I am trying to create a simple pipe/fork function, so that child process modifies the value of text, then it is printed by the parent.
I have checked a similar question on Modify variable in child process, but I am unable to print the text variable in the parent.
int main()
{
pid_t childp;
char text[100];
pipe(text);
childp = fork();
if (childp ==0){
strncpy(text, "Hello world", 100); // child running
}
else{
printf("%s\n", text); // parent prints "Hello world"
}
return 1;
}
Any help is appreciated (I am very new to C language)
look into this website :
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
int main(void)
{
int fd[2], nbytes;
pid_t childpid;
char string[] = "Hello, world!\n";
char readbuffer[80];
pipe(fd);
if((childpid = fork()) == -1)
{
perror("fork");
exit(1);
}
if(childpid == 0)
{
/* Child process closes up input side of pipe */
close(fd[0]);
/* Send "string" through the output side of pipe */
write(fd[1], string, (strlen(string)+1));
exit(0);
}
else
{
/* Parent process closes up output side of pipe */
close(fd[1]);
/* Read in a string from the pipe */
nbytes = read(fd[0], readbuffer, sizeof(readbuffer));
printf("Received string: %s", readbuffer);
}
return(0);
}
It explain how to properly use C pipes. Take a look to this too.
Add this for error handling :
if (pipe(fd) == -1) {
fprintf(stderr, "Pipe Failed");
return 1;
}
The closest to your code working example I can come up with.
int main()
{
pid_t childp;
char text[100];
int fd[2];
pipe(fd);
childp = fork();
if (childp ==0){
FILE *f=fdopen(fd[1], "w"); // Write into this "file" what you want the other end to read.
fprintf(f, "Hello world\n");
}
else{
FILE *f=fdopen(fd[0], "r"); // We can read from this "file"
fgets(text, 100, f); // Read one line of text from the "file" up to 100 bytes
printf("read <%s> from by new child\n", text)
}
return 1;
}
Note again that this is not a shared buffer. So you need both end to agree on a "protocol". Because everything that is written by one end must be read by the other (otherwise the "write" instruction will be blocked), and everything that is read by one end, must be writter by the other (otherwise the "read" instruction will be blocked).
So, either you use a fixed size message, for example. If you choose 100, you need to write 100 bytes exactly at one end (fill with 0 if needed), and read 100 bytes exactly at the other.
Or you find some protocol so that the reading end knows exactly when to stop reading.
I choose the latter (because it is the closest to your code). By using fgets to read, that stop to read at each newline, and fprintf a message ended by a newline at the writing end.
Related
How can i print every string in C pipes i am stuck.
output like this
Received string: spike
Received string: spike
Received string: spike
but i want like this
Received string: spike
Received string: tom
Received string: jerry
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
int main(void)
{
int fd[2], nbytes;
pid_t childpid;
char string[3][10] = {
"spike",
"tom",
"jerry"};
char readbuffer[80];
pipe(fd);
if ((childpid = fork()) == -1)
{
perror("fork");
exit(1);
}
if (childpid == 0)
{
/* Child process closes up input side of pipe */
close(fd[0]);
/* Send "string" through the output side of pipe */
write(fd[1], string, (strlen(string) + 1));
exit(0);
}
else
{
/* Parent process closes up output side of pipe */
close(fd[1]);
/* Read in a string from the pipe */
nbytes = read(fd[0], readbuffer, sizeof(readbuffer));
for (int i = 0; i < 3; i++)
{
printf("Received string: %s \n", readbuffer);
}
}
return (0);
}
You've made several mistakes.
First of all, the child doesn't write the correct thing into the pipe.
Then you read() a 80 byte buffer, which will contain all 3 strings (asuming you've fixed issue 1). But since you write one byte more than the string is long, you'll have a zero byte in the middle of your readbuffer. Any attempt to print that will simply stop at that zero byte.
It'll be important to add error handling. You can never ever assume that a read or write will be successful.
I've modified your code a little, such that it works better (but not right yet):
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
int main(void)
{
int fd[2], nbytes;
pid_t childpid;
/* added newlines to separate the entries when printing */
char string[3][10] = {
"spike\n",
"tom\n",
"jerry\n"
};
char readbuffer[80];
pipe(fd);
if((childpid = fork()) == -1)
{
perror("fork");
exit(1);
}
if(childpid == 0)
{
/* Child process closes up input side of pipe */
close(fd[0]);
/* Send "string" through the output side of pipe */
/* Added a loop here, so all 3 entries are written */
for (int i = 0; i < 3; ++i) {
write(fd[1], string[i], (strlen(string[i])));
}
exit(0);
}
else
{
/* Parent process closes up output side of pipe */
close(fd[1]);
/* Read in a string from the pipe */
/* no loop needed here for now
But you'd need to read until you reach EOF.
