) == -1) {
return -1;
}
return (ret.st_mode & S_IRUSR)|(ret.st_mode & S_IWUSR)|(ret.st_mode & S_IXUSR)|/*owner*/
(ret.st_mode & S_IRGRP)|(ret.st_mode & S_IWGRP)|(ret.st_mode & S_IXGRP)|/*group*/
(ret.st_mode & S_IROTH)|(ret.st_mode & S_IWOTH)|(ret.st_mode & S_IXOTH);/*other*/
}
int main(){
printf("%0X\n",getChmod("/etc/passwd"));
return 0;
}
Related videos on Youtube
trawww
Updated on September 15, 2022Comments
-
trawww about 1 year
I am writing a c program to be run on UNIX, and attempting to utilize the chmod command. After consulting the man pages, i know that chmod needs two parameters. first is the permission bits, second is the file to be changed. I want to take the bitwise OR of the file's current permission bits and those entered by the user, and feed that to chmod() to change the file's permissions.
I found the
access()
function, but am having trouble figuring out how to use it to get the permission bits of the specified file.What i have right now is:
octalPermissionString = strtol(argv[1], (char**)NULL, 8); if(chmod(argv[2], octalPermissionString | (access(argv[2], octalPermissionString)) < 0) { fprintf(stderr, "Permissions of file %s were not changed.\n"); }
where:
argv[1] contains a string of a three digit decimal number entered by the user to be converted to octal and then be used as the permission bits to be bitwise OR'ed,
argv[2] is the the file to have it's permission changed, also specified by the user.
octalPermissionString is a long to hold the octal conversion of user input.
Is/Are there any other functions that can return the permission bits of a given file?
EDIT: missing close parenthesis
-
David Heffernan almost 10 yearsThere is nothing in C to do this. It's implementation specific.
-
Sinkingpoint almost 10 yearsThe st_mode field of stat() might help...
-
-
trawww almost 10 yearsJust tried it out on my program, and everything seems to be working as I want it. after calling stat and retrieving the bits, I call:
chmod(argv[2], bits | octalPermissionBits);
and I get the output I was looking for. Thank you!