send and recv
- C course
- Advanced
- Lesson 29/83
- 5 min
Learn data exchange with send and recv.
send and recv
send(fd, buf, n, 0) sends n bytes to a connected socket. The return value is the number of bytes sent. recv(fd, buf, n, 0) reads up to n bytes from the socket.
The last parameter 0 is the default flag; options like MSG_NOSIGNAL also exist. recv returns 0 when the connection closes. read and write also work with sockets; send/recv add flag support.
socketpair
socketpair(AF_UNIX, SOCK_STREAM, 0, sv) creates two connected sockets and writes them to sv[0] and sv[1]. What one writes, the other reads.
This is ideal for communication within the same process or between parent and child after fork. You test quickly without setting up server-client: send(sv[0], msg, n, 0) writes, recv(sv[1], buf, n, 0) reads. No network address or bind needed; the connection is ready.
Practice Exchange data with socketpair
Create two sockets with int sv[2]; socketpair(AF_UNIX, SOCK_STREAM, 0, sv);. Write with send(sv[0], "merhaba", 7, 0);. Read with char buf[16]; memset(buf, 0, sizeof(buf)); recv(sv[1], buf, 7, 0);. Print with printf("%s\n", buf);.
Starter code
#include <stdio.h>
#include <string.h>
#include <sys/socket.h>
#include <unistd.h>
// Create two sockets with socketpair
// Write with send, read with recv
// Print buf
int main(void) {
return 0;
}Sample solution
#include <stdio.h>
#include <string.h>
#include <sys/socket.h>
#include <unistd.h>
int main(void) {
int sv[2];
socketpair(AF_UNIX, SOCK_STREAM, 0, sv);
send(sv[0], "merhaba", 7, 0);
char buf[16];
memset(buf, 0, sizeof(buf));
recv(sv[1], buf, 7, 0);
printf("%s\n", buf);
close(sv[0]);
close(sv[1]);
return 0;
}Expected output
merhaba
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match data exchange functions
| Term | Meaning |
|---|---|
send |
Sends data |
recv |
Receives data |
socketpair |
Two connected sockets |
MSG_NOSIGNAL |
Signal block |
2 What does recv return when the connection closes?
- 0 Correct answer
- -1
- 1
- EOF
Explanation: recv returns 0 when the connection closes.
3 send call
send(____, msg, 7, 0);send(sv[0], msg, 7, 0);Explanation: send(sv[0], ...) writes to the first socket.