Using a pipe to read array of strings with C - c

I am learning about using pipes with C right now, and I am having some difficulty writing a list of strings, one-by-one, to the pipe in the child process, and then read them from the pipe in the parent process. Here is my current code:
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/wait.h>
#include <string.h>
int main()
{
int pfd[2];
char buf[1000];
int cfork;
if (pipe(pfd) == -1) {
exit(1);
}
cfork = fork();
if (cfork == -1) {
printf("Fork Failed\n");
exit(1);
}
else if (cfork == 0) {
printf("Child Process\n");
char *fruit[] = {
"Orange", "Apple",
"Banana", "Pear"
};
int num_fruit = 4;
for (int i = 0; i < num_fruit; i++) {
printf("Current fruit: %s\n", fruit[i]);
write(pfd[1], fruit[i], (strlen(fruit[i])));
}
_exit(0);
}
else {
printf("Parent Process\n");
read(pfd[0], buf, sizeof(buf));
printf("Fruit Fetched: %s\n", buf);
wait(NULL);
}
return 0;
}
What I am trying to do, is in the child, read a fruit string, write it to the pipe, and have the parent read this string and print it, until all the strings have been printed. My trouble is that the child, since it's in a loop, just keeps adding each string to the buffer, so the program, as it stands, prints out "OrangeAppleBanana". I am pretty sure I will need a loop in the parent as well, but when I've tried a while loop that waits for some end condition string sent by the child (for example "done"), my program still gets stuck in an infinite loop.
What is the most straightforward way, for someone new to C, to write strings in the child one -by-one, and have the parent process print them out one-by-one?
EDIT
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/wait.h>
#include <string.h>
int main()
{
int pfd[2];
int cfork;
if (pipe(pfd) == -1) {
exit(1);
}
cfork = fork();
if (cfork == -1) {
printf("Fork Failed\n");
exit(1);
}
else if (cfork == 0) {
int numbers[] = {
1, 2,
3, 4
};
int limit = 4;
close(pfd[0]);
for (int i = 0; i < limit; i++) {
printf("Child - Current Number: %d\n", numbers[i]);
write(pfd[1], &numbers[i], sizeof(numbers[i]));
}
close(pfd[1]);
_exit(0);
}
else {
int temp;
int reads = 4;
close(pfd[1]);
for (int i = 0; i < reads; i++) {
read(pfd[0], &temp, sizeof(temp));
printf("Parent - Number Fetched: %d\n", temp);
}
close(pfd[0]);
waitpid(-1, NULL, 0);
}
return 0;
This is my new code, where I use integers instead of strings. Seems to work so far. Still not sure what I was doing wrong with strings though.

I believe your problem is with "strings". Strings in C are null terminated, so when you are sending them via pipe, receiver(parent) doesn't know where a string ends. Strlen does count the number of characters in a string, but not the null charatter. You should do:
write(pfd[1], fruit[i], (strlen(fruit[i]))+1);
Parent can now know when and where to split your string.
The other problem is that you need to loop in the parrent as well. You need to set up a condition in the loop, which checks for EOF. In your example, where you know you are going to receive 4 fruits, you can just loop 4 times.
It's a good practice to close read and write end of pipes you don't need. In your example, child should close the reading end, while parent should close the writing end. You can do this with:
close(pfd[0]); //use this in child
close(pfd[1]); //use this in parent
You should also get used to closing all descriptors you don't need. In your example, you should close the pipe in both child and parent process after you are finished with writing / reading. This way, you could create a read loop condition, which closes after EOF. (When child closes pipe, receiver can end)
As an extra tip, try error reporting with "perror"
http://www.tutorialspoint.com/c_standard_library/c_function_perror.htm

//Child
close(pfd[0]); // Close read end this blocks if parent is reading from pipe
write(pfd[1]...); // write data into pipe
close(pfd[1]); // close write end of pipe now the pipe is ready to read
// Parent
close(pfd[1]); // close write end of pipe blocks if child is writing to pipe.
read(pfd[0] ...);
close(pfd[0]..); // close read end so pipe is ready to write to.

