How to resolve the algorithm Send email step by step in the C programming language

Published on 7 June 2024 03:52 AM
#C

How to resolve the algorithm Send email step by step in the C programming language

Table of Contents

Problem Statement

Write a function to send an email. The function should have parameters for setting From, To and Cc addresses; the Subject, and the message text, and optionally fields for the server name and login details.

(Remember to obfuscate any sensitive data used in examples)

Let's start with the solution:

Step by Step solution about How to resolve the algorithm Send email step by step in the C programming language

Overview: This C program demonstrates how to send an email using the SMTP protocol via Gmail's SMTP server. It constructs an email message with custom headers and content and sends it using the libcurl library.

Code Explanation:

  1. Header Inclusions:

    • The program includes the necessary headers for using libcurl (curl/curl.h), string manipulation (string.h), and input/output (stdio.h).

  2. Email Constants:

    • Three constants are defined for the sender, recipient, and cc addresses: from, to, and cc.

  3. Email Payload Text:

    • An array of character strings (payload_text) represents the email message's header and content. This includes the Date, To, From, Cc, Message-ID, Subject, and body text.

  4. Upload Status Structure:

    • A struct named upload_status is defined to track the number of lines read from the payload_text array.

  5. Payload Source Function (payload_source):

    • This function is the callback used by libcurl to obtain the email payload. It reads lines from the payload_text array and returns them to libcurl for sending.

  6. Main Function:

    • The main function is the entry point of the program.

  7. Initializing libcurl:

    • It initializes the libcurl session using curl_easy_init().

  8. SMTP Configuration:

    • Various SMTP-related options are set using curl_easy_setopt:
      • Username and password for authentication
      • SMTP server address and port (smtp.gmail.com:465)
      • SSL configuration (using certificates)
      • Sender's email address
      • Recipient and cc addresses
      • Upload function to provide the email payload
      • Verbose output for debugging

  9. Sending the Email:

    • curl_easy_perform is called to send the email message to the SMTP server.

  10. Cleanup:

  • After sending the email, the recipient list and the libcurl session are cleaned up.

Usage: To use this program, you need to provide the necessary credentials (username and password) for your Gmail account. You may also need to adjust the certificate path if you're using a self-signed certificate.

Note: You should replace the email addresses and certificate path with your own valid values before running the code.

Source code in the c programming language

#include <curl/curl.h>
#include <string.h>
#include <stdio.h>

#define from    "<sender@duniya.com>"
#define to      "<addressee@gmail.com>"
#define cc      "<info@example.org>"
 
static const char *payload_text[] = {
  "Date: Mon, 13 Jun 2018 11:30:00 +0100\r\n",
  "To: " to "\r\n",
  "From: " from " (Example User)\r\n",
  "Cc: " cc " (Another example User)\r\n",
  "Message-ID: <ecd7db36-10ab-437a-9g3a-e652b9458efd@"
  "rfcpedant.example.org>\r\n",
  "Subject: Sanding mail via C\r\n",
  "\r\n",
  "This mail is being sent by a C program.\r\n",
  "\r\n",
  "It connects to the GMail SMTP server, by far, the most popular mail program of all.\r\n",
  "Which is also probably written in C.\r\n",
  "To C or not to C..............\r\n",
  "That is the question.\r\n",
  NULL
};
 
struct upload_status {
  int lines_read;
};
 
static size_t payload_source(void *ptr, size_t size, size_t nmemb, void *userp)
{
  struct upload_status *upload_ctx = (struct upload_status *)userp;
  const char *data;
 
  if((size == 0) || (nmemb == 0) || ((size*nmemb) < 1)) {
    return 0;
  }
 
  data = payload_text[upload_ctx->lines_read];
 
  if(data) {
    size_t len = strlen(data);
    memcpy(ptr, data, len);
    upload_ctx->lines_read++;
 
    return len;
  }
 
  return 0;
}
 
int main(void)
{
  CURL *curl;
  CURLcode res = CURLE_OK;
  struct curl_slist *recipients = NULL;
  struct upload_status upload_ctx;
 
  upload_ctx.lines_read = 0;
 
  curl = curl_easy_init();
  if(curl) {

    curl_easy_setopt(curl, CURLOPT_USERNAME, "user");
    curl_easy_setopt(curl, CURLOPT_PASSWORD, "secret");

    curl_easy_setopt(curl, CURLOPT_URL, "smtp://smtp.gmail.com:465");

    curl_easy_setopt(curl, CURLOPT_USE_SSL, (long)CURLUSESSL_ALL);
 
    curl_easy_setopt(curl, CURLOPT_CAINFO, "/path/to/certificate.pem");

    curl_easy_setopt(curl, CURLOPT_MAIL_FROM, from);

    recipients = curl_slist_append(recipients, to);
    recipients = curl_slist_append(recipients, cc);
    curl_easy_setopt(curl, CURLOPT_MAIL_RCPT, recipients);

    curl_easy_setopt(curl, CURLOPT_READFUNCTION, payload_source);
    curl_easy_setopt(curl, CURLOPT_READDATA, &upload_ctx);
    curl_easy_setopt(curl, CURLOPT_UPLOAD, 1L);

    curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);

    res = curl_easy_perform(curl);

    if(res != CURLE_OK)
      fprintf(stderr, "curl_easy_perform() failed: %s\n",curl_easy_strerror(res));

    curl_slist_free_all(recipients);

    curl_easy_cleanup(curl);
  }
 
  return (int)res;
}

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