Multiplication of square matrix with fork (); in C - c

Question

I made in C a program in Linux of multiplication of matrices of order 1000 A and B. And now I have to add processes!
And now I have to add 4 processes in the multiplication that will result in array C.
1 process for calculating from 0 to 249;
2 process for calculating from 250 to 499;
3 process for calculating from 500 to 749;
4 process for calculating from 750 to 999;
Multiplication is working correctly;
I do not understand much of processes, and the problem lies in the parts of the processes, I can not do what I need;
Follow the code below:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <time.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/shm.h>
int id;
int main(){
int i;
int j;
int row;
int col;
int order;
long T1;
long T2;
float delta;
int process_1;
int process_2;
int process_3;
int process_4;
printf("Enter the order of the square matrices A and B: ");
scanf("%d", &order);
T1 = clock();
printf("\nThe square matrices A and B, are order matrices %d",order);
order = order - 1;
row = order;
col = order;
float A[row+1][col+1];
float B[row+1][col+1];
for(i = 0; i <= row; i++){
for(j = 0; j <= col; j++){
printf("\n\nEnter the value of the array A[%d][%d]: ",i+1,j+1);
scanf("%f", &A[i][j]);
printf("\nEnter the value of the array B[%d][%d]: ",i+1,j+1);
scanf("%f", &B[i][j]);
}
}
printf("\nThe multiplication of matrices A and B:\n\n");
id = shmget(IPC_PRIVATE, 100, 0600);
process_1 = fork();
process_2 = fork();
process_3 = fork();
process_4 = fork();
int *a;
a = shmat(id,0,0);
printf("\n\nprocess 1:\n\n");
if(process_1 == 0){
int P1 = 0;
if(P1 <= 249){
for(i = 0; i <= row; i++) {
for(j = 0; j <= col; j++) {
float C[row+1][col+1];
for(int AUX = 0; AUX <= order; AUX++) {
C[i][j] += A[i][AUX] * B[AUX][j];
}
printf("%.2f ",C[i][j]);
}
printf("\n");
}
}
exit(0);
}
printf("\n\nprocess 2:\n\n");
if(process_2 == 0){
int P2 = 250;
if(P2 >=250 && P2 <= 499){
for(i = 0; i <= row; i++) {
for(j = 0; j <= col; j++) {
float C[row+1][col+1];
for(int AUX = 0; AUX <= order; AUX++) {
C[i][j] += A[i][AUX] * B[AUX][j];
}
printf("%.2f ",C[i][j]);
}
printf("\n");
}
}
exit(0);
}
printf("\n\nprocess 3:\n\n");
if(process_3 == 0){
int P3 = 0;
if(P3 >=500 && P3 <= 749){
for(i = 0; i <= row; i++) {
for(j = 0; j <= col; j++) {
float C[row+1][col+1];
for(int AUX = 0; AUX <= order; AUX++) {
C[i][j] += A[i][AUX] * B[AUX][j];
}
printf("%.2f ",C[i][j]);
}
printf("\n");
}
}
exit(0);
}
printf("\n\nprocess 4:\n\n");
if(process_4 == 0){
int P4 = 0;
if(P4 >=750 && P4 <= 999){
for(i = 0; i <= row; i++) {
for(j = 0; j <= col; j++) {
float C[row+1][col+1];
for(int AUX = 0; AUX <= order; AUX++) {
C[i][j] += A[i][AUX] * B[AUX][j];
}
printf("%.2f ",C[i][j]);
}
printf("\n");
}
}
exit(0);
}
waitpid(process_1, NULL, 0);
waitpid(process_2, NULL, 0);
waitpid(process_3, NULL, 0);
waitpid(process_4, NULL, 0);
T2 = clock();
delta = (float)(T2-T1)/CLOCKS_PER_SEC;
printf("\n\Time %.5f seconds\n\n",delta);
return 0;
}
How can I fix this?

Answer 1

I would start from changing type int to pid_t for process identifiers, e.g.:
pid_t process_1;
and then would change:
process_1 = fork();
process_2 = fork();
process_3 = fork();
process_4 = fork();
to
pid_t pid = fork();
if (pid) {
process_1 = pid;
pid = fork();
}
if (pid) {
process_2 = pid;
pid = fork();
}
if (pid) {
process_3 = pid;
pid = fork();
}
if (pid)
process_4 = pid;
the idea is that we do fork() only in the parent process and skip forking in a child process. Otherwise your code forks processes as a tree, each child and parent processes call the next fork() after the previous, then their children do the same and so on four times.
The code above does not check if fork() returns the error code (-1). In the ideal world it is strongly recommended.
From the fork(2) man page:
Return Value
On success, the PID of the child process is returned in the parent,
and 0 is returned in the child. On failure, -1 is returned in the
parent, no child process is created, and errno is set appropriately.