Related

How to distinguish one child process from other child processes

I have an assignment for class and I am confused on this part of the requirements. So we need to make a multi process word counter with n number of processes and n will be an input argument for the program. Each process needs to do their own mini word count of a select portion of the inputted file. So essentially the inputted file will be divided into 1/n parts and split between n processes.
I understand how to fork the processes through a for loop and how to use pipes to send the mini word count from the children processes to the parent process, but I unsure of how to tell a certain process to do a select part of the input file.
Would you use their PID values to check which process they are then assign them their task?
This is my code so far.
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#define MSGLEN 64
#define MESSES 3
int main(){
int fd[2];
pid_t pid;
int result;
//Creating a pipe
result = pipe (fd);
if (result < 0) {
//failure in creating a pipe
perror("pipe error\n");
exit (1);
}
//Creating a child process
for(int i = 0; i < MESSES; i++){
if ((pid = fork()) < 0) {
//failure in creating a child
perror ("fork error\n");
exit(2);
}
if(pid == 0)
break;
}
if (pid == 0) {
// ACTUALLY CHILD PROCESS
char message[MSGLEN];
//Clearing the message
memset (message, 0, sizeof(message));
printf ("Enter a message: ");
//scanf ("%s",message);
fgets (message, 1024, stdin);
close(fd[0]);
//Writing message to the pipe
write(fd[1], message, strlen(message));
close(fd[1]);
close(fd[0]);
exit (0);
}
else {
//Parent Process
char message[MSGLEN];
char *ptr;
long wc;
close(fd[1]);
while (1) {
//Clearing the message buffer
memset (message, 0, sizeof(message));
//Reading message from the pipe
if(read(fd[0], message, sizeof(message)) == 0)
exit(0);
printf("Message entered %s\n",message);
/*
Message entered needs to be in the format of number first space then string for it to work
*/
wc = 0;
wc = strtol(message, &ptr, 10);
printf("The number(unsigned long integer) is %ld\n", wc);
printf("String part is %s", ptr);
}
close(fd[0]);
wait(NULL);
// exit(0);
}
return 0;
}
The key thing to remember when using fork is that the parent and child share the same memory and a copy of everything the parent has is passed to the child. At which point the child has now forked the parents data.
In the code below we're counting how many processes we've created. You could if you wanted use this as an argument in the child ie the nth child gets value n.
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#define PROCESS_COUNT 50
int main(void) {
pid_t pid;
size_t pid_count = 0;
//pid_t pid_array[PROCESS_COUNT];
for(int i = 0; i < PROCESS_COUNT; i++) {
if ((pid = fork()) < 0) {
perror ("fork error\n");
exit(2);
}
if (pid == 0) {//child
size_t n = 0;
size_t p = getpid();
while(n++ < 2) {
//Next line is illustration purposes only ie I'm taking liberties by
//printing a pid_t value
printf("child %zu has pid_count == %zu\n", p, pid_count);
sleep(1);
}
exit (0);
}
else {
//Count how many process we've created.
pid_count++;
int status;
waitpid( -1, &status, WNOHANG);
}
}
wait(NULL);
return 0;
}
If you want to get really fancy you can use IPC using pipes or shared memory. There are lots of ways to get data from one process to another, sometimes something as simple as temporary files is more than sufficient. For your problem I'd use mmap but it does not need to be that complicated