I leave that as an exercise
*/
nbytes = read(fd[0], readbuffer, sizeof(readbuffer));
readbuffer[nbytes] = '\0';
printf("Received string: %s \n", readbuffer);
}
return(0);
}
I am trying to understand why my program hangs. The Parent sends input froma
file it reads to the child program, and the child program will send the result of its computation back to it's parent. However, I have trouble sending the message back through a second pipe. The parent seems to hang when reading from the pipe.
From the other posts, I have read it seems to indicate that the parent should wait for the child to finish by using wait or waitpid (which in my case both of them does not resolve my issue).
I have notice by adding print statement that neither the PARENT or the CHILD finishes.. Could someone please explain to me why this is happening?
Why does this not work?
int main(int argc,char** argv) {
char buffer[1];
int i;
int fd1[2]; int fd2[2];
pipe(fd1); pipe(fd2);
pid_t pid;
// FIRST PROCESS.
// -------------------
pid = fork();
if(pid == 0) {
int cnt;
dup2(fd1[0], STDIN_FILENO);
dup2(fd2[1], STDOUT_FILENO);
for (i = 0; i < 2; i++) {
close(fd1[i]);
close(fd2[i]);
}
while(read(STDIN_FILENO, buffer, sizeof(buffer)) > 0) {
fprintf(stderr, "( %s )", buffer);
cnt = cnt + *buffer - 48;
}
write(STDOUT_FILENO, &cnt, sizeof(cnt));
exit(0);
}
// PARENT.
// ------------------------
int file = open(argv[1], O_RDONLY);
// READ THE FILE.
while(read(file, buffer, 1) > 0) {
if (48 <= *buffer && *buffer <= 57) {
// PIPE TO CHILD.
write(fd1[1], buffer, 1);
}
}
// WAIT FOR CHILD TO FINISH SENDING BACK.
// int status = 0;
// waitpid(pid, &status, 0);
// THIS BLOCK DOESN'T RESOLVE ANYTHING. IT HANGS AT WAIT OR WAITPID.
// **** THIS IS THE PART WHERE IT DOESN'T WORK.
while(read(fd2[0], buffer, 1) > 0) {
fprintf(stderr, "RESULT : %s", buffer);
}
// CLOSING PIPES
for (i = 0; i < 2; i++) {
close(fd1[i]);
close(fd2[i]);
}
close(file);
exit(0);
}
You aren't closing enough file descriptors in the parent soon enough.
Rule of thumb: If you
dup2()
one end of a pipe to standard input or standard output, close both of the
original file descriptors returned by
pipe()
as soon as possible.
In particular, you should close them before using any of the
exec*()
family of functions.
The rule also applies if you duplicate the descriptors with either
dup()
or
fcntl()
with F_DUPFD
Now, your child process is following the RoT perfectly. But the corollary for parent processes is that they need to close the unused ends of the pipe, and they must close the write end of a pipe that they use to signal EOF to the reading end of that pipe. This is where your code fails.
Arguably, before reading the file, the parent process should close the read end of the pipe it uses to write to the child, and it should close the write end of the pipe it uses to read from the child.
Then, after reading the whole of the file, it should close the write end of the pipe to the child, before going into the 'read from child' loop. That loop never terminates because the parent still has the write end of the pipe open, so there's a process that could (but won't) write to the pipe.
Also, since the child writes the bytes of an integer onto a pipe, the parent should read the bytes of an integer. Using char buffer[1]; with a %s format is pointless; you need a null terminator for the string, and a single char buffer can't hold both a null byte and any data.
