recv()-ing a String with "\0" and "\n" in it - c

So coded a client which is recieving only chars in the Buffer which I print out afterwards:
//CODE FROM BJ'S GUIDE TO NETWORK PROGRAMMING *1
if ((numbytes = recv(sockfd, buf, MAXDATASIZE-1, 0)) == -1) {
perror("recv");
exit(1);
}
buf[numbytes] = '\0';
printf("%s",buf);
//MY CODE
while (numbytes > 0) {
numbytes = recv(sockfd, buf, MAXDATASIZE-1, 0);
if (numbytes > 0) {
buf[numbytes] = '\0';
printf("\%s",buf);
}
}
buf is a char[512]-array
I want to also recv() "\0" and "\n" and print them out as "\0" and "\n"
as code this would look like this
printf("this contains a \\n byte and a \\0 byte");
I cannot change something server-sided because So i have to edit the recieving side
*1: https://beej.us/guide/bgnet/

I want to also recv() "\0" and "\n" and print them out as "\0" and "\n"
Do not use string functions on buf. Handle it as an array of characters.
while (numbytes > 0) {
numbytes = recv(sockfd, buf, MAXDATASIZE, 0);
for (i = 0; i < numbytes; i++) {
unsigned char ch = buf[numbytes];
if (ch <= '\n') { // Escape characters 0 to 13
putc('\\');
putc("0123456abtrvfn"[ch]);
} else if (ch == '\\') { // I suspect OP will also want this
putc('\\');
putc('\\');
} else {
putc(ch);
}
}
}

Ok we did it by using fwrite() like this
fwrite(buf, numbytes, sizeof(char), stdout);

