Dev Notes

Various Cheat Sheets and Resources by David Egan/Carawebs.

HTTP Server in C


C, HTTP, Sockets
David Egan

Simple http server socket in Linux. This is an educational work in progress, and these are my notes.

Server Socket

The basic procedure:

  • Create socket with socket() call
  • bind() this to an IP and port where it can
  • listen() for connections, then
  • accept() connection and send() or receive() data to/from connected sockets

Note that if struct sockaddr_in serverAddress.sin_addr.s_addr is set to INADDR_ANY the socket is bound to all local interfaces. INADDR_ANY is a constant set to zero, defined in netinet/in.h. This will correspond to an IP address of 0.0.0.0 in the standard IPv4 notation. Note that htonl(INADDR_LOOPBACK) and inet_addr("127.0.0.1") are functionally equivalent.

Simple Example

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <netdb.h> // for getnameinfo()

// Usual socket headers
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>

#include <arpa/inet.h>

#define SIZE 1024
#define BACKLOG 10  // Passed to listen()

void report(struct sockaddr_in *serverAddress);

void setHttpHeader(char httpHeader[])
{
    // File object to return
    FILE *htmlData = fopen("index.html", "r");

    char line[100];
    char responseData[8000];
    while (fgets(line, 100, htmlData) != 0) {
        strcat(responseData, line);
    }
    // char httpHeader[8000] = "HTTP/1.1 200 OK\r\n\n";
    strcat(httpHeader, responseData);
}

int main(void)
{
    char httpHeader[8000] = "HTTP/1.1 200 OK\r\n\n";

    // Socket setup: creates an endpoint for communication, returns a descriptor
    // -----------------------------------------------------------------------------------------------------------------
    int serverSocket = socket(
        AF_INET,      // Domain: specifies protocol family
        SOCK_STREAM,  // Type: specifies communication semantics
        0             // Protocol: 0 because there is a single protocol for the specified family
    );

    // Construct local address structure
    // -----------------------------------------------------------------------------------------------------------------
    struct sockaddr_in serverAddress;
    serverAddress.sin_family = AF_INET;
    serverAddress.sin_port = htons(8001);
    serverAddress.sin_addr.s_addr = htonl(INADDR_LOOPBACK);//inet_addr("127.0.0.1");

    // Bind socket to local address
    // -----------------------------------------------------------------------------------------------------------------
    // bind() assigns the address specified by serverAddress to the socket
    // referred to by the file descriptor serverSocket.
    bind(
        serverSocket,                         // file descriptor referring to a socket
        (struct sockaddr *) &serverAddress,   // Address to be assigned to the socket
        sizeof(serverAddress)                 // Size (bytes) of the address structure
    );

    // Mark socket to listen for incoming connections
    // -----------------------------------------------------------------------------------------------------------------
    int listening = listen(serverSocket, BACKLOG);
    if (listening < 0) {
        printf("Error: The server is not listening.\n");
        return 1;
    }
    report(&serverAddress);     // Custom report function
    setHttpHeader(httpHeader);  // Custom function to set header
    int clientSocket;

    // Wait for a connection, create a connected socket if a connection is pending
    // -----------------------------------------------------------------------------------------------------------------
    while(1) {
        clientSocket = accept(serverSocket, NULL, NULL);
        send(clientSocket, httpHeader, sizeof(httpHeader), 0);
        close(clientSocket);
    }
    return 0;
}

void report(struct sockaddr_in *serverAddress)
{
    char hostBuffer[INET6_ADDRSTRLEN];
    char serviceBuffer[NI_MAXSERV]; // defined in `<netdb.h>`
    socklen_t addr_len = sizeof(*serverAddress);
    int err = getnameinfo(
        (struct sockaddr *) serverAddress,
        addr_len,
        hostBuffer,
        sizeof(hostBuffer),
        serviceBuffer,
        sizeof(serviceBuffer),
        NI_NUMERICHOST
    );
    if (err != 0) {
        printf("It's not working!!\n");
    }
    printf("\n\n\tServer listening on http://%s:%s\n", hostBuffer, serviceBuffer);
}

Protocol Family

When setting up a socket using socket() the following protocols are available:

Name Purpose Man page
AF_UNIX, AF_LOCAL Local communication man unix
AF_INET IPv4 Internet protocols man ip
AF_INET6 IPv6 Internet protocols man ipv6
AF_IPX IPX - Novell protocols  
AF_NETLINK Kernel user interface device man netlink
AF_X25 ITU-T X.25 / ISO-8208 protocol man x25
AF_AX25 Amateur radio AX.25 protocol  
AF_ATMPVC Access to raw ATM PVCs  
AF_APPLETALK AppleTalk man ddp
AF_PACKET Low level packet interface man packet
AF_ALG Interface to kernel crypto API  

You can acces this list by running man socket.

Socket Type

The socket has the indicated type, which specifies the communication semantics. Currently defined types are:

Type Description
SOCK_STREAM Provides sequenced, reliable, two-way, connection-based byte streams. An out-of-band data transmission mechanism may be supported. Used for TCP protocol.
SOCK_DGRAM Supports datagrams - connectionless, unreliable messages of a fixed maximum length. Used for UDP protocol.
SOCK_SEQPACKET Provides a sequenced, reliable, two-way connection-based data transmission path for datagrams of fixed maximum length; a consumer is required to read an entire packet with each input system call.
SOCK_RAW Provides raw network protocol access.
SOCK_RDM Provides a reliable datagram layer that does not guarantee ordering.
SOCK_PACKET Obsolete and should not be used in new programs; see packet(7).

Constructing a Local Address Structure

If serverAddress.sin_addr.s_addr is set to INADDR_ANY the socket is bound to all local interfaces. INADDR_ANY is a constant set to zero, defined in netinet/in.h. This will correspond to an IP address of 0.0.0.0 in the standard IPv4 notation. Note that htonl(INADDR_LOOPBACK) and inet_addr("127.0.0.1") are functionally equivalent.

Bind socket to local address

When a socket is created with socket(), it exists in a name space (address family) but has no address assigned to it. bind() assigns the address specified by serverAddress to the socket referred to by the file descriptor serverSocket. serverAddressLength specifies the size, in bytes, of the address structure pointed to by serverAddress.

This operation is known as “assigning a name to a socket”.

Connecting

The programme runs an infinite loop in which we wait and create a connected socket if a connection is pending.

The accept() function gets the first connection request on the queue of pending connections for the listening socket (in this case denoted by serverSocket). It then creates a new connected socket and returns a file descriptor referring to this socket. The newly created socket is NOT in a listening state. The original socket (serverSocket) is unaffected by this call.

Using getnameinfo

Used in the report() function

int getnameinfo(
    const struct sockaddr *sa,  // pointer to a generic socket address structure (of type sockaddr_in or sockaddr_in6)
    socklen_t salen,            // size of the above
    char *host,                 // Caller allocated buffer, will receive host
    socklen_t hostlen,          // Size of above
    char *serv,                 // Caller allocate buffer, will receive service name
    socklen_t servlen,          // Size of above
    int flags);

References


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