Pipe communication in C problems - c

I'm trying to write some code which uses pipes to communicate between a parent process and it's children. However, my pipe seems to give up after the first time I use it (that is, it stops working after the first use of the pipe). I'm not really sure how to fix this problem, and any help will be greatly appreciated. I also know that some of the coding practice I am using in this are not really ideal (mainly use of sleep).
const int READ = 0;
const int WRITE = 1;
char* COOP = "Criminal cooperates\n";
char* SIL = "Criminal doesn't talk\n";
char* reader(int);
void writer(int, char *c);
int main()
{
int c1pipe1[2];
int c1pipe2[2];
int c2pipe1[2];
int c2pipe2[2];
int c1sentence = 0;
int c2sentence = 0;
int r;
int c;
pipe(c1pipe1);
pipe(c1pipe2);
pipe(c2pipe1);
pipe(c2pipe2);
int C2;
int C1 = fork();
if(C1 > 0)
C2 = fork();
if(C1 < 0 || C2 < 0) //error
{
perror("fork() failed");
exit(1);
}
else if(C1 == 0)
{
close(c1pipe1[WRITE]);
close(c1pipe2[READ]);
for(c = 0; c < 10; c++)
{
r = rand();
//printf("C1 rand = %d\n", r%2);
if(r % 2 == 1)
writer(c1pipe2[WRITE], "1");
else
writer(c1pipe2[WRITE], "0");
sleep(1);
}
exit(0);
}
else if(C2 == 0)
{
close(c2pipe1[WRITE]);
close(c2pipe2[READ]);
for(c = 0; c < 10; c++)
{
r = rand();
//printf("C2 rand = %d\n", r%2);
if(r % 2 == 1)
writer(c2pipe2[WRITE], "1");
else
writer(c2pipe2[WRITE], "0");
sleep(1);
}
exit(0);
}
else //parent
{
int buff1; //stores choice of c1
int buff2; //stores choice of c2
close(c1pipe1[READ]);
close(c1pipe2[WRITE]);
close(c2pipe1[READ]);
close(c2pipe2[WRITE]);
for(c = 0; c< 10; c++)
{
buff1 = atoi(reader(c1pipe2[READ]));
buff2 = atoi(reader(c2pipe2[READ]));
printf("C1's \(%d)\ choice trial %d : %d\n", C1, c+1, buff1);
printf("C2's \(%d)\ choice trial %d : %d\n", C2, c+1, buff2);
if(buff1 && buff2) //c1 and c2 cooperate with police
{
c1sentence = c1sentence + 6;
c2sentence = c2sentence + 6;
}
else if(buff1 || buff2) // one cooperates, one is silent
{
if(buff1) // if c1 cooperates and c2 is silent
{
c1sentence = c1sentence + 0;
c2sentence = c2sentence + 10;
}
else // if c2 cooperates and c1 is silent
{
c1sentence = c1sentence + 10;
c2sentence = c2sentence + 0;
}
}
else if(!(buff1 && buff2)) //both c1 and c2 are silent
{
c1sentence = c1sentence + 1;
c2sentence = c2sentence + 1;
}
sleep(1);
}
printf("C1 is in jail for %d years total\n", c1sentence);
printf("C2 is in jail for %d years total\n", c2sentence);
exit(0);
}
exit(0);
}
void writer(int pipe_write_fd, char *c)
{
open(pipe_write_fd);
char* choice = c;
// Write to the pipe
write(pipe_write_fd, choice, strlen(choice));
// Close the pipe
// (Sends 'end of file' to reader)
close(pipe_write_fd);
}
char* reader(int pipe_read_fd)
{
open(pipe_read_fd);
// Allocate buffer to store
// result of read
int buffer_size = 1024;
char buffer[buffer_size];
// Keep reading until we exhaust
// buffer or reach end of file
int i = 0;
while (i < buffer_size
&& read(pipe_read_fd, &buffer[i], 1) > 0)
{ i++; }
if (i < buffer_size) {
// Add null termination
buffer[i] = '\0';
} else {
// We exhausted buffer
fprintf(stderr, "Warning: buffer full.\n");
buffer[buffer_size-1] = '\0';
}
//printf("%s", buffer);
// Close the pipe
close(pipe_read_fd);
return buffer;
}

