C Web Server Segmentation Fault - c

Question

I am having some trouble understanding why I am getting a segmentation fault in my homemade web server. I am trying to correctly label the content type for .html and .txt documents. I can open an .html file in my web browser just fine when my server is running. However, my server is closing on a segmentation fault when I try to open a .txt document. The code below is where I am getting the issue.
void* thread_runner(void* sockfd)
{
char buffer[256];
int n;
int sock = *(int*)sockfd;
bzero(buffer,256);
char filename[256];
bzero(filename,256);
n = read(sock,filename,255);
if (n < 0) error("ERROR reading from socket");
FILE *fp;
char file[4096];
char* word = strtok(filename, " ");
word = strtok(NULL, " ");
word++;
printf("filename = \"%s\"\n", word);
fp = fopen(word, "r");
if (fp == NULL) {
printf("file pointer is NULL\n");
return 0;
}
char string[4096];
char* extension;
bzero(string, 4096);
bzero(file, 4096);
char* word2;
word2 = strtok(word, ".");
while ((word2 = strtok(NULL, ".")) != NULL)
{
extension = word2;
}
printf("File Extension: %s\n", extension);
if (strncmp(extension, "html", 4096) == 0) {
printf("HTML FILE FOUND\n");
strcat(string, "HTTP/1.1 200 OK\nContent-Type: text/html\nContent-Length: 4096 \nConnection: keep-alive\n\n");
}
else if (strncmp(extension, "txt", 4096) == 0) {
printf("TXT FILE FOUND\n");
strcat(string, "HTTP/1.1 200 OK\nContent-Type: text/plain\nContent-Length: 4096\nConnection: keep-alive\n\n");
}
the error occurs in this loop.
I have used bzero on 'string'.
while (fgets(file, 4096, fp))
{
strcat(string, file);
}
//printf("Sending text:%s", string);
printf("The requested file was: %s.%s\n",word,extension);
n = write(sock,string,4096);
printf("file contents are %s\n", string);
fclose(fp);
if (n < 0) error("ERROR writing to socket");
if (n == 0)
{
close(sock);
}

Answer 1

You're writing past the end of string. string only has 4096 characters in it, but your loop tries to read the entire file into it when it keeps calling strcat() in a loop.
Instead of concatenating the entire file into a variable and then writing it all, use fread() to read the file in chunks and immediately write them to the socket. There's no need to read line by line.
size_t in;
while ((in = fread(file, 1, sizeof file, fp) > 0) {
int n = write(sock, file, in);
if (n < in) {
error("ERROR writing to socket");
}
}
close(sock);
fclose(fp);