Along with various other improvements ('0' instead of 48, for example), you might end up with:
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main(int argc, char** argv)
{
if (argc != 2) {
fprintf(stderr, "Usage: %s filename\n", argv[0]);
exit(EXIT_FAILURE);
}
int fd1[2];
int fd2[2];
char buffer[1];
pipe(fd1);
pipe(fd2);
pid_t pid = fork();
if (pid == 0) {
int cnt = 0;
dup2(fd1[0], STDIN_FILENO);
dup2(fd2[1], STDOUT_FILENO);
for (int i = 0; i < 2; i++) {
close(fd1[i]);
close(fd2[i]);
}
while (read(STDIN_FILENO, buffer, sizeof(buffer)) > 0) {
fprintf(stderr, "(%c)", buffer[0]); // Changed
cnt = cnt + buffer[0] - '0';
}
putc('\n', stderr); // Aesthetics
write(STDOUT_FILENO, &cnt, sizeof(cnt));
exit(0);
}
int file = open(argv[1], O_RDONLY);
if (file < 0) {
fprintf(stderr, "failed to open file '%s' for reading\n", argv[1]);
exit(EXIT_FAILURE);
}
close(fd1[0]); // Added
close(fd2[1]); // Added
while (read(file, buffer, sizeof(buffer)) > 0) {
if ('0' <= buffer[0] && buffer[0] <= '9') {
write(fd1[1], buffer, sizeof(buffer));
}
}
close(file); // Moved
close(fd1[1]); // Added
// Rewritten
int result;
while (read(fd2[0], &result, sizeof(result)) == sizeof(result)) {
fprintf(stderr, "RESULT : %d\n", result);
}
close(fd2[0]); // Added
// Close loop removed
return 0;
}
If that is stored in file pipe71.c and compiled, I get the following outputs when it is run:
$ ./pipe71 pipe71.c
(2)(0)(1)(2)(2)(2)(1)(1)(2)(0)(0)(2)(1)(0)(2)(2)(1)(0)(2)(1)(2)(0)(0)(0)(0)(0)(1)(0)(1)(1)(0)(2)(1)(0)(0)(0)(0)(9)(1)(1)(1)(1)(2)(0)(2)(0)(0)
RESULT : 49
$ ./pipe71 pipe71
(0)(0)(8)(0)(0)(2)(2)(0)(8)(1)(1)(5)(1)(1)(1)(1)(5)(1)(1)(1)(8)(5)(1)(9)(8)(5)(1)(1)(0)(4)(4)(4)(6)(0)(2)(8)(0)(0)(0)(2)(7)(1)(3)(8)(3)(0)(4)(3)(0)(4)(9)(0)(0)(0)(0)(7)(1)(9)(8)(1)(3)(0)
RESULT : 178
$
I am a new to pipes in C.
I am trying to Write "hello" on the pipe from a child process & read the same from parent process, but I am getting unexpected output.
I using this piece of code:
#include<stdio.h>
#include<unistd.h>
#include<unistd.h>
#include<stdlib.h>
int main()
{
pid_t pid;
int fds[2];
int ret;
char ch[20];
ret = pipe(fds);
if(ret == -1)
{
perror("pipe failed");
exit(0);
}
pid = fork();
if (pid == 0)
{
printf("Child process\n");
write(fds[1],"Hello",5);
}
if (pid > 0)
{
printf("Parent Process\n");
read(fds[0],ch,15);
printf("%s\n",ch);
}
return 0;
}
I am getting this as output :
Parent Process
Child process
Helloq.
I can't understand why this extra "q." is coming ??
You are trying to write 6 bytes but are setting the size to 5. You need to also send the '\0' at the end of Hello along.
Just change your write call to
write(fds[1],"Hello",6);
and you should be fine.
Use memset() function in your code before writing data into buffer, which fill memory with a constant byte. like,
memset(ch,'\0',20);
Full code may be help you.
#include<stdio.h>
#include<string.h>
#include<unistd.h>
#include<unistd.h>
#include<stdlib.h>
int main()
{
pid_t pid;
int fds[2];
int ret;
char ch[20];
memset(ch,'\0',20);
ret = pipe(fds);
if(ret == -1)
{
perror("pipe failed");
exit(0);
}
pid = fork();
if (pid == 0)
{
printf("Child process\n");
write(fds[1],"Hello",5);
}
if (pid > 0)
{
printf("Parent Process\n");
read(fds[0],ch,15);
printf("%s\n",ch);
}
}
Since you don't record how many bytes were read off the pipe, your code is printing the garbage that was already in the ch variable. There are numerous ways to deal with it. This code shows two of them. I used memset() to ensure that ch contained some data (and the assignment makes sure it is null terminated).