You need to close more of the pipes. The child processes must close every pipe file descriptor that they are not using. You have 8 pipe file descriptors; each child process has to close 6 of those - at least! You would be very well advised not to create all the pipes up front as you have done - it is complicated to control things and get all the right descriptors closed.
Looking at the code more closely, the parent does not write messages to the child processes, so you have twice as many pipes as you need - you only need one pipe for each child process to write back to the parent with.
You also do not open() already open file descriptors to the pipes...but how did you get the code to compile? You must be missing the correct header (#include <fcntl.h>) for open() and compiling without enough warning options enabled.
Your variables COOP and SIL are unused in the code presented.
Your writer() function not only mistakenly tries to open an already closed file descriptor, it also closes it, which means that there is no way to send back the extra messages after the first. You should only close the file descriptor once finished - after the loop in the main program for each child. This is why you only see one message.
It is also worth getting into the habit of error-checking the return from every system call that can fail. There are a few that can't fail - getpid() is one such. But I/O operations are notorious for failing for reasons outside the direct control of the program (or, in this case, within the control of the program), so you should check that writes succeed. When you get back an EBADF - bad file descriptor - error, you know something is up.
You have similar problems with close() (and open()) in reader(), plus the additional problem that you attempt to return a pointer to a local automatic variable - which is not a good idea, ever. Again, a decent compiler (like GCC) with warnings enabled will tell you about such things. I used this command to compile your program:
gcc -O -std=c99 -Wall -Wextra -Wmissing-prototypes -Wstrict-prototypes \
pipe.c -o pipe
Your child processes are always going to generate the same sequence of (pseudo-)random numbers, which isn't very exciting. You should probably use something like:
srand(getpid());
to ensure they get different sequences.
Your reader() function is both not enthusiastic enough and too enthusiastic about reading the data. You read a single byte at a time, but you then loop to accumulate single bytes, so the code waits around for all 10 results to be known, and then spits everything out at once. Since a 32-bit integer can store a number up to 1,111,111,111 without problem, you would get just one number back from your call to atoi() on the first iteration, which isn't quite what you wanted.
Reads and writes on pipes are atomic - in the sense that if the writing process writes 6 bytes and the reading process attempts to read more than 6 bytes, then the packet of 6 bytes will be returned by a single read, even if there are other bytes in the pipe waiting to be read; those extra bytes will be returned on subsequent calls to read().
So, your reader() function should be passed in a buffer to use, along with its size; the code should attempt to read that buffer size; it should null terminate what it does receive; it can return the pointer to the buffer it was passed; it should error check the returned value from read().
The code for the two child processes is essentially the same - you should use an appropriately parameterized function rather than writing out the code twice.
Putting it all together, you end up with something like this (which works fine for me on MacOS X 10.6.6 with GCC 4.5.2):
#include <errno.h>
#include <string.h>
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <stdarg.h>
#include <stdlib.h>
const int READ = 0;
const int WRITE = 1;
static char* reader(int fd, char *buffer, size_t bufsiz);
static void writer(int fd, const char *c);
static void child_process(int *my_pipe, int *his_pipe);
static void err_exit(const char *fmt, ...)
{
va_list args;
int errnum = errno;
va_start(args, fmt);
vfprintf(stderr, fmt, args);
va_end(args);
if (errnum != 0)
fprintf(stderr, "%d: %s\n", errnum, strerror(errnum));
exit(1);
}
int main(void)
{
int c1pipe[2];
int c2pipe[2];
int c1sentence = 0;
int c2sentence = 0;
int c;
if (pipe(c1pipe) != 0 || pipe(c2pipe) != 0)
err_exit("Failed to open a pipe\n");
int C2 = 0;
int C1 = fork();
if (C1 > 0)
C2 = fork();
if (C1 < 0 || C2 < 0) //error
err_exit("fork() failed\n");
else if (C1 == 0)
child_process(c1pipe, c2pipe);
else if (C2 == 0)
child_process(c2pipe, c1pipe);
else //parent
{
int choice1; //stores choice of c1
int choice2; //stores choice of c2
char buffer1[BUFSIZ];
char buffer2[BUFSIZ];
close(c1pipe[WRITE]);
close(c2pipe[WRITE]);
for (c = 0; c< 10; c++)
{
choice1 = atoi(reader(c1pipe[READ], buffer1, sizeof(buffer1)));
choice2 = atoi(reader(c2pipe[READ], buffer2, sizeof(buffer1)));
printf("C1's (%d) choice trial %d : %d\n", C1, c+1, choice1);
printf("C2's (%d) choice trial %d : %d\n", C2, c+1, choice2);
if (choice1 && choice2) //c1 and c2 cooperate with police
{
c1sentence = c1sentence + 6;
c2sentence = c2sentence + 6;
}
else if (!(choice1 && choice2)) //both c1 and c2 are silent
{
c1sentence = c1sentence + 1;
c2sentence = c2sentence + 1;
}
else if (choice1) // if c1 cooperates and c2 is silent
{
c1sentence = c1sentence + 0;
c2sentence = c2sentence + 10;
}
else // if c2 cooperates and c1 is silent
{
c1sentence = c1sentence + 10;
c2sentence = c2sentence + 0;
}
}
printf("C1 is in jail for %d years total\n", c1sentence);
printf("C2 is in jail for %d years total\n", c2sentence);
}
return(0);
}
static void writer(int pipe_write_fd, const char *c)
{
int len = strlen(c);
if (write(pipe_write_fd, c, len) != len)
err_exit("Write failed\n");
}
static char* reader(int pipe_read_fd, char *buffer, size_t bufsiz)
{
int i = read(pipe_read_fd, buffer, bufsiz-1);
if (i < 0)
err_exit("Read failed\n");
buffer[i] = '\0';
return buffer;
}
static void child_process(int *my_pipe, int *his_pipe)
{
int c;
srand(getpid());
close(my_pipe[READ]);
close(his_pipe[READ]);
close(his_pipe[WRITE]);
for (c = 0; c < 10; c++)
{
writer(my_pipe[WRITE], ((rand() % 2) == 1) ? "1" : "0");
sleep(1);
}
close(my_pipe[WRITE]);
}
Note how the error routine captures errno early - to avoid damaging it. It is one of the perils of using global variables; they may change when you call a function. Don't use them when you can avoid them (but note that you can't avoid using errno completely, in general).

void writer(int pipe_write_fd, char *c)
{
open(pipe_write_fd);
char* choice = c;
// Write to the pipe
write(pipe_write_fd, choice, strlen(choice));
// Close the pipe
// (Sends 'end of file' to reader)
close(pipe_write_fd);
}
I'm not sure which function open you are trying yo use but the usual one takes a filename and returns a file descriptor. In any case you are discarding the return value so I suppose that this doesn't matter.
What is clear is that you close the pipe immediately after the first write so it is "correct" that the next write will fail; the pipe has been closed.
If you fix this problem then the next problem is that reader will, one byte at a time, all of the available input - up to 1024 bytes - before closing the read pipe. As reader is called in a loop, the read attempt in the second iteration will fail.

