Error: Seg Fault for FIle Transfer over TCP socket in C + Multi-threading

Question

Goal: I am trying to write a concurrent TCP server that accepts and responds to multiple client connection requests, each requesting a file transfer.

I think the error lies within the file handling section in start_routine(), need to verify where. (Read more below code)

File: server.c

#define CONNECTIONS 5

FILE *input;
pthread_t tids[CONNECTIONS];
char name[100];
char buff[1024]; //buffer size

void *start_routine(void *arg)
{
    //recv file request
    read((int) arg, name, sizeof(name));

    //file handling
    input = fopen(name, "rb");

    printf("hi\n"); //testing

    while (!feof(input))
        write((int) arg, buff, fread(buff, sizeof(char), sizeof(buff), input));
    
    return NULL;
}

int main(int argc, char *argv[])
{
    struct sockaddr_in serv_addr;

    if (argc != 2)
    {
        printf("Usage: ./a.out <port #>\n");
        exit(0);
    }

    //open socket
    int listenfd = socket(AF_INET, SOCK_STREAM, 0);

    //init sockaddr_in struct
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_port = htons(atoi(argv[1]));
    serv_addr.sin_addr.s_addr = htons(INADDR_ANY);

    //binds listenfd to a specific sockaddr_in
    bind(listenfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr));
    
    for(int i = 0; i <= CONNECTIONS; i++){
        //listen for new connections on listenfd
        listen(listenfd, CONNECTIONS);
        //accepts a connection
        int connfd = accept(listenfd, (struct sockaddr *)NULL, NULL);
        pthread_create(tids, NULL, start_routine, &connfd);
    }

    //cleaning
    close(listenfd);
    fclose(input);

    return 0;
}

File: client.c

int main(int argc, char *argv[])
{
    int n;
    char buff[1024]; //buffer size
    struct sockaddr_in serv_addr;

    if (argc != 4)
    {
        printf("Usage: ./a.out <file required> <ip of server> <port #> \n");
        exit(0);
    }

    //open socket
    int sockfd = socket(AF_INET, SOCK_STREAM, 0);

    //init sockaddr_in struct
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_port = htons(atoi(argv[3]));
    serv_addr.sin_addr.s_addr = inet_addr(argv[2]);

    //connect to <server_ip>: <port>
    connect(sockfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr));

    //file handling
    FILE *request;
    request = fopen("dst.txt", "wb");

    //send file request
    write(sockfd, argv[1], strlen(argv[1]) + 1);

    while ((n = read(sockfd, buff, sizeof(buff))) > 0)
        fwrite(buff, sizeof(char), n, request);

    //cleaning
    fclose(request);
    close(sockfd);

    return 0;
}

Other files in folder: src.txt

Compiled Using:

gcc server.c -o server -lpthread
./server 8080

gcc client.c -o client
./client src.txt 127.0.0.1 8080

(5 copies of client and so compiled each [client1, client2 ... client5] 5 times)

Problem: Results in segmentation fault. :(

While testing out, I found out that the segmentation fault lies within the start_routine(), in the while loop.

I have used the same snippet from another program (only the variables updated) -- need to know where am I going wrong!

Also, for multi-threading, I created copies of the client program to simulate 5 connections to the server. Hope it is the right way.

Appreciate the help!