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
int main(void)
{
pid_t pid;
int fds[2];
int ret;
char ch[20];
memset(ch, 'X', sizeof(ch)-1);
ch[sizeof(ch)-1] = '\0';
ret = pipe(fds);
if (ret == -1)
{
perror("pipe failed");
exit(0);
}
pid = fork();
if (pid == 0)
{
printf("Child process\n");
write(fds[1], "Hello", 5);
}
else if (pid > 0)
{
printf("Parent Process\n");
int nbytes = read(fds[0], ch, 15);
printf("[%s]\n", ch);
printf("[%.*s]\n", nbytes, ch);
ch[nbytes] = '\0';
printf("[%s]\n", ch);
}
else
fprintf(stderr, "fork() failed\n");
return 0;
}
The code records how many bytes were written (truly diligent code would ensure that the correct amount of data was written, too). It prints the data 3 times — once using your original technique, then once using the number of bytes read off the pipe to limit the output, and then null-terminating the data so that it can be written as a simple string.
The %.*s conversion specification uses two values — a number and the string. The number is the maximum number of bytes that will be written. If the string is shorter than that, so be it. If the string is longer, the excess bytes are ignored.
Sample output:
Parent Process
[HelloXXXXXXXXXXXXXX]
[Hello]
[Hello]
Child process
This was the result of piping the program output. Visually, on the terminal, I usually got:
Parent Process
Child process
[HelloXXXXXXXXXXXXXX]
[Hello]
[Hello]
Both outputs are valid. Note how the first printing of the data also includes a number of the X's because there was no null byte read from the pipe.
Another alternative is to have the child write the null of the null-terminated string to the pipe: write(fds[1], "Hello", sizeof("Hello"));. Other options include writing the length of the string on the pipe followed by the data and then reading the length and that many bytes of data. This is a minor variant on a TLV (type, length, value) encoding system — the type is not explicitly specified as it is assumed to be char.
I am developing an application in C.
Parent and child process communicate through pipe.
Before writing to pipe, parent process execute another statements. In sample code, i have used sleep(10) to make delay.
In the child process, it should read the data from the pipe.
But data is not read on the read end of pipe in child process.
int main()
{
int pid;
FILE *fp;
fp = fopen("test.txt","w");
char *buff;
int fd[2];
int count = 0 ;
pipe(fd);
pid = fork();
if(pid == 0)
{
close(fd[1]);
ioctl(fd[0], FIONREAD, &count);
fprintf(fp,"Value of count: %d ",count);
buff = malloc(count);
fprintf(fp,"\n TIME before read: %s",__TIME__);
read(fd[0], buff, count);
fprintf(fp,"\nbuffer: %s\n TIME after read %s", buff, __TIME__);
}
else{
close(fd[0]);
sleep(10); //delay caused by application specific code replaced with sleep
write(fd[1],"THIS is it",10);
}
fclose(fp);
return 0;
}
How to make child process wait till data is written on the other end?
Your pipe is opened in blocking mode, and you do nothing to change that, which is likely what you intended.
However, since the first thing you do is request the size of data waiting on the pipe, then blindly jump into reading that many bytes (which in all likelihood will be zero at the time that code executes since the parent hasn't written anything yet) you don't block, and instead just leave because you requested nothing.
There are a number of ways to do this, including a select-loop. If you would rather block on a read until data is available, then do so on a single byte and fill in the remaining data afterward.
This is by no means an example of how to do this right, but it is a short sample of how you can wait on a single byte, request the read-size of the pipe to get the rest of the data, read it, and continue this until the pipe has no data left and the parent shuts down their end:
I hope you find it helpful.
#include <stdio.h>
#include <unistd.h>
#include <sys/ioctl.h>
int main()
{
int pid = 0;
// create pipe pair
int fd[2];
pipe(fd);
pid = fork();
if (pid == 0)
{
// child side
char *buff = NULL;
char byte = 0;
int count = 0;
// close write side. don't need it.
close(fd[1]);
// read at least one byte from the pipe.
while (read(fd[0], &byte, 1) == 1)
{
if (ioctl(fd[0], FIONREAD, &count) != -1)
{
fprintf(stdout,"Child: count = %d\n",count);
// allocate space for the byte we just read + the rest
// of whatever is on the pipe.
buff = malloc(count+1);
buff[0] = byte;
if (read(fd[0], buff+1, count) == count)
fprintf(stdout,"Child: received \"%s\"\n", buff);
free(buff);
}
else
{ // could not read in-size
perror("Failed to read input size.");
}
}
// close our side
close(fd[0]);
fprintf(stdout,"Child: Shutting down.\n");
}
else
{ // close read size. don't need it.
const char msg1[] = "Message From Parent";
const char msg2[] = "Another Message From Parent";
close(fd[0]);
sleep(5); // simulate process wait
fprintf(stdout, "Parent: sending \"%s\"\n", msg1);
write(fd[1], msg1, sizeof(msg1));
sleep(5); // simulate process wait
fprintf(stdout, "Parent: sending \"%s\"\n", msg2);
write(fd[1], msg2, sizeof(msg2));
close(fd[1]);
fprintf(stdout,"Parent: Shutting down.\n");
}
return 0;
}
Output
Parent: sending "Message From Parent"
Child: count = 19
Child: received "Message From Parent"
Parent: sending "Another Message From Parent"
Parent: Shutting down.
