Pipe to and from child process is not working - c

Question

I am trying to learn pipes and I am trying out this program:
#include<stdio.h>
#include<string.h>
#include<unistd.h>
#include<errno.h>
#include<sys/types.h>
#include<fcntl.h>
#define MAXLINE 100
void main(){
int pipe1[2],pipe2[2];
pid_t childpid;
if(pipe(pipe1)<0){
perror("Unable to create the pipe for pipe1");
exit(-1);
}
if(pipe(pipe2)<0){
perror("Unable to create the pipe for pipe1");
exit(-1);
}
childpid=fork();
printf("The child PID is:%d\n",childpid);
if(childpid==0){
printf("In the child process");
close(pipe1[1]);
close(pipe2[0]);
server(pipe1[0],pipe2[1]);
exit(0);
}
close(pipe1[0]);
close(pipe2[1]);
client(pipe2[0],pipe1[1]);
waitpid(childpid,NULL,0);
exit(0);
}
void client(int readfd,int writefd){
int n,len;
char buff[MAXLINE];
printf("Please enter the name of the file to be read:");
fgets(buff,MAXLINE,stdin);
len=strlen(buff);
if(buff[len-1]=='\n')
len--;
write(writefd,buff,len);
printf("File name written into the pipe\n");
printf("The num of bytes written are:\n",read(readfd,buff,MAXLINE));
while((n-read(readfd,buff,MAXLINE))>0){
printf("Trying to read the content\n");
write(STDOUT_FILENO,buff,n);
}
}
void server(int readfd,int writefd){
int fd,n;
char buff[MAXLINE + 1];
write(writefd,"Yello in the server process",strlen("Yello in the server process"));
if((n=read(readfd,buff,MAXLINE))==0)
perror("End of file while reading");
buff[n]='\0';
if((fd=fopen(buff,O_RDONLY))<0){
snprintf(buff+n,sizeof(buff)-n,"Can't open, %s",strerror(errno));
n=strlen(buff);
write(writefd,buff,n);
}
while( (n=read(fd,buff,MAXLINE))>0){
write(writefd,buff,n);
close(fd);
}
}
The problem is I enter the file name and the program just exits. I tried to gdb the child process by setting set "follow-fork-mode child", and still nothing happens. Any ideas as to where I could be going wrong?
Ok, some more additional debugging info is: I set the follow-fork-mode to child.and it is causing a segmentation fault at the opening of the file.
Program received signal SIGSEGV, Segmentation fault.
[Switching to process 28025]
0x00197f08 in _IO_file_fopen () from /lib/libc.so.6

Answer 1

This code in client() looks suspicious:
while((n-read(readfd,buff,MAXLINE))>0){
Surely, that should be:
while ((n = read(readfd, buff, MAXLINE)) > 0)
{
The change from - to = is the important one, of course; the rest are cosmetic (and one is even controversial).
You should also pay attention to compiler warnings. Given:
int fd,n;
...
if((fd=fopen(buff,O_RDONLY))<0){
There's no way this should be compiling without major warnings; fopen() returns a FILE *, not a file descriptor (int).
You also seem to have some odd communications. The server sends a message to the client before reading the file name from the client. The client, OTOH, does not necessarily read that message separately; it gets a glob of information and reports how many bytes it got. That may have included some of the file as well as the introductory message.
You shouldn't close a file in the loop that is reading from it:
while( (n=read(fd,buff,MAXLINE))>0){
write(writefd,buff,n);
close(fd);
}
The close should be outside the loop.
This code more or less works; it is messier than I'd like, but it does more or less work.
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#define MAXLINE 100
static void server(int readfd, int writefd);
static void client(int readfd, int writefd);
int main(void)
{
int pipe1[2], pipe2[2];
pid_t childpid;
if (pipe(pipe1)<0)
{
perror("Unable to create the pipe for pipe1");
exit(-1);
}
if (pipe(pipe2)<0)
{
perror("Unable to create the pipe for pipe2");
exit(-1);
}
childpid = fork();
printf("The child PID is:%d\n", childpid);
if (childpid == 0)
{
printf("In the child process\n");
close(pipe1[1]);
close(pipe2[0]);
server(pipe1[0], pipe2[1]);
exit(0);
}
close(pipe1[0]);
close(pipe2[1]);
client(pipe2[0], pipe1[1]);
waitpid(childpid, NULL, 0);
return 0;
}
static void client(int readfd, int writefd)
{
int n, len;
char buff[MAXLINE];
printf("Please enter the name of the file to be read:");
fgets(buff, MAXLINE, stdin);
len = strlen(buff);
if (buff[len-1]=='\n')
len--;
write(writefd, buff, len);
printf("File name (%.*s) written into the pipe\n", len, buff);
printf("The num of bytes written are: %d\n", (int)read(readfd, buff, MAXLINE));
while ((n = read(readfd, buff, MAXLINE)) > 0)
{
printf("Trying to read the content\n");
write(STDOUT_FILENO, buff, n);
}
}
static void server(int readfd, int writefd)
{
int fd, n;
char buff[MAXLINE + 1];
fprintf(stderr, "Server: %d\n", (int)getpid());
write(writefd, "Yello in the server process", strlen("Yello in the server process"));
if ((n = read(readfd, buff, MAXLINE))==0)
perror("End of file while reading");
buff[n] = '\0';
if ((fd = open(buff, O_RDONLY)) < 0)
{
snprintf(buff+n, sizeof(buff)-n, "Can't open, %s", strerror(errno));
n = strlen(buff);
write(writefd, buff, n);
}
else
{
while ((n = read(fd, buff, MAXLINE)) > 0)
{
if (write(writefd, buff, n) != n)
{
fprintf(stderr, "Write failed in server\n");
break;
}
}
close(fd);
}
}
Note that the code does not try using a file descriptor that it fails to open. It does not crash; the n-read(...) problem is one major part of the trouble, comments notwithstanding. The misuse of fopen() for open() was another major part of the trouble.