Multiple processes with pipe, unexpected variable change? - c

Question

I know it sounds unreal "unexpected variable change." Since I couldn't spot the problem I used that title...
Firstly, this is my homework. The homework is about reading two matrices from file then create 4 child processes, parent will send (via pipe) the quarter pieces of matrix to child processes. Child process will make their calculations afterwards they will send the calculated indices back to parent process(via pipe again. Trying to achieve bidirectional pipes here.) so parent will print calculated values to stdout and return.
Currently what I have done is read those matrices from file, created 3 processes(was going step by step), send 1/4 of matrices to each 3 processes and tried to check if i succeeded or not.
The problem is "generally" what happens is either I see third_start variable is changed (in the same scope) therefore it doesn't even enter the loop.
Also I observed couple more things;
When redirecting the output to some file the data is looks like %10 of my program written there.
If I supply the matrix with smaller values like 4x4 matrices. The problem with third_start doesn't happen.
I'm sure that I'm doing something stupid.
Here is related part of my code. (Bit messy, it's not the way I send my homeworks.. Please ignore the calculations about matrices.)
int pipe_fds[2]; //first pipe
int pipe_fds2[2];//second pipe
int pipe_fds3[2];//third pipe
int pipe_fds4[2];//fourth pipe
int pip_ret, pip_ret2, pip_ret3, pip_ret4;
int pid2,pid3,pid4,pid5; //assumed p1 is parent and 2,3,4,5 are child processes.
int single_x; //store single_x val;
int single_y;
pip_ret = pipe(pipe_fds);
pid2 = fork();
if(pip_ret == -1)
{
fprintf(stderr, "%s\n", "Unable to create pipe\n");
exit(1);
}
else if(pid2 == 0)
{
if (close(pipe_fds[1]) == -1)
{
perror("close");
exit(EXIT_FAILURE);
}
int first_quarter_x[_pow(2, n) / 2][_pow(2, n) / 2];
int first_quarter_y[_pow(2, n) / 2][_pow(2, n) / 2];
for(i = 0 ; i < _pow(2, n) / 2 ; ++i)
{
for(j = 0 ; j < _pow(2, n) / 2; ++j)
{
if(read(pipe_fds[0], &single_x, sizeof(single_x)) <= 0)
{
perror("read failed ");
exit(EXIT_FAILURE);
}
first_quarter_x[i][j] = single_x;
if(read(pipe_fds[0], &single_y, sizeof(single_y)) <= 0)
{
perror("read failed ");
exit(EXIT_FAILURE);
}
first_quarter_y[i][j] = single_y;
fprintf(stderr,"(2)Child with pid %d received value %d\n",getpid(), single_x);
fprintf(stderr,"(2)Child with pid %d received value %d\n",getpid(), single_y);
}
}
for(i = 0 ; i < _pow(2, n) / 2 ; ++i)
{
for(j = 0 ; j < _pow(2, n) / 2; ++j)
{
if(matrixA[i][j] == first_quarter_x[i][j])
{
fprintf(stderr,"Good2a\n" );
}
else
{
fprintf(stderr,"Bad2a.\n" );
}
}
fprintf(stderr,"\n" );
}
for(i = 0 ; i < _pow(2, n) / 2 ; ++i)
{
for(j = 0 ; j < _pow(2, n) / 2; ++j)
{
if(matrixB[i][j] == first_quarter_y[i][j])
{
printf("Good2b\n" );
}
else
{
printf("Bad2b.\n" );
}
}
fprintf(stderr,"\n" );
}
exit(EXIT_SUCCESS);
}
pip_ret2 = pipe(pipe_fds2);
pid3 = fork();
if(pip_ret2 == -1)
{
fprintf(stderr, "%s\n", "Unable to create pipe for 3.process\n");
exit(1);
}
else if(pid3 == 0)
{
if (close(pipe_fds2[1]) == -1)
{
perror("close");
exit(EXIT_FAILURE);
}
int second_start = _pow(2, n) / 2;
int second_quarter_x[_pow(2, n) / 2][_pow(2, n) / 2];
int second_quarter_y[_pow(2, n) / 2][_pow(2, n) / 2];
for(i = 0 ; i < _pow(2, n) / 2 ; ++i)
{
for(j = second_start ; j < _pow(2, n); ++j)
{
if(read(pipe_fds2[0], &single_x, sizeof(single_x)) <= 0)
{
perror("read failed ");
exit(EXIT_FAILURE);
}
second_quarter_x[i][j] = single_x;
if(read(pipe_fds2[0], &single_y, sizeof(single_y)) <= 0)
{
perror("read failed ");
exit(EXIT_FAILURE);
}
second_quarter_y[i][j] = single_y;
fprintf(stderr,"(3)Child with pid %d received value %d\n",getpid(), single_x);