Related

Using pipes to communicate between two programs

I need the main prog to get two strings from the user and an argument for the other program, call fork() and then in child process I need to write the strings into pipe and send them to the other program which returns an int which I want to pass to parent so I'm trying to use another pipe for it but every time it stops right after inserting the strings.
So the main program: (EDITED)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <unistd.h>
#include <sys/wait.h>
#define LINELEN (80)
char *mygets(char *buf, int len);
int mygeti();
int main(int argc, char *argv[])
{
char *cmpstr[] = {"lexcmp", "lencmp"};
int veclen = sizeof(cmpstr)/sizeof(char *);
char str1[LINELEN + 1];
char str2[LINELEN + 1];
int index;
int pid[2];
int pfd[4][2];
for(int i = 0; i < 4; i++)
{
if(pipe(pfd[i]) < 0)
{
perror("pipe");
return -2;
}
}
pid[0] = fork();
if(pid[0] == 0) // child a
{
close(pfd[0][1]);
close(pfd[2][0]);
dup2(pfd[0][0], STDIN_FILENO);
dup2(pfd[2][1], STDOUT_FILENO);
char *myargs[3];
myargs[0] = "./loopcmp";
myargs[1] = "lexcmp";
myargs[2] = NULL;
if(execvp(myargs[0], myargs) == -1)
{
perror("exec");
return -2;
}
close(pfd[0][0]);
close(pfd[2][1]);
}
else
{
pid[1] = fork();
if(pid[1] == 0) //child b
{
close(pfd[1][1]);
close(pfd[3][0]);
dup2(pfd[1][0], STDIN_FILENO);
dup2(pfd[3][1], STDOUT_FILENO);
char *myargs[3];
myargs[0] = "./loopcmp";
myargs[1] = "lencmp";
myargs[2] = NULL;
if(execvp(myargs[0], myargs) == -1)
{
perror("exec");
return -2;
}
close(pfd[1][0]);
close(pfd[3][1]);
}
else // parent
{
while (1)
{
printf("Please enter first string:\n");
if (mygets(str1, LINELEN) == NULL)
break;
printf("Please enter second string:\n");
if (mygets(str2, LINELEN) == NULL)
break;
do {
printf("Please choose:\n");
for (int i=0 ; i < veclen ; i++)
printf("%d - %s\n", i, cmpstr[i]);
index = mygeti();
} while ((index < 0) || (index >= veclen));
close(pfd[index][0]);
if(write(pfd[index][1], str1, strlen(str1)) == -1)
{
perror("writeToPipe");
return -2;
}
if(write(pfd[index][1], str2, strlen(str2)) == -1)
{
perror("writeToPipe");
return -2;
}
if(index == 0)
{
close(pfd[2][1]);
char rbuf[1];
while(read(pfd[2][0], &rbuf, 1) > 0)
{
write(STDOUT_FILENO, &rbuf, 1);
}
}
if(index == 1)
{
close(pfd[3][1]);
char rbuf[1];
while(read(pfd[3][0], &rbuf, 1) > 0)
{
write(STDOUT_FILENO, &rbuf, 1);
}
}
}
}
}
return 0;
}
char *mygets(char *buf, int len)
{
char *retval;
retval = fgets(buf, len, stdin);
buf[len] = '\0';
if (buf[strlen(buf) - 1] == 10) /* trim \r */
buf[strlen(buf) - 1] = '\0';
else if (retval)
while (getchar() != '\n'); /* get to eol */
return retval;
}
int mygeti()
{
int ch;
int retval=0;
while(isspace(ch=getchar()));
while(isdigit(ch))
{
retval = retval * 10 + ch - '0';
ch = getchar();
}
while (ch != '\n')
ch = getchar();
return retval;
}
The other program - loopcmp: (Here I shouldn't change anything)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define LINELEN (80)
int lencmp(const char *str1, const char *str2);
int lexcmp(const char *str1, const char *str2);
char *mygets(char *buf, int len);
int main(int argc, char *argv[])
{
int(*cmpfunc)(const char *, const char *) = NULL;
char str1[LINELEN + 1];
char str2[LINELEN + 1];
if (argc != 2)
return -1;
if (!strcmp(argv[1], "lexcmp"))
cmpfunc = lexcmp;
else if (!strcmp(argv[1], "lencmp"))
cmpfunc = lencmp;
else
return -1;
while (1)
{
if (mygets(str1, LINELEN) == NULL)
break;
if (mygets(str2, LINELEN) == NULL)
break;
printf("%d\n", cmpfunc(str1, str2));
fflush(stdout);
}
return 0;
}
int lencmp(const char *str1, const char *str2)
{
int val;
val = strlen(str1) - strlen(str2);
if (val < 0)
return 1;
if (val > 0)
return 2;
return 0;
}
int lexcmp(const char *str1, const char *str2)
{
int val;
val = strcmp(str1, str2);
if (val < 0)
return 1;
if (val > 0)
return 2;
return 0;
}
char *mygets(char *buf, int len)
{
char *retval;
retval = fgets(buf, len, stdin);
buf[len] = '\0';
if (buf[strlen(buf) - 1] == 10) /* trim \r */
buf[strlen(buf) - 1] = '\0';
else if (retval) while (getchar() != '\n'); /* get to eol */
return retval;
}
This is what I get:
Picture
and what I actually need it to print the interger returned from the exec of the child and then start again and get new two strings and so on till the user exits. what am I doing wrong? I can only modify the main program (the first one)
The first thing to do is ensure you are closing all unnecessary file descriptors in each process.
This means anything relating to the lexcmp child process should be closed in the lencmp child process, and vice versa. The parent needs to close the read ends of both "TO" pipes, and the write end of both "FROM" pipes.
Each of these closures should happen exactly once, where appropriate.
As is, in the parent, you are calling close(pfd[index][0]);, close(pfd[2][1]);, and close(pfd[3][1]); in a loop.
After calling dup2, you should immediately close the first argument (the original pipe end). As is, in the the children, you are attempting to close them after execvp is called, which leads into the next issue...
If execvp succeeds, it NEVER returns, as it will completely replace the process image. Anything expected to run after it is really operating in a failure state. So
if(execvp(myargs[0], myargs) == -1)
{
perror("exec");
return -2;
}
could be written as
execvp(myargs[0], myargs)
perror("exec");
return -2;
to the same effect.
Aside: the large if .. else if .. else structure of main is a bit hard to read, and not needed since the body of each if statement results in the child processes being replaced, or exiting on error.
