popen for linux cmd output

Hi,

The following C program on Linux, does a set of operations based on whether user is "root" or an ordinary user.

Using popen() function, I've tried to manipulate Linux cmd output in the program:

Code:

---

    int main()
{
        FILE *fp;
        char ch[130],user[]="root";
        int i;

        fp = popen("id -u -n","r");

        while (fgets(ch,sizeof ch,fp))
        {
        printf("\n String from fgets = %s \n String in user = %s",ch,user);

        i=strcmp(user,ch);
        printf("\n i= %d\n",i);
        i=strlen(user);
        printf("\n i= %d\n",i);
        i=strlen(ch);
        printf("\n i= %d\n",i);

        if ((strcmp(user,ch))==0)
                printf("\n Welcome to Administrator Login \n");
        else       
                printf("\n Welcome to User Login \n");
        }
        pclose(fp);
        return 0;
}

---

Even, the user is root, but still its giving wrong output.

# icc chusr.c -o chusr

# ./chusr

String from fgets = root

String in user = root
i= -1

i= 4

i= 5

Welcome to User Login

I'm not getting, why strcmp function gave "-1" value here.

Is there a function to read word by word from fp?
What is the best method to manipulate data between program to OS & vice-versa.

Quote:

---

Originally Posted by sangamesh

I'm not getting, why strcmp function gave "-1" value here.

---

Because ch and user are different lengths.

Why don't you do the thing id does by yourself? It will be quite a lot simpler and cleaner. getuid(2) and geteuid(2).

Thanks fronty for your suggestion. Its working with the function getuid(). Is root's uid (0) same in all linux flavours?

Also can some one let's know, why length of ch is 5? why not 4? :confused:

Quote:

---

fgets() reads in at most one less than size characters from stream and
stores them into the buffer pointed to by s. Reading stops after an
EOF or a newline. If a newline is read, it is stored into the buffer.
A '\0' is stored after the last character in the buffer.

---

There from man fgets.

Quote:

---

Originally Posted by sangamesh

Is root's uid (0) same in all linux flavours?

---

Super user's uid is 0 on all posix-ish operating systems.