Segmentation error that is still unresolved in C(Linux) - c

Question

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <libio.h>
#include <sys/wait.h>
#include <sys/types.h>
#include <sys/shm.h>
#include <ctype.h>
int main(int argc, char** argv) {
int shmID;
char* shmptr, *array, *filearray;
FILE* infile = fopen(argv[1], "r");
if (!infile) {
printf("No file exists\n");
}
int length = 0;
char ch;
ch = getc(infile);
while ((ch = getc(infile)) != EOF) {
filearray[length] = ch;
length++;
}
length++;
fclose(infile);
shmID = shmget(IPC_PRIVATE, length, IPC_CREAT | 0666);
if (shmID < 0) {
printf("There is an error while creating memory \n");
}
int pid = fork();
if (pid > 0) {
shmptr = shmat(shmID, NULL, 0);
if (shmptr == (char * ) - 1) {
printf("There is an error while attaching memory \n");
}
int j = 0;
while ( * array != '!') {
if (array[j] >= 'A' && array[j] <= 'Z') {
array[j] = tolower(array[j]);
} else if (array[j] >= 'a' && array[j] <= 'z') {
array[j] = toupper(array[j]);
} else if (array[j] >= '0' && array[j] <= '9') {
j--;
}
j++;
}
* shmptr = '#';
shmdt(shmptr);
shmctl(shmID, IPC_RMID, NULL);
} else if (pid == 0) {
shmptr = shmat(shmID, NULL, 0);
if (shmptr == (char * ) - 1) {
printf("There is an error while attaching memory \n");
}
array = shmptr;
int x;
for (x = 0; x <= length; x++) {
* array = filearray[x];
array++;
}
* array = '!';
while ( * shmptr != '#') {
sleep(1);
}
int k = 0;
//here we restore the values back into file
while (*array != '!') {
printf("%c", array[k]);
k++;
}
shmdt(shmptr);
} else if (pid < 0) {
printf("Error\n");
}
return 0;
}
This is the code. What I intend to do is take the data from the file and input it into an array. We do this so that we have a temporary data point to store into. I then apply the appropriate checks to see if there is an error while creating the shared memory or the fork command.
After this we enter the child where we intend to
Attach memory
Check error for 1.
Get the array(tried with another char array poiinter but still ran into problems) pointed at shmptr which should initially hold NULL at first but with a length of L(number of characters in file)
Copy the values from the file array into the array(acts as a moving head similar to in a link list) and then attach as a final block a ! to tell the parent the array is over.
Use a # as a char to be added to array so we can know waiting period is over.
In he parent:
Attach memory
Get array
Upper,lower case and check if value is an integer(remove integer by j-- goes back 1 location, then j++ moves back to same location)
attach a # at the end.
As child was rerun it saw this # and should print the array
Hope I was clear. Thank you for the help.

Answer 1

Several problems with your code:
You check if the file open worked, but then you proceed still with the NULL pointer in such a case.
It should be:
FILE *infile = fopen(argv[1], "r");
if(!infile)
{
printf("No file exists\n");
return 1;
}
Pointers are not initialized but you already access them, which invokes undefined behavior.
You need to initialize these pointers before you access them.
char *shmptr;
char *array;
char *filearray;
You can determine the filesize like this:
fseek(infile, 0L, SEEK_END);
length = ftell(infile);
fseek(infile, 0L, SEEK_SET);
Now you can allocate the memory for the file and read it in one go, instead of that loop:
filearray = malloc(length);
if (!filearray)
{
// error
return 1;
}
if(fread(filearray, length, 1, infile) != 1)
{
// error
return 1;
}
As said above you access uninitialized pointers you do this also for array but fro your example it is not clear where this comes from, so you have to allocate memory for it, and initialize it, before you read it.