The next issues have to do with deadlocking, which most typically occurs when two intercommunicating processes attempt blocking reads from one another at the same time.
Your child processes expect input in a very specific way: 2 lines at a time, creating a pair of strings. The two write calls, in the form of,
write(pfd[index][1], strX, strlen(strX))
do not write any newlines, thus the children wait forever, never to send any data back, and the parent will wait forever, never receiving any data.
Aside: mygets is severely flawed, in a few ways, including being unable to detect EOF or I/O failures (this function is a SIGSEGV in waiting). One of the more obnoxious failings is that the comment here
if (buf[strlen(buf) - 1] == 10) /* trim \r */
is just plain wrong. ASCII decimal 10 is '\n', the line feed, or newline character. '\r', or carriage return, would be decimal 13. This is why using character constants 'A' instead of integer constants 65 is highly encouraged.
The side effect here, generally speaking, is your strings are stripped of a trailing newline character.
The second deadlock occurs when you go to read the child process' response.
Firstly, this example
char rbuf[1];
while(read(pfd[N][0], &rbuf, 1) > 0)
{
write(STDOUT_FILENO, &rbuf, 1);
}
is malformed. Either remove the & operators, OR change char rbuf[1]; to char rbuf;. Fixing this, and the newline problem from above, will result in the parent process reading data back from the child.
The problem then becomes that a while (read(...) > 0) loop will continuously block execution of the calling process, waiting for more data to be available.
This means another deadlock when the child process has already moved on to trying to read another pair of lines from the parent process.
A simple solution is to attempt a single, reasonably large read in the parent, and rely on the behaviour of fflush(stdout); in the child to flush the pipe to the parent.
Here is a functional -ish example, with minimal changes made. This program still has some problems, such as: the parent process generally has no idea of the status of the child processes, and relying signal propagation (^C) from the terminal to end the process tree gracefully, since loopcmp does not handle EOF (should really discuss this with whoever wrote loopcmp.c / mygets).
Additionally, mygeti is flawed as well, as an invalid input cannot be distinguished from a valid input of 0. It also does not handle EOF, or prevent signed integer overflow.
Some more robust abstraction (functions and structures) around creating child processes would help a lot to clean this up further.
This should help you progress, though.
#define _POSIX_C_SOURCE 200809L
#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/wait.h>
#include <unistd.h>
#define LINELEN (80)
char *mygets(char *buf, int len);
int mygeti();
void close_pipe(int fd[2])
{
close(fd[0]);
close(fd[1]);
}
int main(void)
{
char *cmpstr[] = {"lexcmp", "lencmp"};
int veclen = sizeof(cmpstr)/sizeof(char *);
char str1[LINELEN + 1];
char str2[LINELEN + 1];
int index;
int pid[2];
int pfd[4][2];
/* pfd[0] is TO lexcmp
* pfd[1] is TO lencmp
* pfd[2] is FROM lexcmp
* pfd[3] is FROM lencmp
*/
for(int i = 0; i < 4; i++)
if(pipe(pfd[i]) < 0) {
perror("pipe");
return -2;
}
pid[0] = fork();
if (pid[0] == 0) {
/* child lexcmp */
close_pipe(pfd[1]);
close_pipe(pfd[3]);
close(pfd[0][1]);
close(pfd[2][0]);
dup2(pfd[0][0], STDIN_FILENO);
dup2(pfd[2][1], STDOUT_FILENO);
close(pfd[0][0]);
close(pfd[2][1]);
char *args[] = { "./loopcmp", "lexcmp", NULL };
execvp(*args, args);
perror("exec");
return -2; /* This only returns from the child */
}
pid[1] = fork();
if (pid[1] == 0) {
/* child lencmp */
close_pipe(pfd[0]);
close_pipe(pfd[2]);
close(pfd[1][1]);
close(pfd[3][0]);
dup2(pfd[1][0], STDIN_FILENO);
dup2(pfd[3][1], STDOUT_FILENO);
close(pfd[1][0]);
close(pfd[3][1]);
char *args[] = { "./loopcmp", "lencmp", NULL };
execvp(*args, args);
perror("exec");
return -2; /* This only returns from the child */
}
/* parent */
close(pfd[0][0]);
close(pfd[1][0]);
close(pfd[2][1]);
close(pfd[3][1]);
while (1) {
printf("Please enter first string: ");
if (mygets(str1, LINELEN) == NULL)
break;
printf("Please enter second string: ");
if (mygets(str2, LINELEN) == NULL)
break;
do {
printf("Please choose (");
for (int i=0 ; i < veclen ; i++)
printf(" [%d] %s", i, cmpstr[i]);
printf(" ): ");
index = mygeti();
} while ((index < 0) || (index >= veclen));
if (0 >= dprintf(pfd[index][1], "%s\n%s\n", str1, str2)) {
fprintf(stderr, "Failed to write to child %d\n", index);
perror("dprintf");
return -2;
}
char buf[64];
ssize_t bytes = read(pfd[index + 2][0], buf, sizeof buf - 1);
if (-1 == bytes) {
perror("read from child");
return -2;
}
buf[bytes] = 0;
printf("Result: %s", buf);
}
}
char *mygets(char *buf, int len)
{
char *retval;
retval = fgets(buf, len, stdin);
buf[len] = '\0';
if (buf[strlen(buf) - 1] == 10) /* trim \r */
buf[strlen(buf) - 1] = '\0';
else if (retval)
while (getchar() != '\n'); /* get to eol */
return retval;
}
int mygeti()
{
int ch;
int retval=0;
while(isspace(ch=getchar()));
while(isdigit(ch))
{
retval = retval * 10 + ch - '0';
ch = getchar();
}
while (ch != '\n')
ch = getchar();
return retval;
}
Note the use of dprintf. If not available for whatever reason, just make sure to write a single newline after each string.
Final aside: with the way fgets works, the + 1 to the string buffer sizes are rather meaningless (although they are indirectly required here due to mygets performing its own, poorly designed buf[len] = '\0'). fgets writes at most len - 1 non-null bytes, always leaving room for the null terminating byte, which it places.