fprintf(stderr,"(3)Child with pid %d received value %d\n",getpid(), single_y);
}
}
for(i = 0 ; i < _pow(2, n) / 2 ; ++i)
{
for(j = second_start ; j < _pow(2, n) ; ++j)
{
if(second_quarter_x[i][j] == matrixA[i][j])
{
fprintf(stderr,"Good3a \n" );
}
else
{
fprintf(stderr,"Bad3a\n" );
}
}
fprintf(stderr,"\n" );
}
for(i = 0 ; i < _pow(2, n) / 2 ; ++i)
{
for(j = second_start ; j < _pow(2, n) ; ++j)
{
if(second_quarter_y[i][j] == matrixB[i][j])
{
fprintf(stderr,"Good3b \n" );
}
else
{
fprintf(stderr,"Bad3b\n" );
}
}
fprintf(stderr,"\n" );
}
exit(EXIT_SUCCESS);
}
pip_ret3 = pipe(pipe_fds3);
pid4 = fork();
if(pip_ret3 == -1)
{
fprintf(stderr, "%s\n", "Unable to create pipe for 4.process\n");
exit(1);
}
else if(pid4 == 0)
{
if (close(pipe_fds3[1]) == -1)
{
perror("close");
exit(EXIT_FAILURE);
}
int third_start = _pow(2, n) / 2;
printf("THIRD START IS %d\n",third_start ); //here it prints normal.
int third_quarter_x[_pow(2, n) / 2][_pow(2, n) / 2];
int third_quarter_y[_pow(2, n) / 2][_pow(2, n) / 2];
for(i = third_start ; i < _pow(2, n) ; ++i)
{
for(j = 0 ; j < _pow(2, n) / 2; ++j)
{
if(read(pipe_fds3[0], &single_x, sizeof(single_x)) <= 0)
{
perror("read failed ");
exit(EXIT_FAILURE);
}
third_quarter_x[i][j] = single_x;
if(read(pipe_fds3[0], &single_y, sizeof(single_y)) <= 0)
{
perror("read failed ");
exit(EXIT_FAILURE);
}
third_quarter_y[i][j] = single_y;
fprintf(stderr,"(4)Child with pid %d received value %d\n",getpid(), single_x);
fprintf(stderr,"(4)Child with pid %d received value %d\n",getpid(), single_y);
}
}
printf("THIRD START IS %d\n",third_start ); //then it prints something anormal...
for(i = third_start ; i < _pow(2, n) ; ++i)
{
for(j = 0 ; j < _pow(2, n) / 2; ++j)
{
if(third_quarter_x[i][j] == matrixA[i][j])
{
fprintf(stderr,"Good4a \n" );
}
else
{
fprintf(stderr,"Bad4a\n" );
}
}
printf("\n" );
}
for(i = third_start ; i < _pow(2, n) ; ++i)
{
for(j = 0 ; j < _pow(2, n) / 2; ++j)
{
if(third_quarter_x[i][j] == matrixB[i][j])
{
fprintf(stderr,"Good4b \n" );
}
else
{
fprintf(stderr,"Bad4b\n" );
}
}
fprintf(stderr,"\n" );
}
exit(EXIT_SUCCESS);
}
//parent start
if (close(pipe_fds[0]) == -1)
{
perror("close");
exit(EXIT_FAILURE);
}
fprintf(stderr,"First quarter beginning \n");
for(i = 0 ; i < _pow(2, n) / 2 ; ++i)
{
for(j = 0 ; j < _pow(2, n) / 2; ++j)
{
single_x = matrixA[i][j];
single_y = matrixB[i][j];
write(pipe_fds[1], &single_x, sizeof(single_x));
write(pipe_fds[1], &single_y, sizeof(single_y));
fprintf(stderr,"Parent with pid %d sent value to 2 %d\n",getpid(), single_x);
fprintf(stderr,"Parent with pid %d sent value to 2 %d\n",getpid(), single_y);
}
}
wait(NULL);
fprintf(stderr,"First quarter end\n \n");
fprintf(stderr,"Second quarter beginning\n");
if (close(pipe_fds2[0]) == -1)
{
perror("close");
exit(EXIT_FAILURE);
}
int sec = _pow(2,n) / 2;
for(i = 0 ; i < _pow(2, n) / 2 ; ++i)
{
for(j = sec ; j < _pow(2, n); ++j)
{
single_x = matrixA[i][j];
single_y = matrixB[i][j];
write(pipe_fds2[1], &single_x, sizeof(single_x));
write(pipe_fds2[1], &single_y, sizeof(single_y));
fprintf(stderr,"Parent with pid %d sent value to 3 %d\n",getpid(), single_x);
fprintf(stderr,"Parent with pid %d sent value to 3 %d\n",getpid(), single_y);
}
}
wait(NULL);
fprintf(stderr,"Second quarter end\n");
fprintf(stderr,"Third quarter beginning\n");
if (close(pipe_fds3[0]) == -1)
{
perror("close");
exit(EXIT_FAILURE);
}
int third = _pow(2,n) / 2;
for(i = third ; i < _pow(2, n) ; ++i)
{
for(j = 0 ; j < _pow(2, n) / 2; ++j)
{
single_x = matrixA[i][j];
single_y = matrixB[i][j];
write(pipe_fds3[1], &single_x, sizeof(single_x));
write(pipe_fds3[1], &single_y, sizeof(single_y));
fprintf(stderr,"Parent with pid %d sent value to 4 %d\n",getpid(), single_x);
fprintf(stderr,"Parent with pid %d sent value to 4 %d\n",getpid(), single_y);
}
}
wait(NULL);
fprintf(stderr,"Third quarter end\n");

