I am running an embedded C program on a Digi 9P 9215 module with a Linux op. system. One of the tasks I am running is telnet. I use the supplied telnet daemon and communicate with the rest of the system through shared memory to update a system menu, store in flash etc. I want to change the login password (I have set up a user admin which invokes my telnet interface) by running a shell that takes the new password from a file that I write just before running the shell. Everything appears to work except the final massage from running $passwd admin is that the password is unchanged! I am assuming it is something to do with file ownership/permissions but I cannot seem to set anything up that works. The C code and files are below;
C code
fp = fopen(PASSWORD_FILE, "w+");
//store the new password in our structure
sprintf(pwdform.pwd1,"%s\n\r",command);
sprintf(pwdform.pwd2,"%s\n\r",command);
//write to the newpwd file
fwrite(&pwdform.pwd1[0],sizeof(char),sizeof(pwdform),fp);
fclose(fp);
//execute our shell file, chpwd - this will call newpwd
sprintf(sysCmd,"sh /etc/chpwd\n\r");
fpVal = system(sysCmd);

newpwd file
ptf_pwd
ptf_pwd

chpwd file

#file to change telnet admin password
chmod a+xrw /etc/newpwd
passwd admin < /etc/newpwd

If I follow exactly the same process from the console keyboard everything works, and the password is changed, but when executed from my program, everything appears to work except the password remains unchanged! Thus my conclusion it is something to do with permissions.

Any pointers to help me solve this would be most welcome! Thanks

passwd uses kbd device in raw mode, as opposed to "cooked", google it

I had 30-40 machines that I used to admin and occasionally changed their passwords
I used a program call "expect" written on top of tcl (only good program in tcl I ever saw)
You can write scripts as a series of "expect $", send "passwd", expect "Old Password: ", send "foobar" , etc, etc

Thanks BSD. Guess I need to figure out how to install expect on my system then! Unfortunately, as a novice, that is rather more difficult than I'm sure it should be.