Working with pipes in Unix C

I am having serious trouble working with pipes in C. I'm supposed to take in arguments from the command line (example: ./myprogram 123 45 67), read the arguments one character at a time into a buffer, send the character to the child process to be counted, and then return the total number of characters read to the parent process. My code is as follows(note: the comments are what I'm supposed to be doing):
// Characters from command line arguments are sent to child process
// from parent process one at a time through pipe.
// Child process counts number of characters sent through pipe.
// Child process returns number of characters counted to parent process.
// Parent process prints number of characters counted by child process.
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <signal.h>
static int toChild[2];
static int fromChild[2];
static char buffer;
int main(int argc, char **argv)
{
int status;
int nChars = 0;
pid_t pid;
pipe(toChild);
pipe(fromChild);
if ((pid = fork()) == -1) {
printf("fork error %d\n", pid);
return -1;
}
else if (pid == 0) {
close(toChild[1]);
close(fromChild[0]);
// Receive characters from parent process via pipe
// one at a time, and count them.
int count = 0;
printf("child about to read\n");
while(read(toChild[0], &buffer, 1)){
count++;
}
// Return number of characters counted to parent process.
write(fromChild[1], &count, sizeof(count));
close(toChild[0]);
close(fromChild[1]);
printf("child exits\n");
}
else {
close(toChild[0]);
close(fromChild[1]);
// -- running in parent process --
printf("CS201 - Assignment 3 - Chris Gavette\n");
write(toChild[1], &argv[1], 1);
// Send characters from command line arguments starting with
// argv[1] one at a time through pipe to child process.
read(fromChild[0], &nChars, 1);
// Wait for child process to return. Reap child process.
// Receive number of characters counted via the value
// returned when the child process is reaped.
close(toChild[1]);
close(fromChild[0]);
waitpid(pid, &status, 0);
printf("child counted %d chars\n", nChars);
printf("parent exits\n");
return 0;
}
}
The child process seems to hang even though I've closed both ends of both pipes.
For starters, this is wrong.
write(toChild[1], &count, 1)
It will eventually contribute to your problem. count is a int, not char or unsigned char. You need to send sizeof(count). Also, the read-function upon hitting an error will return EOF, which is non-zero, so your child exit condition is not appropriate. it should look something like this:
while(read(toChild[0], &buffer, 1) == 1)
Finally, your parent process should cycle through each argument in argv[] sending each as a strlen sized buffer.
I'm nearly certain this is what you're trying to do. Note that in order to maintain sanity in knowing which descriptor is used for a specific purpose, I prefer using a #define to note what each process uses for reading and writing. This can be extended to any number of processes, btw, which I'm sure is not too far down the line for your next assignment:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <signal.h>
// P0_READ - parent read source
// P0_WRITE - parent write target
// P1_READ - child read source
// P1_WRITE - child write target
#define P0_READ 0
#define P1_WRITE 1
#define P1_READ 2
#define P0_WRITE 3
#define N_PIPES 4
int main(int argc, char **argv)
{
int fd[N_PIPES], count = 0, i;
pid_t pid;
char c;
if (pipe(fd) || pipe(fd+2))
{
perror("Failed to open pipe(s)");
return EXIT_FAILURE;
}
// fork child process
if ((pid = fork()) == -1)
{
perror("Failed to fork child process");
return EXIT_FAILURE;
}
// child process
if (pid == 0)
{
// close non P1 descriptors
close(fd[P0_READ]);
close(fd[P0_WRITE]);
// get chars from input pipe, counting each one.
while(read(fd[P1_READ], &c, 1) == 1)
count++;
printf("Child: count = %d\n", count);
write(fd[P1_WRITE], &count, sizeof(count));
// close remaining descriptors
close(fd[P1_READ]);
close(fd[P1_WRITE]);
return EXIT_SUCCESS;
}
// parent process. start by closing unused descriptors
close(fd[P1_READ]);
close(fd[P1_WRITE]);
// send each arg
for (i=1; i<argc; ++i)
write(fd[P0_WRITE], argv[i], strlen(argv[i]));
// finished sending args
close(fd[P0_WRITE]);
// Wait for child process to return.
wait(NULL);
// wait for total count
if (read(fd[P0_READ], &count, sizeof(count)) == sizeof(count))
printf("Parent: count = %d\n", count);
// close last descriptor
close(fd[P0_READ]);
return 0;
}
Input
./progname argOne argTwo
Output
Child: count = 12
Parent: count = 12
Edit: Single Pipe with Child Return Status
It seems from the comments of the original question your assignment may call for reaping the return status of the child process as the result count rather than returning it in a pipe. In doing so, you can do this with a single pipe-descriptor pair. I prefer the first method, but this works as well:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <signal.h>
// P0_WRITE - parent write target
// P1_READ - child read source
#define P1_READ 0
#define P0_WRITE 1
#define N_PIPES 2
int main(int argc, char **argv)
{
int fd[N_PIPES], count = 0;
pid_t pid;
char c;
if (pipe(fd))
{
perror("Failed to open pipe(s)");
return EXIT_FAILURE;
}
// fork child process
pid = fork();
if (pid == -1)
{
perror("Failed to fork child process");
return EXIT_FAILURE;
}
if (pid == 0)
{
// close non P1 descriptors
close(fd[P0_WRITE]);
// Return number of characters counted to parent process.
while(read(fd[P1_READ], &c, 1) == 1)
++count;
close(fd[P1_READ]);
printf("Child: count = %d\n", count);
return count;
}
// parent process. start by closing unused descriptors
close(fd[P1_READ]);
// eacn each arg entirely
for (int i=1; i<argc; ++i)
write(fd[P0_WRITE], argv[i], strlen(argv[i]));
// finished sending args
close(fd[P0_WRITE]);
// Wait for child process to return.
if (wait(&count) == -1)
{
perror("Failed to wait for child process");
return EXIT_FAILURE;
}
printf("Parent: count = %d\n", WEXITSTATUS(count));
return 0;
}
The results are the same, but note this is a biach to to debug as most debuggers will signal-trip on your child process and the real exit status is lost. On my Mac, for example, running this under Xcode trips:
Failed to wait for child process: Interrupted system call
while running from the command line gives:
Child: count = 12
Parent: count = 12
One of the many reasons I prefer the two-pipe methodology.

