char struct and comparison 1

[simanek]

hi all,

I'm using the getpwnam method of the pwd library and in it, it has a passwd struct. A member of this struct is called char *pw_shell. When I call getpwnam("simanek&quot I am returned a passwd struct that i call pw. Now, I'm trying to see if my shell is /bin/ksh (which it is) but for some reason, I cannot compare them. The following is my 'if' statement for comparison.

if(pw->pw_shell == "/bin/ksh&quot

this fails no matter what i do. However, when I do printf(pw->pw_shell) "/bin/ksh" (no quotes) is printed to my console. Any ideas?
Thanks. Mike
~~~~
simanek@uiuc.edu
"It's a Swingline!"
~~~~

 

[programsecrets]

Try using this instead:

if (!strcmp(pw->pw_shell, "/bin/ksh&quot)
{
}

;-)

 

[michaelkrauklis]

Just for further reference, the comparison was failing because it was comparing pointer values, not the actual char values. pw->pw_shell gives you a pointer. If you wanted to do the actual comparison by yourself you would need to dereference the pointer and check each char, eg:

char *pt="/bin/ksh";
bool comp=true;
for(int i=0;i<8&&comp;i++){
  if(*(pt+i)==(pw->pw_shell)[i]){
    comp=false;
  }
}
if(comp){
  ...code...
}

This is a little redundant and unecessary but helps show why your comparison fails I hope. Obviously there is the need for more error checking, such as the event that your string is the same but longer than the string to check:

if(comp&&*(pw->pw_shell+8)!=NULL){
  comp=false;
}

In order for your comparison to return true the pointers would both have to be pointing to the exact same char in memory. Fun with pointers! good luck MYenigmaSELF
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