File: tcpecho.c
   1 /*
   2 The MIT License (MIT)
   3 
   4 Copyright © 2020-2025 pacman64
   5 
   6 Permission is hereby granted, free of charge, to any person obtaining a copy of
   7 this software and associated documentation files (the “Software”), to deal
   8 in the Software without restriction, including without limitation the rights to
   9 use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
  10 of the Software, and to permit persons to whom the Software is furnished to do
  11 so, subject to the following conditions:
  12 
  13 The above copyright notice and this permission notice shall be included in all
  14 copies or substantial portions of the Software.
  15 
  16 THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  17 IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  18 FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  19 AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  20 LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  21 OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  22 SOFTWARE.
  23 */
  24 
  25 /*
  26 You can build this command-line app by running
  27 
  28 cc -Wall -s -O2 -o ./tcpecho ./tcpecho.c
  29 */
  30 
  31 #include <arpa/inet.h>
  32 #include <fcntl.h>
  33 #include <netinet/in.h>
  34 #include <stdbool.h>
  35 #include <stdio.h>
  36 #include <stdlib.h>
  37 #include <string.h>
  38 #include <sys/socket.h>
  39 #include <sys/types.h>
  40 #include <unistd.h>
  41 
  42 #ifdef _WIN32
  43 #include <windows.h>
  44 #endif
  45 
  46 const char* info = ""
  47 "tcpecho [port] [words...]\n"
  48 "\n"
  49 "\n"
  50 "Start a TCP server which always replies with a plain-text line made from\n"
  51 "the arguments given after the port number, joined by single spaces.\n"
  52 "\n"
  53 "\n"
  54 "Options, all of which can start with either 1 or 2 dashes:\n"
  55 "\n"
  56 "\n"
  57 "  -h          show this help message\n"
  58 "  -help       show this help message\n"
  59 "";
  60 
  61 // is_help_option simplifies control-flow for func main
  62 bool is_help_option(char* s) {
  63     return (s[0] == '-') && (
  64         strcmp(s, "-h") == 0 || strcmp(s, "-help") == 0 ||
  65         strcmp(s, "--h") == 0 || strcmp(s, "--help") == 0
  66     );
  67 }
  68 
  69 int main(int argc, char** argv) {
  70 #ifdef _WIN32
  71     setmode(fileno(stdin), O_BINARY);
  72     // ensure output lines end in LF instead of CRLF on windows
  73     setmode(fileno(stdout), O_BINARY);
  74     setmode(fileno(stderr), O_BINARY);
  75 #endif
  76 
  77     // handle any of the help options, if given
  78     if (argc > 1 && is_help_option(argv[1])) {
  79         puts(info);
  80         return 0;
  81     }
  82 
  83     if (argc < 2) {
  84         char* msg = "you forgot the port number as the leading argument";
  85         fprintf(stderr, "\x1b[31m%s\x1b[0m\n", msg);
  86         return 1;
  87     }
  88 
  89     char* end;
  90     int port = strtol(argv[1], &end, 10);
  91     if (*end != 0 || port < 1 || port > 65535) {
  92         char* msg;
  93         msg = "isn't a valid port number";
  94         fprintf(stderr, "\x1b[31m%s %s\x1b[0m\n", argv[1], msg);
  95         msg = "maybe you forgot the port number as the leading argument";
  96         fprintf(stderr, "\x1b[31m%s\x1b[0m\n", msg);
  97         return 1;
  98     }
  99 
 100     // count spaces between words
 101     size_t msglen = argc - 2;
 102     // count word lengths
 103     for (size_t i = 2; i < argc; i++) {
 104         msglen += strlen(argv[i]);
 105     }
 106     // remember the final line-feed
 107     msglen++;
 108 
 109     // also remember that strings are ended with an extra null byte
 110     char* msg = malloc(msglen + 1);
 111     if (msg == NULL) {
 112         const char* m = "can't get enough memory for the response message";
 113         fprintf(stderr, "\x1b[31m%s\x1b[0m\n", m);
 114         return 1;
 115     }
 116 
 117     for (size_t i = 2, pos = 0; i < argc; i++) {
 118         if (i > 2) {
 119             msg[pos] = ' ';
 120             pos++;
 121         }
 122 
 123         for (size_t j = 0; argv[i][j] != 0; j++) {
 124             msg[pos] = argv[i][j];
 125             pos++;
 126         }
 127     }
 128 
 129     msg[msglen - 1] = '\n';
 130     msg[msglen] = 0;
 131 
 132     struct sockaddr_in srv;
 133     memset(&srv, 0, sizeof(srv));
 134 
 135     int fd = socket(AF_INET, SOCK_STREAM, 0);
 136     srv.sin_family = AF_INET;
 137     srv.sin_addr.s_addr = htonl(INADDR_ANY);
 138     srv.sin_port = htons(port);
 139 
 140     bind(fd, (struct sockaddr*)&srv, sizeof(srv));
 141     listen(fd, 1);
 142 
 143     while (true) {
 144         int conn = accept(fd, NULL, NULL);
 145         send(conn, msg, msglen, 0);
 146         close(conn);
 147     }
 148 
 149     free(msg);
 150     return 0;
 151 }