[英]Reading a named pipe while multiple processes are writing to it
什么樣的方法才是解決該問題的正確方法?
例如,我有一個名為write.c
的程序,該程序具有4個子進程,並且這些子進程將其PID寫入單個全局命名管道。
另一個名為read.c
程序應讀取此PID。
我有一種類似下面的方法,但是這種方法存在一些問題。它無法讀取所有PID,有時無法讀取其中的3個,有時甚至可以讀取其中的2個。 :
writer.c
:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
int main(){
int fd;
char * myfifo = "/tmp/myfifo"; //FIFO file
char buffer[50];
mkfifo(myfifo, 0666); //creating the FIFO
for(int i=0;i<4;i++){ //creating 4 child process
if(fork() == 0) {
fd = open(myfifo, O_WRONLY); //each child process opens the FIFO for writing their own PID.
sprintf(buffer, "%d", getpid()); //each child process gets pid and assign it to buffer
printf("write:%s\n", buffer); // each child process prints to see the buffer clearly
write(fd, buffer, strlen(buffer)+1); //each child process writes the buffer to the FIFO
close(fd);
exit(0);
}
}
for(int i=0;i<4;i++) { //waiting the termination of all 4 child processes.
wait(NULL);
}
//parent area
}
reader.c
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/stat.h>
#include <time.h>
#include <string.h>
#include <fcntl.h>
int main(int argc, char **argv) {
int fd1;
// FIFO file path
char * myfifo = "/tmp/myfifo";
// Creating the named file(FIFO)
mkfifo(myfifo, 0666);
char str1[80]; //str2[80];
while (1)
{
// First open in read only and read
fd1 = open(myfifo,O_RDONLY);
read(fd1, str1, 80);
// Print the read string and close
printf("read: %s\n", str1);
close(fd1);
}
}
這行將空字節寫入fifo:
write(fd, buffer, strlen(buffer)+1);
因此,如果管道中有兩個pid,則會讀取以下字符串:
1234\02345\0
並且printf
將只打印到第一個\\0
為止:
1234
要解決此問題,將PID轉換為二進制文件而不是格式化和解析文本會更容易:
if(fork() == 0) {
fd = open(myfifo, O_WRONLY);
pid_t pid = getpid();
write(fd, &pid, sizeof(pid));
close(fd);
exit(0);
}
fd1 = open(myfifo,O_RDONLY);
pid_t pid;
while (1) // whatever is your termination condition
{
read(fd1, &pid, sizeof(pid));
printf("read: %d\n", pid);
}
close(fd1);
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