Related

Strtok() returning NULL randomly

I am trying to create a client that communicate with a server by sending 2 types of messages:
The word QUIT that communicate to the server to close the connection.
An operation with the following syntax: operator first_operand second_operand. For example: + 3 3, - 5 6 etc. (the operands must be positive integers, and there must be only 2 operands).
If the server receive an operation, it executes it and returns the result to the client. The problem is that the first operation I send returns the right result, while the following ones work randomly (sometimes they return the right result, other times the function strtok() doesn't get the second operand and returns NULL...).
This is code of the client that process the message written by the user in the prompt and that scan the message to check if the operation is written with the correct syntax (WARNING: the code is written in an extremely unprofessional and unclean way).
The code part that creates the problem is inside the while(1).
#define MAXLENGTH 256
int main (int argc, char *argv[]) {
int simpleSocket = 0;
int simplePort = 0;
int returnStatus = 0;
char first[10], second[10];
char* operator;
char buffer[MAXLENGTH] = "";
char message[50];
char terminationCommand[] = "QUIT\n";
char space[2] = " ";
struct sockaddr_in simpleServer;
if (3 != argc) {
fprintf(stderr, "Usage: %s <server> <port>\n", argv[0]);
exit(1);
}
/* create a streaming socket */
simpleSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
if (simpleSocket == -1) {
fprintf(stderr, "Could not create a socket!\n");
exit(1);
} else {
fprintf(stderr, "Socket created!\n");
}
/* retrieve the port number for connecting */
simplePort = atoi(argv[2]);
/* setup the address structure */
/* use the IP address sent as an argument for the server address */
//bzero(&simpleServer, sizeof(simpleServer));
memset(&simpleServer, '\0', sizeof(simpleServer));
simpleServer.sin_family = AF_INET;
//inet_addr(argv[2], &simpleServer.sin_addr.s_addr);
simpleServer.sin_addr.s_addr=inet_addr(argv[1]);
simpleServer.sin_port = htons(simplePort);
/* connect to the address and port with our socket */
returnStatus = connect(simpleSocket, (struct sockaddr *)&simpleServer, sizeof(simpleServer));
if (returnStatus == 0) {
fprintf(stderr, "Connect successful!\n\n");
} else {
fprintf(stderr, "Could not connect to address!\n");
close(simpleSocket);
exit(1);
}
/* get the message from the server */
returnStatus = read(simpleSocket, buffer, sizeof(buffer));
if (returnStatus > 0) {
printf("%s\n", &buffer[3]);
} else {
fprintf(stderr, "Return Status = %d \n", returnStatus);
}
memset(&buffer, '\0', sizeof(buffer));
printf("You can execute 2 commands:\n");
printf("1. Operations ( +, -, *, /, % ) with the following syntax: operator + first operand + second operand.\n");
printf("Example: + 5 2 \n");
printf("2. Termination of the connection with the following syntax: QUIT + press Enter.\n");
while(1) {
printf("\nEnter a command:\n");
fgets(message, 1000, stdin);
// the if with the termination command works fine
if (strcmp(message, terminationCommand) == 0) {
if (send(simpleSocket, message, strlen(message), 0) < 0) {
printf("Send failed.");
return 1;
}
returnStatus = read(simpleSocket, buffer, sizeof(buffer));
if (returnStatus > 0) {
printf("%s\n", &buffer[4]);
} else {
fprintf(stderr, "Return Status = %d \n", returnStatus);
}
close(simpleSocket);
exit(1);
}
operator = strtok(message, space);
if (strcmp(operator, "+") == 0 || strcmp(operator, "-") == 0 || strcmp(operator, "/") == 0 || strcmp(operator, "%") == 0 || strcmp(operator, "*") == 0) {
char *first_operand = strtok(NULL, space);
if (first_operand != NULL) {
if (strcmp(first_operand, "ANS") == 0)
strcpy(first, "ANS");
else
strcpy(first, first_operand);
printf("%s\n", operator);
printf("%s\n", first);
char *second_operand = strtok(NULL, space);
printf("%s\n", second_operand);
if (second_operand != NULL && strtok(NULL, space) == NULL && (atoi(first) > 0 || strcmp(first, "ANS") == 0)) {
if (strcmp(second_operand, "ANS\n") == 0)
strcpy(second, "ANS");
else {
strcpy(second, second_operand);
}
if (atoi(second) > 0 || strcmp(second, "ANS") == 0) {
printf("OK\n");
char operation[] = "";
strcat(operation, operator);
strcat(operation, " ");
strcat(operation, first);
strcat(operation, " ");
strcat(operation, second);
if (send(simpleSocket, operation, strlen(operation), 0) < 0) {
printf("Send failed.");
return 1;
}
returnStatus = read(simpleSocket, buffer, sizeof(buffer));
if (returnStatus > 0) {
printf("%s\n", buffer);
} else {
fprintf(stderr, "Return Status = %d \n", returnStatus);
}
}
}
}
}
// after everything I reset the buffers I use to memorize the message and the elements of the message
memset(&buffer, '\0', sizeof(buffer));
memset(&first, '\0', sizeof(first));
memset(&second, '\0', sizeof(second));
memset(&message, '\0', sizeof(message));
memset(operator, '\0', sizeof(operator));
}
}
Can someone tell me why the second strtok() acts weird 90% of the times? What am I doing wrong?
There are multiple issues in you program:
You send newline terminated messages and you assume on the other end the read will return exactly the bytes sent by the other party, which is an incorrect assumption for the TCP/IP communications, only the order of bytes received is guaranteed, but the messages can be split on the way and received in chunks different from the sending sequence. You should instead read the socket into a buffer and only handle it once you receive a newline.
In your case, there is another problem which is more pressing: the buffer into which you read the data is not null terminated, so you should not pass it to standard C functions such as strtok().