New to IPC, can't get my pipe to work

Sorry for the length of this post... I've encountered about a zillion problems in this. Up front I'll say I'm a student and my professor is a worthless resource. So, all I want to to do is have producer fork, then the parent producer will count some stuff in a file and send two ints to consumer, which was launched by the child process. I've tested everything, the fork and the file stuff works and I have printf statements all over the place so I know what is being done and where the code is at.
When I added the
if (pipe(pipefd) == -1) {
perror("pipe");
}
it caused my parent to just terminate. It reaches "parent pipe open" but then it dies. I checked with $ ps to see if it was just hung, but it's not there; it just dies. If I take that snippet out, it runs to the end but I presume if that code isn't there, then it's not actually aware that pipefd is a pipe... right?
I did search on this site and found another example of this and followed what he did as well as the answer and mine just refuses to work. I'm pretty sure it's a trivially easy thing to fix but I've run out of ideas of what to try :(
I don't really want to post all my code because it'll be a huge wall of text but I don't want to accidentally cut something out that turns out to be important either.
producer.c
#include <stdio.h> /* printf, stderr, fprintf */
#include <sys/types.h> /* pid_t */
#include <unistd.h> /* _exit, fork, execl */
#include <stdlib.h> /* exit */
#include <errno.h> /* errno */
#include <string.h> /* strlen */
#include <sys/wait.h> /* wait */
#define SLEEP_TIME 8
int main (int argc, char *argv[]){
//PID
pid_t local_pid;
local_pid = fork();
//Logic to determine if the process running is the parent or the child
if (local_pid == -1) {
/* Error:
* When fork() returns -1, an error happened
* (for example, number of processes reached the limit).
*/
fprintf(stderr, "can't fork, error %d\n", errno);
exit(EXIT_FAILURE);
} else if (local_pid == 0) {
//Child specific code
int child;
char *temp[] = {NULL};
printf("Child PID found\n");
child = execv("./consumer", temp);
_exit(0);
} else {
//Parent specific code
printf("Parent running\n");
//open file
FILE * randStrings;
randStrings = fopen("randStrings.txt", "r");
int file_length;
int num_of_e = 0;
int c; //using this as a char
//until eof
while (feof(randStrings) == 0) {
c = fgetc(randStrings);
//calculate length of file
file_length++;
//count e chars
if (c == 'e') {
num_of_e++;
}
}
//close file
fclose(randStrings);
//send bundle to child
int a[2];
a[0] = num_of_e;
a[1] = file_length;
printf("num of e = %i\n", a[0]);
printf("len = %i\n", a[1]);
//set up parent pipe
int pipefd[2];
if (pipe(pipefd) == -1) {
perror("pipe");
printf("x\n");
}
printf("parent pipe open\n");
close(pipefd[0]); //close the read end
write(pipefd[1], &a[0], sizeof(int));
write(pipefd[1], &a[1], sizeof(int));
close(pipefd[1]);
printf("parent pipe closed\n");
//wait for child to finish running
wait(NULL);
printf("parent out\n");
//terminate
}
}
and consumer.c
#include <stdio.h> /* printf, stderr, fprintf */
#include <sys/types.h> /* pid_t */
#include <unistd.h> /* _exit, fork, execl */
#include <stdlib.h> /* exit */
#include <errno.h> /* errno */
#define SLEEP_TIME 5
int main (int argc, char *argv[]){
sleep(SLEEP_TIME);
printf("Child program launched\n");
//receive bundle
int pipefd[2];
int buf[2];
if (pipe(pipefd) == -1) {
perror("pipe");
printf("child x\n");
}
close(pipefd[1]); //child closes write end
buf[0] = 0;
buf[1] = 0;
/*int i = 0; // i dont like this
while (read(pipefd[0], &buf[i], sizeof(int)) > 0) {
i++;
}*/
printf("child reading pipe\n");
read(pipefd[0], &buf[0], sizeof(int));
read(pipefd[0], &buf[1], sizeof(int));
close(pipefd[0]);
//buf should have the stuff in it
int num_of_e = buf[0];
int file_length = buf[1];
printf("child num of e = %i\n", num_of_e);
printf("child len = %i\n", file_length);
//open file
FILE * resultStrings;
resultStrings = fopen("resultStrings.txt", "w");
for (int i = 0; i < num_of_e; i++) {
//write num_of_e e chars
fputc('e', resultStrings);
}
//or if no e chars, write - chars
if (num_of_e == 0) {
for (int i = 0; i < file_length; i++) {
//write file_length '-' chars
fputc('-', resultStrings);
}
}
//close file
fclose(resultStrings);
printf("child out\n");
}
if you're still here after all that, you deserve a thank you just due to the length of this.
You're doing it wrong. The whole mechanism works because a child process inherits the parent's open file descriptors.
It should go like this:
Open the pipe with pipe(pipefd)
fork()
Parent (producer):
closes the read side (pipefd[0])
writes to the write side (pipefd[1])
Child (consumer):
closes the write side (pipefd[1])
reads from the read side (pipefd[0]) or calls exec
You are opening distinct pipes in both the parent and child process (after you've forked.) It needs to happen before you fork.
Now since you're execing, the new process needs to be aware of read-only pipe. There are a couple ways you could do this:
Pass it the file descriptor number (pipefd[0]) on the command line
dup2(1, fd) it to be the stdin of the newly exec'd process

