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From: maw@auc.UUCP (Michael A. Walker)
Newsgroups: comp.lang.c
Subject: Re: getpwnam
Message-ID: <32131@auc.UUCP>
Date: Sun, 16-Aug-87 13:34:42 EDT
Article-I.D.: auc.32131
Posted: Sun Aug 16 13:34:42 1987
Date-Received: Sun, 16-Aug-87 22:49:53 EDT
References: <8725@brl-adm.ARPA>
Organization: Atlanta University, Atlanta, GA
Lines: 125
Summary: getpwnam:  the lazy way(sometimes)

In article <8725@brl-adm.ARPA>, oxy!bagpiper@csvax.caltech.EDU (Michael Paul Hunter) writes:
> 
> In uudecode.c there is a function called getpwnam. (struct passwd getpwnam)
> Since I am not using a true unix (primix 2.0 yuch) I need to know what this
> function does so that I can try to rewrite it.  Could anybody please either
> send me a functional speck of getpwnam, or, could somebody send me a piece
> of code that does what unix getpwname does, but on a PR1ME 9955.

	The function getpwnam() returns a pointer to the entry in the password
file for a specified login name.  The following is an example of how it might
be used in a program(probably only in UN*X enviornments:

		$cat sample.c

		#include <stdio.h>
		#include <pwd.h>

		main(argc, argv)
		int argc;
		char *argv[];
		{
			struct passwd *getpwnam(), *pwentry;

			if (argc < 2) {
				fprintf(stderr, "Usage: user name expected\n");
				exit (0);
			}

			pwentry = getpwnam(argv[1]);
			if(pwentry == (struct passwd *)NULL ){
				fprint(stderr,"Can't find %s in pwd file\n",
					argv[1];
				exit (0);
			} else
				printf("%d\n",pwentry-pw_uid);
		}

The above program would produce an output like this:

$sample foo
127
$
where foo is the specified login.

However, to mimic getpwnam(), one can very simply write a program that will 
search the password file and retrieve the desired information.  Here's an
example that retrieves all of the users login name:

#include <stdio.h>

/*
 * Field number of name in passwd
 */
#define FIELD	4

main()
{
	FILE *fp; /*declares a pointer to a file*/
	char buf[120], name[80];
	int m1, m2, count;

	/* Open password file */
	if((fp = fopen("/etc/passwd", "r")) == NULL){
		perror("Opening /etc/passwd"); /*print this error message to
						stderr*/
		exit(1); /* Abort program */
	}
	
	printf("List of Users\n");
	/*
	 * Read in a line from passwd file
	 */
	while(fgets(buf, 119, fp) != NULL){  /* read in a line of code at
						a time until the end of the
        					file*/
		m1 = 0;
		count = 0;
		/*
		 * Search for fourth colon (beginning of name field)
		 */
		while(count < FIELD && buf[m1] != NULL){
			if(buf[m1] == ':') ++count;
			++m1;
		}
		m2 = m1 + 1;
		/*
		 * Search for next colon (end of name field)
		 */
		while(buf[m2] != ':' && buf[m1] != NULL)
			++m2;
		/*
		 * Terminate string
		 */
		buf[m2] = NULL;
		/*
		 * Copy to name variable
		 */
		strcpy(name, buf+m1);  /* copy the context of buffer to name */
		printf("%s\n", name);
	}
	printf("Done\n");
}

This code can be easily modified get the login name of one user.  I believe 
this code is generic enough to run in any environment.  At most, you would
have to change the name and directory of the password file(from /etc/passwd
to /yoursystem/password_file), and the strcpy function if it is not available
on your system.  I am not very well versed in primix, but if you don't have
the fopen(), perror(), and fgets() functions, very good implementations of 
these are given in Kernighan & Ritchie's[1] C programming book.

I hope this helps.

-----mike

				   REFERENCE

1.	Brian W. Kernighan, Dennis M. Ritchie, "The C Programming Language,"
		Prentice-Hall, Englewood Cliffs, NJ., 1978.

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