tcp server blocking read in C

I want to implement a simple TCP server with blocking read, that receives messages sent from a client character by character until a separator. Once a message is received, it has to wait until the next message appears. Here is my pseudocode:
// Messages sent from the client
char *message1 = "mssg1\n"
char *message2 = "mssg2\n"
// On server side
char buffer;
char completeMessage[5]
while(1){
while(buffer != '\n'){
recv(sock, &buffer, 1, 0); // 1 is the read size
if(buffer != '\n') {
printf("buffer: %c\n", buffer);
completeMessage[n] = buffer;
count ++;
}
else{
printf("Complete message: %s\n", completeMessage);
count = 0;
}
}
}
And the result is the following:
buffer: m
buffer: s
buffer: s
buffer: g
buffer: 1
Complete message: mssg1
buffer:
buffer:
buffer:
buffer:
buffer:
buffer:
// Error due to buffer overflow
I don't know why recv instead of waiting for the next message character (blocking read), it continues reading blank spaces. My questions are the following:
Is recv really a socket blocking read function?
Is there something wrong or missing in the code?
Any other suggestions for implementing this?
Is recv really a socket blocking read function?
Yes, unless you made the handle non-blocking.
Is there something wrong or missing in the code?,
You're not checking what recv returns. 0 indicates EOF, and -1 indicates an error.
You don't check how full your buffer is, so you risk buffer overflows.
You're not terminating the string in completeMessage with a NUL as required by printf %s.
Any other suggestions for implementing this?
You shouldn't read a character at a time!
#define BUFFER_SIZE (64*1024)
char* extract_string(const char* start, const char* end) {
size_t len = end - start;
char* dst = malloc(len+1);
if (dst == NULL)
return NULL;
memcpy(dst, src, len);
dst[len] = '\0';
return dst;
}
{
char buf_start[BUFFER_SIZE];
char* buf_end = buf_start + BUFFER_SIZE;
char* window_start = buf_start;
char* window_end = buf_start;
while (1) {
if (window_end == buf_end) { // No more space.
fprintf(stderr, "Overly large message");
return 0;
}
ssize_t rv = recv(sock, window_end, buf_end-window_end, 0);
if (rv == -1) { // Error.
perror("recv");
return 0;
}
if (rv == 0) { // EOF.
return 1;
}
while (rv--) {
if (*(window_end++) == '\n') {
char* msg = extract_string(window_start, window_end-1); // Excl LF.
if (msg == NULL) {
fprintf(stderr, "Out of memory");
return 0;
}
// Do something with msg
printf("Complete message: %s\n", msg);
free(msg);
window_start = window_end;
}
}
memmove(buf_start, window_start, window_end-window_start);
window_end -= (window_start - buf_start);
window_start = buf_start;
}
}
There are quite a number of problems with your code, namely that you are ignoring the return value of recv(), you are not null-terminating your buffer before printing it, and you are not protecting yourself from a buffer overflow.
Try something more like this instead:
char ch, *tmp, *message = NULL;
int ret, length = 0, allocated = 0;
while (1)
{
ret = recv(sock, &ch, 1, 0);
if (ret <= 0)
{
if (ret < 0)
printf("Read error: %d\n", errno); // or WSAGetLastError() on Windows
else
printf("Client disconnected\n");
break;
}
if (ch == '\n')
{
if ((length > 0) && (message[length-1] == '\r'))
--length;
printf("Complete message: '%.*s'\n", length, message);
length = 0;
}
else
{
printf("ch: %c\n", ch);
if (length == allocated)
{
if (length >= 5000) // some max length of your choosing...
{
printf("Message length too large!\n");
break;
}
// just for example. You should use a more robust growth algorithm in production code...
tmp = (char*) realloc(message, allocated + 10);
if (!tmp)
{
printf("Memory allocation failed\n");
break;
}
message = tmp;
allocated += 10;
}
message[length] = ch;
++length;
}
}
free(message);
Alternatively, don't read char-by-char. Read as much data as you can from the socket on any given read and store it all in a growing buffer, and then scan that buffer for complete messages, eg:
char *buffer = (char*) malloc(100);
if (!buffer)
{
printf("Memory allocation failed\n");
}
else
{
int ret, offset, remaining, inbuf = 0, allocated = 100;
char *ptr;
while (1)
{
if (inbuf == allocated)
{
if (inbuf >= 5000) // some max length of your choosing...
{
printf("Buffer length too large!\n");
break;
}
// just for example. You should use a more robust growth algorithm in production code...
tmp = (char*) realloc(buffer, allocated + 100);
if (!tmp)
{
printf("Memory allocation failed\n");
break;
}
buffer = tmp;
allocated += 100;
}
ret = recv(sock, buffer+inbuf, allocated-inbuf, 0);
if (ret <= 0)
{
if (ret < 0)
printf("Read error: %d\n", errno); // or WSAGetLastError() on Windows
else
printf("Client disconnected\n");
break;
}
printf("Received: %.*s\n", ret, buffer+inbuf);
inbuf += ret;
while (ptr = (char*)memchr(buffer, '\n', inbuf))
{
offset = (ptr-buffer);
if ((offset > 0) && (buffer[offset-1] == '\r'))
--offset;
printf("Complete message: '%.s'\n", offset, buffer);
++ptr;
remaining = (inbuf - (ptr - buffer));
if (remaining > 0)
memmove(buffer, ptr, remaining);
inbuf = remaining;
}
}
free(buffer);
}

