1. Set non blocking in `example/main.c`.

2. Add some description of `ff_socket()` and `ff_write()`.
3. Ref #709.

example/main.c

@@ -104,13 +104,19 @@ int loop(void *arg)
             } while (available);
         } else if (event.filter == EVFILT_READ) {
             char buf[256];
-            size_t readlen = ff_read(clientfd, buf, sizeof(buf));
-
-            ff_write(clientfd, html, sizeof(html) - 1);
+            ssize_t readlen = ff_read(clientfd, buf, sizeof(buf));
+            ssize_t writelen = ff_write(clientfd, html, sizeof(html) - 1);
+            if (writelen < 0){
+                printf("ff_write failed:%d, %s\n", errno,
+                    strerror(errno));
+                ff_close(clientfd);
+            }
         } else {
             printf("unknown event: %8.8X\n", event.flags);
         }
     }
+
+    return 0;
 }
 
 int main(int argc, char * argv[])
@@ -129,6 +135,10 @@ int main(int argc, char * argv[])
         exit(1);
     }
 
+    /* Set non blocking */
+    int on = 1;
+    ff_ioctl(sockfd, FIONBIO, &on);
+
     struct sockaddr_in my_addr;
     bzero(&my_addr, sizeof(my_addr));
     my_addr.sin_family = AF_INET;

lib/ff_api.h

@@ -64,6 +64,15 @@ int ff_sysctl(const int *name, u_int namelen, void *oldp, size_t *oldlenp,
 
 int ff_ioctl(int fd, unsigned long request, ...);
 
+/*
+ * While get sockfd from this API, and then need set it to non-blocking mode like this,
+ * Otherwise, sometimes the socket interface will not work properly, such as `ff_write()`
+ *
+ *    int on = 1;
+ *    ff_ioctl(sockfd, FIONBIO, &on);
+ *
+ *  See also `example/main.c`
+ */
 int ff_socket(int domain, int type, int protocol);
 
 int ff_setsockopt(int s, int level, int optname, const void *optval,
@@ -87,6 +96,21 @@ int ff_getsockname(int s, struct linux_sockaddr *name,
 ssize_t ff_read(int d, void *buf, size_t nbytes);
 ssize_t ff_readv(int fd, const struct iovec *iov, int iovcnt);
 
+
+/*
+ * Write data to the socket sendspace buf.
+ *
+ * Note:
+ * The `fd` parameter need set non-blocking mode in advance if F-Stack's APP.
+ * Otherwise if the `nbytes` parameter is greater than
+ * `net.inet.tcp.sendspace + net.inet.tcp.sendbuf_inc`,
+ * the API will return -1, but not the length that has been sent.
+ *
+ * You also can modify the value of  `net.inet.tcp.sendspace`(default 16384 bytes)
+ * and `net.inet.tcp.sendbuf_inc`(default 16384 bytes) with `config.ini`.
+ * But it should be noted that not all parameters can take effect, such as 32768 and 32768.
+ * `ff_sysctl` can see there values while APP is running.
+ */
 ssize_t ff_write(int fd, const void *buf, size_t nbytes);
 ssize_t ff_writev(int fd, const struct iovec *iov, int iovcnt);