C program read data from FIFO - c

Question

I have two programs to write and read a FIFO. One is read(O_RDONLY) a FIFO. Another is write data into this FIFO. This is code:
Read: The executable file is named read.
#include<errno.h>
#include<sys/stat.h>
#include<fcntl.h>
#include<stdio.h>
#include<string.h>
int main(int argc, char *argv[])
{
int fd, nread;
const char *pipe_file = "/tmp/test_fifo";
char buf[0x100] ;
fd = open(pipe_file, O_RDONLY);
while(1) {
printf("\n"); /* */
memset(buf, 0, 100);
nread = read(fd, buf, 0x100);
printf("data is %s", buf);
sleep(1);
}
return 0;
}
Write: The executable file is named write.
#include<errno.h>
#include<sys/stat.h>
#include<fcntl.h>
#include<stdio.h>
#include<string.h>
int main(int argc, char *argv[])
{
int fd;
int num = 1234;
char *s = "changzhi";
const char *pipe_file = "/tmp/test_fifo";
fd = open(pipe_file, O_WRONLY);
write(fd, s, strlen(s));
return 0;
}
The FIFO named /tmp/test_fifo is already exists. When I run read to read FIFO and run write to write the FIFO, everything goes ok. But, the read can not read data when there is no printf("\n"); in read code. I don not know why. Could someone help me ? Thanks a lot!

Answer 1

Your second printf() is buffered until the \n is written. The read is not blocked :-)
Rewrite your loop:
while(1) {
memset(buf, 0, 100);
nread = read(fd, buf, 0x100);
printf("data is %s\n", buf);
sleep(1);
}

Answer 2

With cleanly compiling code under stringent compilation options:
$ gcc -O3 -g -std=c11 -Wall -Wextra -Wmissing-prototypes -Wstrict-prototypes \
> -Wold-style-definition -Werror write.c -o write
$ gcc -O3 -g -std=c11 -Wall -Wextra -Wmissing-prototypes -Wstrict-prototypes \
> -Wold-style-definition -Werror read.c -o read
$
the code basically works for me. I've added error checking. The slightly odd argv[argc-argc] prints argv[0] but avoids a warning (error because of -Werror) about argc being unused.
read.c
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
int main(int argc, char **argv)
{
int fd, nread;
const char *pipe_file = "/tmp/test_fifo";
char buf[0x100];
if ((fd = open(pipe_file, O_RDONLY)) < 0)
{
fprintf(stderr, "%s: Failed to open FIFO %s\n", argv[argc-argc], pipe_file);
return 1;
}
printf("%s: open successful\n", argv[0]);
while(1) {
printf("\n"); /* */
memset(buf, 0, 100);
nread = read(fd, buf, 0x100-1);
if (nread <= 0)
break;
printf("data is %s\n", buf);
sleep(1);
}
return 0;
}
write.c
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
int main(int argc, char *argv[])
{
int fd;
char *s = "changzhi";
const char *pipe_file = "/tmp/test_fifo";
if ((fd = open(pipe_file, O_WRONLY)) < 0)
{
fprintf(stderr, "%s: failed to open FIFO %s\n", argv[argc-argc], pipe_file);
return 1;
}
printf("%s: open successful\n", argv[0]);
if (write(fd, s, strlen(s)) <= 0)
{
fprintf(stderr, "%s: failed to open FIFO %s\n", argv[argc-argc], pipe_file);
return 1;
}
printf("%s: write successful\n", argv[0]);
return 0;
}
Sample output
$ ./read & ./write
[1] 85542
./write: open successful
./write: write successful
./read: open successful
data is changzhi
$
[1]+ Done ./read
$ ./write & ./read
[1] 85544
./write: open successful
./read: open successful
./write: write successful
data is changzhi
[1]+ Done ./write
$
The programs report an error sensibly if the FIFO does not exist.