forking processes, reading a file but not showing its content - c

Question

I have the following code where I have to enter 2 arguments as an input where: The 1st argument is a number. If this number is multiple of 2, then 15 child processes will be created with fork. If the number is multiple of 3, 10 processes will be created. If the number is multiple of 5, 7 processes will be created. The 2nd argument is a file where its size (in bytes) will be divided into the number of processes created and each child process is going to read one part of the file, they save that part into a variable and finally the parent shows all the text in that variable. For example, I run the program with ./p 5 /home/directoryFile.c. So I'm having 7 child processes and let's say the filesize is 700 bytes. That means every child process should read 100 bytes, they save it into a variable (appending the content) and finally the parent shows all the content together. The problem is that the variable textToSend that should show all the content in the parent doesn't show anything... I believe there should be a problem with the sprintf line in the child.
//gcc Test.c -o p
//./p 5 /home/directoryFile.c
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <dirent.h>
#include <sys/wait.h>
int main(int argc, char *argv[])
{
int studentId, children = 0, j, i, childNumber[15], fdFile, fdread;
float bytesToRead;
char directory[50];
char *buffer = malloc(256), *textToSend = malloc(256);
system("clear");
if(argc-1 < 1)
{
printf("\nSome arguments are missing\n");
return EXIT_FAILURE;
}
studentId = atoi(argv[1]);
strcpy(directory,argv[2]);
if((studentId%2) == 0)
{
children = 15;
}
else
{
if((studentId%3) == 0)
{
children = 10;
}
else
{
if((studentId%5) == 0)
{
children = 7;
}
else
{
printf("\nStudentId is not multiple of 2, 3 o 5\n");
return EXIT_FAILURE;
}
}
}
struct stat fileInfo;
stat(argv[2],&fileInfo);
bytesToRead = fileInfo.st_size / children;
printf("children: %d\n",children);
printf("File Size: %lld\n",(long long int) fileInfo.st_size);
printf("Bytes: %.2f\n",bytesToRead);
fdFile = open(directory,O_RDONLY);
if(fdFile == -1)
{
printf("\nError opening the fileo\n");
return EXIT_FAILURE;
}
for(i=0;i<children;i++)
{
childNumber[i] = fork();
if(childNumber[i] == -1)
{
printf("\nError creating the child process\n");
return EXIT_FAILURE;
}
if(childNumber[i] == 0)
{
fdread = read(fdFile,buffer,bytesToRead);
if(fdread == -1)
{
printf("\nError reading the file\n");
return EXIT_FAILURE;
}
printf("%s",buffer);
//printf("\n\n------------------------\n\n");
sprintf(textToSend,"%s%s",textToSend,buffer);
return EXIT_SUCCESS;
}
else
{
//waitpid(childNumber[i],NULL,WNOHANG);
}
}
printf("\nThis is the content of the file: %s\n",textToSend);
close(fdFile);
for(j=0;j<children;j++)
{
wait(NULL);
}
return EXIT_SUCCESS;
}
What could happen? The buffer is showing a part of the file correctly...
Could it be the sprintf function?

Answer 1

You are using sprintf to print to a local buffer which is not shared among your processes.
When you fork your child receives a full private copy of all of the parents memory which is private. All memory is private by default unless specifically allocated as shared.
If you allocate it as shared
char * textToSend = mmap(NULL, 256, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_SHARED, -1, 0);
Then you will have the same memory accessible from all processes.
This will not account for the fact that you have to tell all other processes that the start of the buffer has moved, so that would require an extra shared variable and synchronised access. At the moment you are not appending text with sprintf, you are overwriting any previous content.
In the end if you wish to pool together results from different processes you may want to use pipes (pipe(2)) rather then shared memory.