Is there a way to make this code concise?

I'm currently learning and practicing c, but the exercise I'm doing wants each functions to have 25 lines limit (without changing { } or using single-line if statements!)
Please help if there's a way to make this even shorter.
void ft_write_file(void)
{
char c;
int fd;
int i;
i = 0;
if ((fd = open("write_exam", O_WRONLY | O_TRUNC | O_CREAT, 00777)) == -1)
{
ft_putstr("map error");
return ;
}
while (read(0, &c, 1))
{
write(fd, &c, 1);
if (c == '\n')
break ;
ft_allocate_g_var(i, c, 0);
i++;
}
int j = 0;
while (j < g_line)
{
while (read(0, &c, 1))
{
write(fd, &c, 1);
if (c == '\n')
break ;
}
j++;
}
close(fd);
}
To start with, don't try to do two things in one function. And try to write your functions with sensible arguments instead of hard-coding their subjects.
For example, your function is really doing two things:
Finding and potentially creating the output file (with a hard-coded name).
Copying the entire contents of one stream (hard-coded to stdin) to another stream.
So you could break that down: (prototypes only)
/* Returns fd or -1 on error */
int open_output(const char* name);
/* Returns number of bytes copied or -1 on error */
ssize_t copy_fd(int fd_dest, int fd_source);
Then your driver could be:
ssize_t copy_stdin_to_file(const char *name)
{
int fd = open_output(name);
if (fd < 0)
{
ft_putstr("Could not open output file");
return -1;
}
ssize_t copied = copy_fd(fd, 0);
if (copied < 0) {
ft_putstr("Could not write data.");
return copied;
}
}
A simple way would be to declare all the variables at the top in one line, for exemple :
char c; int fd; int i; i = 0;
Except from that I dont know, hope it can help a bit a least !

Bad file number in simple main function with pipes

I am still a baby in C world and I was doing some "system" programming in order to do some exercises when I stumbled upon some error which is obvious, but I can't find the problem within my application
This is the code
const int __WRITE_ERROR = 44;
const int __READ_ERROR = 43;
const int READ = 0;
const int WRITE = 1;
const int MAX = 1024;
int main(int argc, char *argv[]) {
int fd[2], n;
char buff[MAX];
if(pipe(&fd[2]) < 0)
exit(__PIPE_ERROR);
printf("Hello, from pipe: write: %d and read: %d\n", fd[WRITE], fd[READ]);
if(write(fd[WRITE], "Hello World\n", 12) != 12) {
printf("Explanation: %i", errno); // <- constantly comes here with errno 9 for some reason.
exit(__WRITE_ERROR);
}
if((n = read(fd[READ], buff, MAX)) != 0)
exit(__READ_ERROR);
write(1, buff, n);
exit(0);
}
Could you help me out, cause I ran out of options, thanks.
There is a problem with:
if (pipe(&fd[2]) < 0)
It should be instead:
if (pipe(fd) < 0)
The former is passing to pipe() the address of one element past of the bounds of the array fd (i.e.: the &fd[2]).
Also, read() and write() return the number of bytes read or written, respectively. However, if an error occurs -1 is returned for both functions.
As someone already pointed out first mistake was
if (pipe(&fd[2]) < 0)
had to be
if (pipe(fd) < 0)
then read was failing because of wrong conition
if((n = read(fd[READ], buff, MAX)) != 0)
should have been
if((n = read(fd[READ], buff, MAX)) <= 0)