Child: count = 27
Child: received "Another Message From Parent"
Child: Shutting down.
I think after
ioctl(fd[0], FIONREAD, &count);
the count is 0.
read(fd[0], buff, count) will get no data.
try
read(fd[0], buff, 10)
The problem is with getting number of bytes written to the pipe. You are getting it right after the fork(). If the read process executes first, it will contain no data (and the count will be zero). If the write process execute first, it will contain some data.
How to make child process wait till data is written on the other end?
Since you opened the pipe in blocking mode, you should read as much data as possible, and not try to get the size of written data.
Here is your modified example that waits for a full message :
#include <stdio.h>
#include <sys/ioctl.h>
int main()
{
int pid;
FILE *fp;
fp = fopen("test.txt","w");
char *buff = malloc(1024);
int fd[2];
int count = 0 ;
pipe(fd);
pid = fork();
if(pid == 0)
{
close(fd[1]);
int i = 0;
while ( i < 10 )
{
fprintf(fp,"\n TIME before read: %s \n",__TIME__);
read(fd[0], buff+i, 1);
++ i;
}
fprintf(fp,"Full message received!\nbuffer: %s\n TIME after read %s\n", buff, __TIME__);
}
else{
close(fd[0]);
sleep(10); //delay caused by application specific code replaced with sleep
write(fd[1],"THIS is it",10);
}
fclose(fp);
return 0;
}
I have a task in Linux and I can't get it work.
I have a program that receives a text file as parameter. It then creates a child process using fork() and sends to the child process, line by line the content of the text file received as parameter. The child process needs to count the lines and return to the parent process the number of lines received.
This is what I have until now, but somewhat the child process does not receive all the lines. For my test I used a text file with 9 lines. The parent sent 9 lines as strings but the child process received only 2 or 3 of them.
What am I doing wrong?
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(int argc, char *argv[])
{
char string[80];
char readbuffer[80];
int pid, p[2];
FILE *fp;
int i=0;
if(argc != 2)
{
printf("Syntax: %s [file_name]\n", argv[0]);
return 0;
}
fp = fopen(argv[1], "r");
if(!fp)
{
printf("Error: File '%s' does not exist.\n", argv[1]);
return 0;
}
if(pipe(p) == -1)
{
printf("Error: Creating pipe failed.\n");
exit(0);
}
// creates the child process
if((pid=fork()) == -1)
{
printf("Error: Child process could not be created.\n");
exit(0);
}
/* Main process */
if (pid)
{
// close the read
close(p[0]);
while(fgets(string,sizeof(string),fp) != NULL)
{
write(p[1], string, (strlen(string)+1));
printf("%s\n",string);
}
// close the write
close(p[1]);
wait(0);
}
// child process
else
{
// close the write
close(p[1]);
while(read(p[0],readbuffer, sizeof(readbuffer)) != 0)
{
printf("Received string: %s\n", readbuffer);
}
// close the read
close(p[0]);
}
fclose(fp);
}
A pipe is a unidirectional interprocess communication channel. You have to create 2 pipes, one to speak to the child process, the other to read data back.
Remember to close the unused side of the pipe on both processes.
You are sending the null terminator to the other process:
write(p[1], string, (strlen(string)+1));
That makes the result confusing because when you print what you've received, you only see up to the null terminator.
If you do this instead:
write(p[1], string, strlen(string));
you should get what you expect.
You're not counting the number of lines, you're counting the number of times read(2) returns.
When using pipes, read(2) will pull as much data as possible from the pipe: min(pipe_available, space_available). It doesn't care for newlines, 0 bytes etc. Simple tricks to make it work:
Use a loop to walk readbuffer and look for \n
Use fdopen + fgets (I have a feeling this is probably flawed)
look into manpage of pipe ( man 2 pipe ), the program you're trying to write is as an example there, compare it with yours :)