Answer 1

int pipe_fds2[2];//second pipe
Indirect help: numbered variables are almost always better represented as arrays. You probably want an array of structures
struct child_t {
int datapipe[2], resultpipe[2];
pid_t pid;
int result;
} children[4];
Now you can iterate over your array, and for each element call pipe and fork. Each child deals only with its pipe, but the parent can iterate over all children. In a matrix, I imagine it matters which part of the matrix produces which result, and this structure would keep those pieces of information related.
By organizing your data that way, you eliminate the temptation to duplicate code, because a loop on an array is easier. Less code means fewer bugs, so you get your homework done sooner.

Answer 2

You're accessing outside the arrays.
int third_start = _pow(2, n) / 2;
printf("THIRD START IS %d\n",third_start ); //here it prints normal.
int third_quarter_x[_pow(2, n) / 2][_pow(2, n) / 2];
int third_quarter_y[_pow(2, n) / 2][_pow(2, n) / 2];
The indexes of both dimensions of third_quarter_x and third_quarter_y go from 0 to third_start-1. But then you have the following loops:
for(i = third_start ; i < _pow(2, n) ; ++i)
{
for(j = 0 ; j < _pow(2, n) / 2; ++j)
{
The values of i are outside the range of indexes for the first dimension. Since you're writing outside the array, you're causing undefined behavior.
The loops should both go from 0 to third_start.
You have a similar problem in the previous process with second_start, except it's doing it in the j loop.