Forked process isn't calling function in C

C beginner here. The function send_chars_to_reducers does not appeared to be getting called inside the forked processes created in fork_mappers function.
C Code
#include <sys/wait.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <time.h>
#include <errno.h>
#define BUFFER_SIZE 1024
#define ALPHA_OFFSET 97
#define LETTERS 26
const int NUM_OF_MAPPERS = 4;
const int NUM_OF_REDUCERS = 26;
const int PIPE_READ_END = 0;
const int PIPE_WRITE_END = 1;
const int PIPE_BUFFER_SIZE = 1000;
int mapper_pipes[4][2];
int reducer_pipes[26][2];
void pipe_wrapper(int pipefd[]) {
int ret = pipe(pipefd);
if (ret == -1) {
perror("Error. Failed when trying to create pipes.");
exit(EXIT_FAILURE);
}
}
void create_mapper_pipes(void) {
int i;
for (i = 0; i < NUM_OF_MAPPERS; i++) {
pipe_wrapper(mapper_pipes[i]);
}
}
void create_reducer_pipes(void) {
int i;
for (i=0; i < NUM_OF_REDUCERS; i++) {
pipe_wrapper(reducer_pipes[i]);
}
}
// Prints an error msg and exits if one occurs. Else, returns the system call value.
int print_if_err(int syscall_val, const char* syscall_name) {
if (syscall_val < 0) {
perror(syscall_name);
exit(errno);
} else {
//No syscall error we can return
return syscall_val;
}
}
void send_chars_to_reducers(void) {
printf("hello from send_chars_to_reducers\n");
}
void fork_mappers(void) {
/* Constants useful to all children */
char ibuf[PIPE_BUFFER_SIZE]; // input pipe buffer
int rlen = 0;
int i;
for (i=0; i<NUM_OF_MAPPERS; i++) {
pid_t mapper_pid = print_if_err(fork(), "fork");
close(mapper_pipes[i][PIPE_WRITE_END]);
if (mapper_pid == 0) {
rlen = print_if_err(read(mapper_pipes[i][PIPE_READ_END], ibuf, 1000), "read");
while(rlen > 0) {
send_chars_to_reducers();
printf("read line from forked_mappers, p%d: %s\n", i, ibuf);
rlen = print_if_err(read(mapper_pipes[i][PIPE_READ_END], ibuf, 1000), "read");
}
_exit(0);
}
}
}
void fork_reducers(void) {
printf("hello from fork_reducers\n");
int i;
for (i = 0; i < NUM_OF_REDUCERS; i++) {
pid_t reducer_pid = print_if_err(fork(), "fork");
if (reducer_pid == 0) {
while (1 == 1) {
}
}
}
}
void send_lines_to_mappers(void) {
int wlen = 0;
char obuf[PIPE_BUFFER_SIZE];
int ob_size;
int count = 0;
char buff[BUFFER_SIZE]; // a buffer for each line of the file
FILE *input_file = fopen("input.txt", "r");
// read the input file line by line
while(fgets(buff, BUFFER_SIZE, input_file) > 0) {
printf("read line from send_lin_to_mappers: %s\n", buff);
ob_size = sizeof buff;
switch(count) {
case 0 :
write(mapper_pipes[0][PIPE_WRITE_END], buff, ob_size);
close(mapper_pipes[0][PIPE_WRITE_END]);
break;
case 1 :
write(mapper_pipes[1][PIPE_WRITE_END], buff, ob_size);
close(mapper_pipes[1][PIPE_WRITE_END]);
break;
case 2 :
write(mapper_pipes[2][PIPE_WRITE_END], buff, ob_size);
close(mapper_pipes[2][PIPE_WRITE_END]);
break;
case 3 :
write(mapper_pipes[3][PIPE_WRITE_END], buff, ob_size);
close(mapper_pipes[3][PIPE_WRITE_END]);
break;
default :
printf("you did something wrong in send_lines_to_mappers loop");
}
count++;
}
fclose(input_file);
}
int main(void) {
// Setup the mapper pipes
create_mapper_pipes();
create_reducer_pipes();
fork_reducers();
fork_mappers();
send_lines_to_mappers();
return 0;
}
Output
hello from fork_reducers
read line from send_lin_to_mappers: I like coding in C.
read line from send_lin_to_mappers: I like manually allocating memory, and opening the registers window in Visual Studio to see the values of the eax register and blitting graphics to the screen and all the stuff that Dr. Dobbs wrote about in the 90s.
read line from send_lin_to_mappers: My programming friends seem to believe that understanding this level of programming is good in a hand-wavy, theoretical sense, but when you consider all the web development, Java frameworks, and existing libraries most programmers today rely on, it's hard to really pin down a solid answer to the question "Why learn C?"
read line from send_lin_to_mappers: This is my attempt to answer that question, and I believe it comes down to the basic programming concept of abstraction.
The problem is the number of processes which share handles.
Each reducer has the pipes open
Each mapper has the pipes open, so the pipes are never properly closed ( the reducers are in a busy loop.
if (mapper_pid == 0) {
int j;
for( j = 0; j < NUM_OF_MAPPERS; j++ ){
close( mapper_pipes[j][PIPE_WRITE_END]);
if( j != i ){
close(mapper_pipes[j][PIPE_READ_END]);
}
}
I commented out the reducer fork's and then fixed the mapper forks to close all the pipe handles in the child except the one we want to read from.
This started to make the program work.
From pipe documentation, you should close the unused bits of a pipe. But you have pre-opened all the pipes (probably better localizing them), and have therefore got to close all the unused handles in the sub-processes to have the OS tidy up the resource correctly

