Sending and receiving a file (server / client) in C using a socket in Unix

First of all, thank you all for reading this and helping, I am very grateful. Secondly, I'm sorry, but I'm still new to this site, and English is not my first language, so I could make some mistakes in formatting and language. I apologize in advance. In addition, my knowledge of C is not so good, but I want to learn and improve.
Now, to the point:

What I need to do is create a client and server, and the server will listen for incoming connections.
Then I have a client that sends a rather large text file to the server (I know that it will be only one file).
Then the server will perform some operation on the file (it will run the script in the file to be sent, which creates another file in the output called "output.txt"). Then the server will need to send the output.txt file back to the client.

Now I have a way to make the client and server (I read the beej manual and some other materials on this site), even if I probably made some mistakes. I need help with a server that returns a file, then calls a script and sends another file to the client. So far I have done this server and client ... I really don't know how to do this.
On the other hand, I made these files using what I found on this site and on the Internet, I hope that they are not too dirty, because I am not so good a programmer.

This is client.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h> 
#include <pthread.h>


#define SOCKET_PORT "50000"
#define SOCKET_ADR "localhost"
#define filename "/home/aryan/Desktop/input.txt"


void error(const char *msg)
{
    perror(msg);
    exit(0);
}


int main()
{
/* Making the client */
int sockfd, portno, n;
struct sockaddr_in serv_addr;
struct hostent *server;

char buffer[256];

portno = atoi(SOCKET_PORT);

sockfd = socket(AF_INET, SOCK_STREAM, 0);
if (sockfd < 0) 
    error("ERROR opening socket");

server = gethostbyname(SOCKET_ADR);

if (server == NULL) 
{
    fprintf(stderr,"ERROR, no such host\n");
    exit(0);
}

bzero((char *) &serv_addr, sizeof(serv_addr));
serv_addr.sin_family = AF_INET;
bcopy((char *)server->h_addr, 
     (char *)&serv_addr.sin_addr.s_addr,
     server->h_length);
serv_addr.sin_port = htons(portno);
if (connect(sockfd,(struct sockaddr *) &serv_addr,sizeof(serv_addr)) < 0) 
    error("ERROR connecting");

/* Time to send the file */
FILE *pf;
unsigned long fsize;

pf = fopen(filename, "rb");
if (pf == NULL) 
{
    printf("File not found!\n");
    return 1;
}
else 
{
    printf("Found file %s\n", filename);

    fseek(pf, 0, SEEK_END);
    fsize = ftell(pf);
    rewind(pf);

    printf("File contains %ld bytes!\n", fsize);
    printf("Sending the file now");
}

while (1) 
{
    // Read data into buffer.  We may not have enough to fill up buffer, so we
    // store how many bytes were actually read in bytes_read.
    int bytes_read = fread(buffer, sizeof(char),sizeof(buffer), pf);
    if (bytes_read == 0) // We're done reading from the file
        break;

    if (bytes_read < 0) 
    {
        error("ERROR reading from file"); 
    }

    // You need a loop for the write, because not all of the data may be written
    // in one call; write will return how many bytes were written. p keeps
    // track of where in the buffer we are, while we decrement bytes_read
    // to keep track of how many bytes are left to write.
    void *p = buffer;
    while (bytes_read > 0) 
    {
        int bytes_written = write(sockfd, buffer, bytes_read);
        if (bytes_written <= 0) 
        {
            error("ERROR writing to socket\n");
        }
        bytes_read -= bytes_written;
        p += bytes_written;
    }
}       

printf("Done Sending the File!\n");
printf("Now Closing Connection.\n");

fclose(pf);
close(sockfd);
return 0;
}

This is server.c:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h> 
#include <sys/socket.h>
#include <netinet/in.h>
#include <signal.h>
#include <pthread.h>

#define SOCKET_PORT 50000
#define filename "/home/aryan/Desktop/output.txt"


void error(const char *msg)
{
    perror(msg);
    exit(1);
}

void* client_thread_proc(void* arg)
{
char buffer[256];
struct sockaddr_in serv_addr, cli_addr;
int n;
FILE *fp;

int thisfd = (int)arg;
printf("Server %d: accepted = %d\n", getpid(), thisfd);

if (thisfd < 0) 
{ 
    printf("Accept error on server\n");
    error("ERROR on accept"); 
    return NULL;
}

printf("Connection %d accepted\n", thisfd);

fp = fopen(filename, "a+b");
if (fp == NULL) 
{
    printf("File not found!\n");
    return NULL;
}
else 
{
    printf("Found file %s\n", filename);
}

/* Time to Receive the File */
while (1)
{
    bzero(buffer,256);
    n = read(thisfd,buffer,255);
    if (n < 0) error("ERROR reading from socket");

    n = fwrite(buffer, sizeof(char), sizeof(buffer), fp);
    if (n < 0) error("ERROR writing in file");

    n = write(thisfd,"I am getting your file...",25);
    if (n < 0) error("ERROR writing to socket");
} /* end child while loop */

fclose(fp);

return NULL;
}

void serve_it(int Client)
{
    void* arg = (void*)Client;
    pthread_t new_thread;
    pthread_create( &new_thread, NULL, &client_thread_proc, arg);
}

/* Making Server */
int main()
{
int sockfd, newsockfd, portno;
socklen_t clilen;
char buffer[256];
struct sockaddr_in serv_addr, cli_addr;
int n;
FILE *fp;

signal (SIGCHLD, SIG_IGN);

sockfd = socket(AF_INET, SOCK_STREAM, 0);
if (sockfd < 0) 
    error("ERROR opening socket");

bzero((char *) &serv_addr, sizeof(serv_addr));

serv_addr.sin_family = AF_INET;
serv_addr.sin_addr.s_addr = INADDR_ANY;
serv_addr.sin_port = htons(SOCKET_PORT);

if (bind(sockfd, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0) 
      error("ERROR on binding");

listen(sockfd,5);

clilen = sizeof(cli_addr);

while (1)
{
    printf("Server %d accepting connections\n", getpid());

    newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, &clilen);

    serve_it(newsockfd);
}  // serving loop


close(sockfd);   
return 0; 
}

, , ...
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, . !