Read HTML Response using SSL_read() in C, HTTP/1.0

I read HTML response from a website using SSL_read() in C. I used HTTP/1.0 in request message. Here is my function:
void Reveive_response(char *resp, SSL *ssl, int body_required) {
char header[1048576];
char body[1048576];
int bytes; // number of bytes actually read
int received = 0; // number of bytes received
int i, line_length;
char c[1];
memset(header, '\0', sizeof(header)); // header assign = '\0'
memset(body, '\0', sizeof(body)); // body assign = '\0'
/***********Try to read byte by byte***********/
i = 0;
line_length = 0; // to check length of each line
do {
bytes = SSL_read(ssl, c, 1); // read 1 byte to c[0]
if (bytes <= 0) break; // read fall or connection closed
if (c[0] == '\n') { // if '\n'
if (line_length == 0) break; // empty line, so end header
else line_length = 0; // else reset for new line
} else if ( c[0] != '\r') line_length++; // inc length
header[i++] = c[0]; // add to header
received += bytes; // count
} while (1);
fprintf(f, "%s\n", header);
//printf("Received...%d\n", received);
printf("\n##### Header DONE #####\n");
strcpy(resp, header); // return via resp
/***********************************************/
/********Then try to read body if needed********/
char *buf = malloc(1024*sizeof(char));
if (body_required) {//read body
do {
memset(buf, '\0', 1024*sizeof(char));
bytes = SSL_read(ssl, buf, 1024);
if (bytes <= 0) break;
//printf("Wait : ");
//getchar();
printf("%s", buf);
strcat(body, buf);
} while (1);
printf("\n##### Body DONE #####\n");
}
fprintf(f, "%s\n", body);
free(buf);
/***********************************************/
fprintf(f, "=============================\n");
}
The header is OK. But when I want to read body, I received this:
I don't know where I went wrong. Any ideas? Please help.