reading from a pipe in c

Scenario: Say I have 8 files that I want to sort all the data of numbers it has in order from least to greatest. Only leaf proceses can sort all the numbers that a file contains. These leaf processes must send the sorted data to a parent process via pipes. This parent process will compare the data is receives and send which ever number is smaller to the next process up. It will do this until all number in the pipe is empty.
So think of it as a tree. We have one master process. With 8 files to sort, the Master process will spawn 2 processes off of it (a left and a right). Those two new processes will spawn their own processes. This will happen until there are 8 leaf processes at the bottom. Internal nodes can only hold onto one number. These will pass their number along a series of pipes until they reach the master process. The master process will output its piped contents to a file.
I've included the code here (as it is a bit lengthy but straightforward).
This works if I have 2 files to sort. So we have 1 master process and then two children. The two children sort their file's numbers and then pass them up. The master process then prints out the data in order from the pipes. However if I add some complexity (4 files), the leaf processes still send their data up, however when the master process begins to read the from the internal nodes pipes, it thinks it is empty and finishes the program without any data.
Any idea why the master process is thinking that its left and right pipes are empty?
Like I said, works great when there is one parent and 2 children. Anymore processes and it fails. (assuming that processing will happen in powers of 2).
NOTE: perror is being used for debugging purposes.
full program here [very messy as I have been doing a lot with it but it will compile.
The updated code in Pastebin is not a compilable function - let alone the complete program. That makes it hard to say what's really wrong.
However, one of the problems is in the code fragment:
if (pipe(upPipe) < 0 || pipe(leftPipe) < 0 || pipe(rightPipe) < 0)
...error exit...
if ((leftPID = fork()) < 0)
...error exit...
if(leftPID == 0){
fMax = ((fMax)/2);
dup2(leftPipe[WRITE], upPipe[WRITE]);
pipe(leftPipe);
pipe(rightPipe);
The call to dup2() is odd; you carefully map the write channel of the left pipe to the write channel of the up pipe.
The two pipe() calls after the dup2() fairly promptly screw up everything in the left child, opening 4 more file descriptors but losing the previous values stored in leftPipe and rightPipe.
You need to make your problem statement clearer. I cannot fathom from what you've got what you're supposed to have. There's a call to convertToInt() which takes no arguments and returns no value; what on earth is that doing? There's a call to freeMem(); it is not clear what that does.
z.c:42: error: ‘numberChar’ undeclared (first use in this function)
z.c:42: error: ‘sizeNumbers’ undeclared (first use in this function)
z.c:43: warning: implicit declaration of function ‘readFile’
z.c:43: error: ‘fileNames’ undeclared (first use in this function)
z.c:45: warning: implicit declaration of function ‘convertToInt’
z.c:46: error: ‘i’ undeclared (first use in this function)
z.c:46: error: ‘numbs’ undeclared (first use in this function)
z.c:47: error: ‘numbers’ undeclared (first use in this function)
z.c:48: warning: implicit declaration of function ‘freeMem’
Sorry, your question is unanswerable because you are not giving us:
The accurate requirements.
The code you've actually got compiling.
Your code does not have a good clean break up of the functions. Do you use a VCS (version control system - such as git)? If not, you should. I made the changed version below - which is essentially a complete rewrite - in 9 check-ins, and should probably have made more smaller check-ins than that. But using a VCS was crucial to me; it allowed me to make changes with confidence, knowing I would not lose anything valuable. And I didn't have to comment code out; I removed the stuff I didn't want. The solution below is 261 lines; the original was about 687 lines in total, including a lot of commented out code; when I'd finished stripping out the comments, etc, it came down to 469 lines.
When I got your code running (and reporting on which files were being opened by each child), I found that there were 2 processes opening each of files 2 and 3 (and since the data files didn't exist at the time, they failed at that point).
The revised code has an almost clean structure; the odd bit is the 'convertToString()' phase which reads binary integers off a pipe and converts them to ASCII output again. It works; I'm not convinced it is elegant. Instead of using an array of hard-coded file names, it takes an arbitrary list of file names from the command line (it does not have to be 8; it has been tested with 0 through 8, and I've no reason to think it won't handle 20 or more). I did a fair amount of testing with:
./piped-merge-sort [1-8]
There is copious diagnostic output. I've used two functions that I find vastly helpful in my work - I have them packaged up with some other related code in a more complex package, but the simple versions of err_error() and err_remark() functions really help me. Note that these versions report the PID of the reporting process for each call. They're also careful to pre-format the message into a string and then write the string in one print to standard error; otherwise, I was getting a lot of interleaved output which was confusing at best.