How to pipe stdin to a child and execl cat in C

In the code below, I am simply trying to send a file via stdin to a child process which will exec the cat OS command. The code compiles fine. Here is how I call it from the command line:
$ ./uniquify < words.txt
However, when I run it I get a seg fault error. I am really having a hard time understanding how the flow if information is supposed to work through pipes to children. I am trying to make the code as simple as possible, so I can understand it, but it is not yet making sense. Any help would be appreciated.
#include <unistd.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#define NUM_CHILDREN 2
int main(int argc, char *argv[])
{
pid_t catPid;
int writeFds[NUM_CHILDREN];
int catFds[2];
int c = 0;
FILE *writeToChildren[NUM_CHILDREN];
//create a pipe
(void) pipe(catFds);
if ((catPid = fork()) < 0) {
perror("cat fork failed");
exit(1);
}
//this is the child case
if (catPid == 0) {
//close the write end of the pipe
close(catFds[1]);
//close stdin?
close(0);
//duplicate the read side of the pipe
dup(catFds[0]);
//exec cat
execl("/bin/cat", "cat", (char *) 0);
perror("***** exec of cat failed");
exit(20);
}
else { //this is the parent case
//close the read end of the pipe
close(catFds[0]);
int p[2];
//create a pipe
pipe(p);
writeToChildren[c] = fdopen(p[1], "w");
} //only the the parent continues from here
//close file descriptor so the cat child can exit
close(catFds[1]);
char words[NUM_CHILDREN][50];
//read through the input file two words at a time
while (fscanf(stdin, "%s %s", words[0], words[1]) != EOF) {
//loop twice passing one of the words to each rev child
for (c = 0; c < NUM_CHILDREN; c++) {
fprintf(writeToChildren[c], "%s\n", words[c]);
}
}
//close all FILEs and fds by sending and EOF
for (c = 0; c < NUM_CHILDREN; c++) {
fclose(writeToChildren[c]);
close(writeFds[c]);
}
int status = 0;
//wait on all children
for (c = 0; c < (NUM_CHILDREN + 1); c++) {
wait(&status);
}
return 0;
}
Since your question seems to be about understanding how pipes and forks work, I hope below programs can help you. Please notice that this is for illustration only. It wouldn't qualify for commercial implementation, but I wanted to keep it short!
You can compile the two programs as follows:
cc pipechild.c -o pipechild
cc pipeparent.c -o pipeparent
Then execute with ./pipeparent
pipeparent.c source
/* pipeparent.c */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#define MESSAGE "HELLO!\n"
#define INBUFSIZE 80
#define RD 0 // Read end of pipe
#define WR 1 // Write end of pipe
int main(void)
{
int ptocpipe[2]; // Parent-to-child pipe
int ctoppipe[2]; // Chile-to-parent pipe
pid_t childpid; // Process ID of child
char inbuf[80]; // Input from child
int rd; // read() return
int rdup; // dup():ed stdin for child
int wdup; // dup():ed stdout for child
char *eol; // End of line
// Create pipe for writing to child
if (pipe(ptocpipe) < 0) {
fprintf(stderr, "pipe(ptocpipe) failed!\n");
return 2;
}
// Create pipe for writing back to parent
if (pipe(ctoppipe) < 0) {
fprintf(stderr, "pipe(ctoppipe) failed!\n");
return 2;
}
// Verify that one of the pipes are working by filling it first
