I want to define a 2D array of very big size. But it is giving me segmentation fault?
#include <stdio.h>
int main () {
int i;
int temp[4000][5000];
for (i = 0; i < 5; i++)
{
printf ("Hello World\n");
}
}
Can anyone please suggest me some other way? Is there some problem with memory initialization? Thanks in advance
You can allocate the whole table in only one array but you won't be able to access array data with indices using two square brackets:
int * temp = malloc(4000*5000*sizeof(int));
to access the element (i,j) where previously you wrote temp[i][j], now you should now compute the index the following way:
temp[i*5000+j];
and do not forget to free the memory allocated for your table afterward:
free(temp);
int temp[4000][5000];
That's a VERY BIG array, way bigger than the normal size of stack, you get a segmentation fault because of stack overflow. Consider using dynamic allocation instead.
You need to use dynamic allocated arrays for such big arrays.
Try:
int* temp[4000];
for(i = 0; i < 4000; ++i) temp[i] = malloc(5000 * sizeof(int));
...
for(i = 0; i < 4000; ++i) free(temp[i]).
Whole program with error checking:
int main () {
int i, j;
int* temp[4000];
for (i = 0; i < 4000; ++i)
{
temp[i] = malloc(5000 * sizeof(int));
if (temp[i] == NULL)
{
for (j = 0; j < i; ++j) free(temp[i]);
exit(1);
}
}
for (i = 0; i < 5; i++)
{
printf ("Hello World\n");
}
for (i = 0; i < 4000; ++i) free(temp[i]);
}
Here you can find function which would use single malloc call to allocate two dimension array.
And simpler version of my own:
int main () {
int i, j;
int* temp[4000];
int* array = malloc(4000 * 5000 * sizeof(int));
if (malloc_tmp == NULL) exit(1);
for (i = 0; i < 4000; ++i)
{
temp[i] = array + (i * 5000);
}
for (i = 0; i < 5; i++)
{
printf ("Hello World\n");
}
free(temp[0]);
}
Related
I am new to threads and I have a program that uses threads to find the minimum number out of a 2d array and later on, it finds the distance that the other elements of the array have from the minimum number and stores them in another array.
The user should enter the size of the array and the number of threads he wants to use.
I tried the program below for 1d array and it worked just fine. When I converted it to work for a 2d array it started crashing and throwing a segmentation fault. I, however, cannot find which part of the 2d declaration is wrong.
Any help is really appreciated.
Here is my code:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>
#include <string.h>
#include <pthread.h>
struct Parameters
{
// input
int s,p; //n is size of array, p is number of threads
int** array; //array with elements
int start;
int end;
// output
int smallest;
int pos; //position if minimum
int** B; //array that holds the distances
};
void* min(void* args)
{
struct Parameters* p = (struct Parameters*)args;
int **array = p->array;
int **B1 = p->B;
int start = p->start;
int end = p->end;
int smallest = array[start][start];
int pos = p->pos;
int distance;
//find the smallest
for (int i = start; i < end; i++)
{
for(int j = start; j < end; j++)
{
if (array[i][j] < smallest)
{
smallest = array[i][j];
pos = i;
}
}
}
//find the distances
for(int i = 0; i < ((struct Parameters*)args) -> s; i++)
{
for(int j = 0; j < ((struct Parameters*)args) -> s; j++)
{
distance = abs(pos - i);
B1[i][j] = distance;
}
}
params->smallest = smallest;
params->B = B1;
return NULL;
}
int main()
{
int smallest,pos;
int s,p;
struct Parameters *ptr = (struct Parameters *)malloc(sizeof(struct Parameters));
if(ptr == NULL)
{
printf("Not enough. Try again \n");
exit(0);
}
printf("Type s\n");
scanf("%d",&(ptr->s));
printf("Type p\n");
scanf("%d", &(ptr->p));
// declare an array of threads and associated parameter instances
pthread_t threads[(ptr->p)];
struct Parameters thread_parameters[(ptr->p)] ;
int arr[ptr->s][ptr->s];
int B2[ptr->s][ptr->s];
// intialize the array
for(int i=0; i< ptr->s; i++)
{
for(int j=0; j< ptr->s; j++)
{
printf("Type a \n");
scanf("%d",&arr[i][j]);
}
}
// smallest needs to be set to something
smallest = arr[0][0];
// start all the threads
for (int i = 0; i < ptr->p; i++)
{
memcpy(arr, thread_parameters[i].array, sizeof(arr));
thread_parameters[i].s = ptr->s;
memcpy(Bb, thread_parameters[i].B, sizeof(B2));
thread_parameters[i].start = i * (ptr->s / ptr->p);
thread_parameters[i].end = (i+1) * (ptr->s / ptr->p);
pthread_create(&threads[i], NULL, min, &thread_parameters[i]);
}
// wait for all the threads to complete
for (int i = 0; i < ptr->p; i++)
{
pthread_join(threads[i], NULL);
}
// Now aggregate the "smallest" and "largest" results from all thread runs
for (int i = 0; i < ptr->p; i++)
{
if (thread_parameters[i].smallest < smallest)
{
smallest = thread_parameters[i].smallest;
}
}
printf("Smallest is %d\n", smallest);
thread_parameters[ptr->p].B[ptr->s][ptr->s];
for (int i = 0; i < 1; i++)
{
for(int j = 0; j < ptr->s;j++)
{
for(int k = 0; k < ptr->s; k++)
{
printf("Element %d is %d away from min\n",j,thread_parameters[i].B[j][k]);
}
}
}
return 0;
}
Thank you!!
