Invoking the MTA directly: /usr/lib/sendmail
The mail program is an example of a Mail User Agent, a
program intended to be invoked by the user to send and receive mail, but
which does not handle the actual transport. A program for transporting
mail is called an MTA, and the most commonly found MTA on Unix
systems is called sendmail. There are other MTAs in use, such as
MMDF, but these generally include a program that emulates the
usual behaviour of sendmail.
Historically, sendmail has usually been found in `/usr/lib',
but the current trend is to move library programs out of `/usr/lib'
into directories such as `/usr/sbin' or `/usr/libexec'. As a
result, one normally invokes sendmail by its full path, which is
system-dependent.
To understand how sendmail behaves, it's useful to understand the
concept of an envelope. This is very much like paper mail; the
envelope defines who the message is to be delivered to, and who it is
from (for the purpose of reporting errors). Contained in the envelope
are the headers, and the body, separated by a blank line.
The format of the headers is specified primarily by RFC 822; see also
the MIME RFCs.
There are two main ways to use sendmail to originate a message:
either the envelope recipients can be explicitly supplied, or
sendmail can be instructed to deduce them from the message
headers. Both methods have advantages and disadvantages.
5.2.2.1 Supplying the envelope explicitly
The recipients of a message can simply be specified on the command line.
This has the drawback that mail addresses can contain characters that
give system() and popen() considerable grief, such as
single quotes, quoted strings etc. Passing these constructs successfully
through shell interpretation presents pitfalls. (One can do it by
replacing any single quotes by the sequence single-quote backslash
single-quote single-quote, then surrounding the entire address with
single quotes. Ugly, huh?)
Some of this unpleasantness can be avoided by eschewing the use of
system() or popen(), and resorting to fork() and
exec() directly. This is sometimes necessary in any event; for
example, user-installed handlers for SIGCHLD will usually break
pclose() to a greater or lesser extent.
Here's an example:
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sysexits.h>
/* #include <paths.h> if you have it */
#ifndef _PATH_SENDMAIL
#define _PATH_SENDMAIL "/usr/lib/sendmail"
#endif
/* -oi means "dont treat . as a message terminator"
* remove ,"--" if using a pre-V8 sendmail (and hope that no-one
* ever uses a recipient address starting with a hyphen)
* you might wish to add -oem (report errors by mail)
*/
#define SENDMAIL_OPTS "-oi","--"
/* this is a macro for returning the number of elements in array */
#define countof(a) ((sizeof(a))/sizeof((a)[0]))
/* send the contents of the file open for reading on FD to the
* specified recipients; the file is assumed to contain RFC822 headers
* & body, the recipient list is terminated by a NULL pointer; returns
* -1 if error detected, otherwise the return value from sendmail
* (which uses <sysexits.h> to provide meaningful exit codes)
*/
int send_message(int fd, const char **recipients)
{
static const char *argv_init[] = { _PATH_SENDMAIL, SENDMAIL_OPTS };
const char **argvec = NULL;
int num_recip = 0;
pid_t pid;
int rc;
int status;
/* count number of recipients */
while (recipients[num_recip])
++num_recip;
if (!num_recip)
return 0; /* sending to no recipients is successful */
/* alloc space for argument vector */
argvec = malloc((sizeof char*) * (num_recip+countof(argv_init)+1));
if (!argvec)
return -1;
/* initialise argument vector */
memcpy(argvec, argv_init, sizeof(argv_init));
memcpy(argvec+countof(argv_init),
recipients, num_recip*sizeof(char*));
argvec[num_recip + countof(argv_init)] = NULL;
/* may need to add some signal blocking here. */
/* fork */
switch (pid = fork())
{
case 0: /* child */
/* Plumbing */
if (fd != STDIN_FILENO)
dup2(fd, STDIN_FILENO);
/* defined elsewhere -- closes all FDs >= argument */
closeall(3);
/* go for it: */
execv(_PATH_SENDMAIL, argvec);
_exit(EX_OSFILE);
default: /* parent */
free(argvec);
rc = waitpid(pid, &status, 0);
if (rc < 0)
return -1;
if (WIFEXITED(status))
return WEXITSTATUS(status);
return -1;
case -1: /* error */
free(argvec);
return -1;
}
}
5.2.2.2 Allowing sendmail to deduce the recipients
The `-t' option to sendmail instructs sendmail to
parse the headers of the message, and use all the recipient-type headers
(i.e. To:, Cc: and Bcc:) to construct the list of
envelope recipients. This has the advantage of simplifying the
sendmail command line, but makes it impossible to specify
recipients other than those listed in the headers. (This is not usually
a problem.)
As an example, here's a program to mail a file on standard input to
specified recipients as a MIME attachment. Some error checks have been
omitted for brevity. This requires the `mimencode' program from the
metamail distribution.
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
/* #include <paths.h> if you have it */
#ifndef _PATH_SENDMAIL
#define _PATH_SENDMAIL "/usr/lib/sendmail"
#endif
#define SENDMAIL_OPTS "-oi"
#define countof(a) ((sizeof(a))/sizeof((a)[0]))
char tfilename[L_tmpnam];
char command[128+L_tmpnam];
void cleanup(void)
{
unlink(tfilename);
}
int main(int argc, char **argv)
{
FILE *msg;
int i;
if (argc < 2)
{
fprintf(stderr, "usage: %s recipients...\n", argv[0]);
exit(2);
}
if (tmpnam(tfilename) == NULL
|| (msg = fopen(tfilename,"w")) == NULL)
exit(2);
atexit(cleanup);
fclose(msg);
msg = fopen(tfilename,"a");
if (!msg)
exit(2);
/* construct recipient list */
fprintf(msg, "To: %s", argv[1]);
for (i = 2; i < argc; i++)
fprintf(msg, ",\n\t%s", argv[i]);
fputc('\n',msg);
/* Subject */
fprintf(msg, "Subject: file sent by mail\n");
/* sendmail can add it's own From:, Date:, Message-ID: etc. */
/* MIME stuff */
fprintf(msg, "MIME-Version: 1.0\n");
fprintf(msg, "Content-Type: application/octet-stream\n");
fprintf(msg, "Content-Transfer-Encoding: base64\n");
/* end of headers -- insert a blank line */
fputc('\n',msg);
fclose(msg);
/* invoke encoding program */
sprintf(command, "mimencode -b >>%s", tfilename);
if (system(command))
exit(1);
/* invoke mailer */
sprintf(command, "%s %s -t <%s",
_PATH_SENDMAIL, SENDMAIL_OPTS, tfilename);
if (system(command))
exit(1);
return 0;
}
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