// in one end and then reading it from the other. The OS will
// buffer the contents for us. Note, this is not at all necessary,
// it's just to illustrate how it works!
write(ptocpipe[WR], MESSAGE, strlen(MESSAGE));
read(ptocpipe[RD], inbuf, INBUFSIZE);
if (strlen(inbuf) != strlen(MESSAGE)) {
fprintf(stderr, "Failed to flush the toilet!\n");
return 6;
} else {
printf("Wrote to myself: %s", inbuf);
}
// Next, we want to launch some interactive program which
// replies with exactly one line to each line we send to it,
// until it gets tired and returns EOF to us.
// First, we must clone ourselves by using fork(). Then the
// child process must be replaced by the interactive program.
// Problem is: How do we cheat the program to read its stdin
// from us, and send its stdout back to us?
switch (childpid = fork()) {
case -1: // Error
fprintf(stderr, "Parent: fork() failed!\n");
return 3;
case 0: // Child process
// Close the ends we don't need. If not, we might
// write back to ourselves!
close(ptocpipe[WR]);
close(ctoppipe[RD]);
// Close stdin
close(0);
// Create a "new stdin", which WILL be 0 (zero)
if ((rdup = dup(ptocpipe[RD])) < 0) {
fprintf(stderr, "Failed dup(stdin)\n");
return 4;
}
// Close stdout
close(1);
// Create a "new stdout", which WILL be 1 (one)
if ((wdup = dup(ctoppipe[WR])) < 0) {
fprintf(stderr, "Failed dup(stdout)\n");
return 5;
}
// For debugging, verify stdin and stdout
fprintf(stderr, "rdup: %d, wdup %d\n", rdup, wdup);
// Overload current process by the interactive
// child process which we want to execute.
execlp("./pipechild", "pipechild", (char *) NULL);
// Getting here means we failed to launch the child
fprintf(stderr, "Parent: execl() failed!\n");
return 4;
}
// This code is executed by the parent only!
// Close the ends we don't need, to avoid writing back to ourself
close(ptocpipe[RD]);
close(ctoppipe[WR]);
// Write one line to the child and expect a reply, or EOF.
do {
write(ptocpipe[WR], MESSAGE, strlen(MESSAGE));
if ((rd = read(ctoppipe[RD], inbuf, INBUFSIZE)) > 0) {
// Chop off ending EOL
if ((eol = rindex(inbuf, '\n')) != NULL)
*eol = '\0';
printf("Parent: Read \"%s\" from child.\n", inbuf);
}
} while (rd > 0);
fprintf(stderr, "Parent: Child done!\n");
return 0;
}
pipechild.c source
/* pipechild.c
* Note - This is only for illustration purpose!
* To be stable, we should catch/ignore signals,
* and use select() to read.
*/
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <strings.h>
#include <string.h>
#define MAXCOUNT 5 // Maximum input lines toread
#define INBUFSIZE 80 // Buffer size
int main(void)
{
char buff[INBUFSIZE];
int remains = MAXCOUNT;
pid_t mypid;
char *eol;
mypid = getpid(); // Process-ID
fprintf(stderr, "Child %d: Started!\n", mypid);
// For each line read, write one tostdout.
while (fgets(buff, INBUFSIZE, stdin) && remains--) {
// Chop off ending EOL
if ((eol = rindex(buff, '\n')) != NULL)
*eol = '\0';
// Debug to console
fprintf(stderr, "Child %d: I got %s. %d remains.\n",
mypid, buff, 1 + remains);
// Reply to parent
sprintf(buff, "Child %d: %d remains\n", mypid, 1 + remains);
write(1, buff, strlen(buff));
}
fprintf(stderr, "Child %d: I'm done!\n", mypid);
return 0;
}