Message receive program only printing every other message

I am have implemented the two programs from section 7.6 of http://beej.us/guide/bgipc/output/html/multipage/mq.html.
I have extended it so that there are two receiving programs and which one it goes to is determined by the message type.
The problem arises in the receiving program, B and C. They are supposed to print out the messages entered into program A everytime, however they only print the messages every other time.
This is where the message is sent, it reads the first 6 chars and if it is URGENT it sets the the message type.
buf.mtype = 2;
while(fgets(buf.mtext, sizeof buf.mtext, stdin) != NULL) {
int len = strlen(buf.mtext);
strncpy(typeTest, buf.mtext, 6);
if(strncmp(typeTest, "URGENT", 6) == 0){
buf.mtype = 1;
}
printf("This is the message %s \n", buf.mtext);
/* ditch newline at end, if it exists */
if (buf.mtext[len-1] == '\n') buf.mtext[len-1] = '\0';
if (msgsnd(msqid, &buf, len+1, 0) == -1) /* +1 for '\0' */
perror("msgsnd");
}
This is where the message is received, then the if statement checks the type to then print out.
for(;;) { /* Spock never quits! */
if (msgrcv(msqid, &buf, sizeof buf.mtext, 0, 0) == -1) {
perror("msgrcv");
exit(1);
}
if(buf.mtype == 2){
printf("spock: \"%s\"\n", buf.mtext);
}
}
Can anyone shed some light on why it only prints out every other message?
Thanks.
In your program A you must set buf.mtype to 2 if the input is not "URGENT..." You must do that in the loop, every time.
while(fgets(buf.mtext, sizeof buf.mtext, stdin) != NULL) {
int len = strlen(buf.mtext);
strncpy(typeTest, buf.mtext, 6);
if(strncmp(typeTest, "URGENT", 6) == 0){
buf.mtype = 1;
}
else buf.mtype= 2; // always set the default
printf("This is the message %s \n", buf.mtext);
/* ditch newline at end, if it exists */
if (buf.mtext[len-1] == '\n') buf.mtext[len-1] = '\0';
if (msgsnd(msqid, &buf, len+1, 0) == -1) /* +1 for '\0' */
perror("msgsnd");
}
In your programs B and C you must set msgtyp to 1 or 2 for each program to get the right message from the queue, for example:
int main(argc, argv)
{
int msgtype;
if (*argv[1]=='A')
msgtype= 1;
else if (*argv[1]=='B')
msgtype= 2;
else
msgtype= 0;
...
for(;;) { /* Spock never quits! */
if (msgrcv(msqid, &buf, sizeof buf.mtext, msgtype, 0) == -1) {
perror("msgrcv");
exit(1);
}
if(buf.mtype == msgtype){
printf("spock: \"%s\"\n", buf.mtext);
}
}
return 0;
}

C Socket client prints out a strange output

I'm trying to send a .txt file to a Linux socket client from a Linux server client (I use the loopback interface). I tried to a send a string, i.e. "OK", and everything worked fine, but when I try to send a file, the client prints out a strange output. Obviously I've done all the previous steps like socket, connect, accept, listen etc.
This is the server-side code:
printf("Sending file\n);
if ((fp=fopen(filename, "r"))!=NULL){
while ( (nbytes = fread(sendline, sizeof(char), 512, fp) > 0)){
printf("%s\n",sendline);
sent = writen(clientfd, sendline, nbytes);
}
close(fp);
}else
perror("Open file");
The 'writen' function is:
ssize_t writen(int fd, const void *vptr, size_t n){
/* Write "n" bytes to a descriptor. */
size_t nleft;
ssize_t nwritten;
const char *ptr;
ptr = vptr;
nleft = n;
while (nleft > 0) {
if ( (nwritten = write(fd, ptr, nleft)) <= 0) {
if (nwritten < 0 && errno == EINTR)
nwritten = 0; /* and call write() again */
else
return(-1); /* error */
}
nleft -= nwritten;
ptr += nwritten;
}
return(n);
}
The client-side code is:
while (fgets(sendline, 10000,stdin) != NULL)
{
sendto(sockfd,sendline,strlen(sendline),0,
(struct sockaddr *)&servaddr,sizeof(servaddr));
read(sockfd,recvline,10000);
fputs(recvline,stdout);
recvline[n]=0;
if((recvline[0]=='-')&&(recvline[1]=='E')&&(recvline[2]=='R')&&(recvline[3]=='R')){
close(sockfd);
exit(1);
}
}
The strange client that I get is in the image.
Client Output
So what's my mistake? Why do I receive this kind of output and how could I fix it?
Couple of issues,
In server code, parenthesis is at incorrect place,
while ( (nbytes = fread(sendline, sizeof(char), 512, fp) > 0)){
should be
while ( (nbytes = fread(sendline, sizeof(char), 512, fp)) > 0){
---------^ parenthesis close here
And in client side, set '\0' in recvline before printing as
n = read(sockfd,recvline,10000);
recvline[n] = '\0'
fputs(recvline,stdout);
You never check how many bytes read returned (or whether it failed). Even if it does return n bytes as you assume, you don't null-terminate the buffer until after you print it, so fputs(recvline,stdout) will print whatever garbage it finds in the uninitialized buffer on the stack.

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