c - 阻塞 UDP 套接字 C

Question

这个问题已经有答案了:

Why does printf not flush after the call unless a newline is in the format string? (10 个回答)

已关闭 6 年前。

有人可以解释一下为什么在调用 ConnectionSocket() 函数之前没有执行这个 printf 吗？

我知道套接字默认情况下是阻塞的，所以它会等待直到收到一些东西。但此时的函数还没有执行，为什么不先在屏幕上打印“test”呢？

#include <stdio.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <string.h>
#include <stdlib.h>
#include <wiringPi.h>
#include <stdbool.h>
#include <sys/time.h>
/* Global variables */
unsigned char buffer[1];
struct sockaddr_in serverAddr, clientAddr;
struct timeval timeout={10,0}; //set timeout for 10 seconds 
int udpSocket, slen = sizeof(clientAddr);


int main(){
    printf("test"); // -> for the first time executed when receiving a byte
    initialize_pins();
    ConnectionSocket();
    loop();

    return 0;
}

void ConnectionSocket(){

    /*Create UDP socket*/
    udpSocket = socket(AF_INET, SOCK_DGRAM, 0);

    /*Configure settings in address struct*/
    serverAddr.sin_family = AF_INET;
    serverAddr.sin_port = htons(7891);
    serverAddr.sin_addr.s_addr = inet_addr("192.168.0.184");
    memset(serverAddr.sin_zero, '\0', sizeof serverAddr.sin_zero);

    /* set receive UDP message timeout */
    setsockopt(buffer,SOL_SOCKET,SO_RCVTIMEO,(char*)&timeout,sizeof(struct timeval));

    bind(udpSocket, (struct sockaddr *)&serverAddr, sizeof(serverAddr));

}

Answer 1

printf() 将要写入的字符串存储在缓冲区中，如果缓冲区已满，或者插入了 '\n' 或者调用 fflush()，或者如果标准函数调用 fflush() 或到达程序末尾。

我用 write() 编写了一个直接在屏幕上打印的示例。

#include <stdio.h>
#include <unistd.h>

int     main(void)
{
    printf("1");                    // stored in a buffer but not printed on screen
    write(STDOUT_FILENO, "2", 1);   // directly printed on screen (2)
    printf("3");                    // stored in a buffer but not printed on screen
    write(STDOUT_FILENO, "4", 1);   // directly printed on screen (4)
    fflush(stdout);                 // flushing stdout, 1 and 3 are printed
    write(STDOUT_FILENO, "5", 1);   // directly printed on screen (5)
    printf("6\n");                  // stored in a buffer then the buffer is printed on the screen because of '\n' (6\n)
    printf("7");                    // stored in a buffer but not printed on screen

    return (0);                     // end of the program, 7 is printed
}

输出为 241356\n7