fork() and pipe()

I need help with this sample application. When I run it, it gets stuck after the child process prints "Child sending!".
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <stdlib.h>
#include <string.h>
#define INPUT 0
#define OUTPUT 1
int main()
{
int fd1[2];
int fd2[2];
int pid;
if (pipe(fd1) < 0)
exit(1);
if (pipe(fd2) < 0)
exit(1);
if ((pid = fork()) < 0)
{
perror("fork");
exit(1);
}
else if (pid == 0)
{
close(fd1[INPUT]);
close(fd2[OUTPUT]);
char *str = "Hello World!";
printf("Child sending!\n");
write(fd1[OUTPUT], str, strlen(str));
char *bufferc = (char *)malloc(1000);
char *readbufferc = (char *)malloc(80);
int rdc;
int gotdata = 0;
while (gotdata == 0)
while ((rdc = read(fd2[INPUT], readbufferc, sizeof(readbufferc))) > 0)
{
strncat(bufferc,readbufferc,rdc);
gotdata = 1;
}
printf("Child received: %s",bufferc);
free(readbufferc);
free(bufferc);
exit(0);
}
else
{
close(fd1[OUTPUT]);
close(fd2[INPUT]);
int rd;
char *buffer = (char *)malloc(1000);
char *readbuffer = (char *)malloc(80);
int gd = 0;
while (gd == 0)
while ((rd = read(fd1[INPUT],readbuffer, sizeof(readbuffer))) > 0)
{
strncat(buffer, readbuffer,rd);
gd = 1;
}
printf("Parent received: %s\n",buffer);
free(readbuffer);
printf("Parent sending!");
write(fd2[OUTPUT], buffer, strlen(buffer));
free(buffer);
}
return 0;
}
On a side note, is there a way to debug when I use fork because gdb automatically goes to the parent process
After the child writes to the parent, it must close the write end of the pipe so the parent knows it has reached EOF.
There are many bugs in your code. Why are you using fd2 without initializing it? Remove it.
Now its stuck at "Child sending" because pipe read is a blocking call and you are putting it in a while loop which will never return. Please refer to man page of pipe.
If you want to break that while loop, close all write ends of that pipe.
Also to debug child process, use gdb command follow-fork-mode as child before call to fork() while debugging.
Several things wrong:
fd2 is just never initialized.
The parent will never exit this:
while ((rd = read(fd1[INPUT],readbuffer, sizeof(readbuffer))) > 0)
{
strncat(buffer, readbuffer,rd);
gd = 1;
}
If there is no data to read, read will block and just not return. The only thing that would make it exit is if the connection was closed and the child doesn't close it.
You are calling read() in the expectation that if there is nothing to read, it will return with zero bytes read. However, what you are seeing is because read() is waiting for some data before returning. To address this, you need to do one of two things:
set your socket to do non-blocking reads (not recommended)
use select() or poll() to see whether there is some data to read before you read it
Also, several other points:
don't cast the returns from malloc()
check that malloc() does not return NULL
replace the whole gotdata thing with a break instruction

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