'Nuff said - here's the code:
#include <assert.h>
#include <errno.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/wait.h>
#include <unistd.h>
/* Error reporting */
static void err_vremark(char *fmt, va_list args)
{
char buffer[256];
int errnum = errno;
int buflen = snprintf(buffer, sizeof(buffer), "%d: ", (int)getpid());
buflen += vsnprintf(buffer + buflen, sizeof(buffer) - buflen, fmt, args);
if (errnum != 0)
buflen += snprintf(buffer + buflen, sizeof(buffer) - buflen,
": errno = %d (%s)", errnum, strerror(errnum));
fprintf(stderr, "%s\n", buffer);
}
static void err_error(char *fmt, ...)
{
va_list args;
va_start(args, fmt);
err_vremark(fmt, args);
va_end(args);
exit(1);
}
static void err_remark(char *fmt, ...)
{
va_list args;
va_start(args, fmt);
err_vremark(fmt, args);
va_end(args);
}
enum { READ = 0 };
enum { WRITE = 1 };
enum { BUFFER_SIZE = 256 };
static int *a_data = 0;
static int a_used = 0;
static int a_size = 0;
void readFile(char const *fileName);
void freeMem(void);
void sortArray(void);
int intcmp(void const *n1, void const *n2);
static void sortMergeFiles(int fd, int number, char **names);
static void sortOneFile(int fd, const char *file);
static void convertToString(int fd, FILE *fp);
int main(int argc, char **argv)
{
int m_pipe[2];
pid_t pid;
if (pipe(m_pipe) < 0)
err_error("Failed to create master pipe");
if ((pid = fork()) < 0)
err_error("Failed to fork master");
else if (pid == 0)
{
close(m_pipe[READ]);
sortMergeFiles(m_pipe[WRITE], argc - 1, &argv[1]);
close(m_pipe[WRITE]);
}
else
{
close(m_pipe[WRITE]);
convertToString(m_pipe[READ], stdout);
close(m_pipe[READ]);
}
return 0;
}
static void convertToString(int fd, FILE *fp)
{
int value;
while (read(fd, &value, sizeof(int)) == sizeof(int))
fprintf(fp, "%d\n", value);
}
static int readInteger(int fd, int *value)
{
if (read(fd, value, sizeof(int)) != sizeof(int))
return EOF;
return 0;
}
static void writeInteger(int fd, int value)
{
if (write(fd, &value, sizeof(int)) != sizeof(int))
err_error("Failed to write integer to fd %d", fd);
}
static void mergeFiles(int fd_in1, int fd_in2, int fd_out)
{
int value_1;
int value_2;
int eof_1 = readInteger(fd_in1, &value_1);
int eof_2 = readInteger(fd_in2, &value_2);
while (eof_1 != EOF && eof_2 != EOF)
{
err_remark("v1: %d; v2: %d", value_1, value_2);
if (value_1 <= value_2)
{
writeInteger(fd_out, value_1);
eof_1 = readInteger(fd_in1, &value_1);
}
else
{
writeInteger(fd_out, value_2);
eof_2 = readInteger(fd_in2, &value_2);
}
}
while (eof_1 != EOF)
{
err_remark("v1: %d", value_1);
writeInteger(fd_out, value_1);
eof_1 = readInteger(fd_in1, &value_1);
}
while (eof_2 != EOF)
{
err_remark("v2: %d", value_2);
writeInteger(fd_out, value_2);
eof_2 = readInteger(fd_in2, &value_2);
}
}
static void sortMergeFiles(int fd, int number, char **names)
{
assert(number >= 0);
if (number == 0)
return;
else if (number == 1)
sortOneFile(fd, names[0]);
else
{
err_remark("Non-Leaf: processing %d files (%s .. %s)", number, names[0], names[number-1]);
int mid = number / 2;
int l_pipe[2];
int r_pipe[2];
pid_t l_pid;
pid_t r_pid;
if (pipe(l_pipe) < 0 || pipe(r_pipe) < 0)
err_error("Failed to create pipes");
if ((l_pid = fork()) < 0)
err_error("Failed to fork left child");
else if (l_pid == 0)
{
close(l_pipe[READ]);
close(r_pipe[READ]);
close(r_pipe[WRITE]);
sortMergeFiles(l_pipe[WRITE], mid, names);
close(l_pipe[WRITE]);
exit(0);
}
else if ((r_pid = fork()) < 0)
err_error("Failed to fork right child");
else if (r_pid == 0)
{
close(r_pipe[READ]);
close(l_pipe[READ]);
close(l_pipe[WRITE]);
sortMergeFiles(r_pipe[WRITE], number - mid, names + mid);
close(r_pipe[WRITE]);
exit(0);
}
else
{
close(l_pipe[WRITE]);
close(r_pipe[WRITE]);
mergeFiles(l_pipe[READ], r_pipe[READ], fd);
close(l_pipe[READ]);
close(r_pipe[READ]);
err_remark("Non-Leaf: finished %d files (%s .. %s)", number, names[0], names[number-1]);
}
}
}
static void addNumberToArray(int number)
{
assert(a_used >= 0 && a_used <= a_size);
if (a_used == a_size)
{
int n_size = (a_size + 1) * 2;
int *n_data = realloc(a_data, sizeof(*n_data) * n_size);
if (n_data == 0)
err_error("Failed to allocate space for %d numbers", n_size);
a_data = n_data;
a_size = n_size;
}
a_data[a_used++] = number;
}
/* Could be compressed to write(fd, a_data, a_used * sizeof(int)); */
/* Arguably should check for write errors - but not SIGPIPE */
static void writeArray(int fd)
{
for (int i = 0; i < a_used; i++)
{
err_remark("Write: %d", a_data[i]);
write(fd, &a_data[i], sizeof(int));
}
}
void readFile(char const *fileName)
{
char buffer[BUFFER_SIZE];
FILE *fp;
fp = fopen(fileName, "r");
if (fp == NULL)
err_error("Failed to open file %s for reading", fileName);
while (fgets(buffer, sizeof(buffer), fp) != NULL)
{
char *nl = strchr(buffer, '\n');
if (nl != 0)
*nl = '\0';
err_remark("Line: %s", buffer);
addNumberToArray(atoi(buffer));
}
fclose(fp);
}
int intcmp(const void *n1, const void *n2)
{
const int num1 = *(const int *) n1;
const int num2 = *(const int *) n2;
return (num1 < num2) ? -1 : (num1 > num2);
}
void sortArray(void)
{
qsort(a_data, a_used, sizeof(int), intcmp);
}
void freeMem(void)
{
free(a_data);
}
static void sortOneFile(int fd, const char *file)
{
err_remark("Leaf: processing file %s", file);
readFile(file);
sortArray();
writeArray(fd);
freeMem();
err_remark("Leaf: finished file %s", file);
}

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