The issue with your code might also come from :
memcpy(arr, thread_parameters[i].array, sizeof(arr));
...
memcpy(Bb, thread_parameters[i].B, sizeof(B2));
as thread_parameters[i].array and thread_parameters[i].B are not allocated, if you are only reading the array it might b fine to only pass them by address
thread_parameters[i].array = arr
but for thread_parameters[i].B you would need to allocate the arrays and perform a deep copy (memcpy would not work)
The below text does not answer the question but does provide some insight on VLA usage
One reason for causing the segmentation with a declaration of a Variable Length Array is that the value is to large to allocate the array on the stack (some compiler choose this option, this choice might have performance reason).
The is not much option to recover cleanly from failure to allocate memory on the stack as there is little way to clean up stack memory during runtime within the same stack context.
You can mitigate the issue by allocating your 2D arrays on the heap instead, some of the strategies are available here(thanks #Lundin) and here.
int** alloc_2d_int_array(size_t rows, size_t cols) {
int **result = malloc(rows * sizeof(int *));
if(result == NULL) {
// could not allocate more memory
return NULL;
}
size_t row_size = cols * sizeof(int);
for(int i=0; i < rows; ++i) {
result[i] = malloc(row_size);
if(result[i] == NULL) {
// could not allocate more memory
// cleanup
return NULL;
}
}
return result;
}
the above implementation have not been tested, but does compile, there are still risk of integer overflow.
Then use the above define function as following:
int **arr = alloc_2d_int_array(ptr->s, ptr->s);
int **B2 = alloc_2d_int_array(ptr->s, ptr->s);
easier implementation (see here(thanks #Lundin))
int **arr = malloc(sizeof(int[ptr->s][ptr->s]);
int **B2 = malloc(sizeof(int[ptr->s][ptr->s]);
Im trying to build a program that parsing a 2d dynamic array to other program by using shared memory.I search a lot but im a bit confused because im not familiar at this one.
My code so far:
int main (int argc, char* argv []){
int rows,columns;
if( argc < 3 ){
printf("Need The size of the 2d array\n");
return 0;
}
rows = atoi(argv[1]);
columns = atoi(argv[2]);
time_t t;
srand((unsigned) time(&t));
key_t key = ftok(".",'a');
size_t size = sizeof(key_t) + (rows * columns + 2 + rows) * sizeof(int);
int shmid = shmget(key,size,IPC_CREAT|IPC_EXCL|S_IRWXU);
int *memory = shmat(shmid, NULL, 0);
printf("Shared Memory Key: %d\n", key);
int *argsflag = memory;
int *resflag= memory + 1;
int *res = memory + 2;
int **array = (int **) memory + (rows*columns);
for(int i = 0; i < rows ; i++) {
for(int j = 0; j < columns; j++) {
array[i][j] = rand() % 100;
}
}
for(int i = 0; i < rows ; i++) {
for(int j = 0; j < columns; j++) {
printf("%d ",array[i][j]);
}
printf("\n");
}
shmctl(shmid,IPC_RMID,NULL);
shmdt(memory);
return(0);
}
Im getting a Segmentation fault (core dumped) and i dont know why.Also by searching i find a solution with struct but i dint get how i can build that.
You cannot have a int** point at a 2D array. It can only point to the first element in a 1D array of int*.
Furthermore, what's the logic of memory + (rows*columns)? You end up setting the pointer to the last item of the array, rather than the first.
Try this instead:
void* memory = shmat( ...
...
int (*array)[columns] = memory;
...
array[i][j] = ... ;
Where int (*array)[columns] is an array pointer, which ends up point at the first array in the 2D array.
For details, see Correctly allocating multi-dimensional arrays.
I am trying to randomly fill a 2d array with values then multiply them, but for some odd reason when I run my code, on the last iteration, I get a segmentation fault. I have tried decreasing the number I am passing it and everything, but the fault still persists. Here is the code I am trying to execute, any help is much appreciated, thanks.
#include <stdlib.h>
#include <stdio.h>
int main(int argc, char *argv[])
{
FILE *matrixFile;
int n = atoi(argv[1]); // the number of matrices
int i, j; // must declare outside of for loop due to resolve C99 mode error
double arrA[n][n];// = CreateRandomMatrix(n);
double arrB[n][n];
double sumArr[n][n];
matrixFile = fopen("home/acolwell/Documents/CPE631_HW2_Number1/results.txt", "w+");
printf("Usage: %s <size of nxn matrices>\n", argv[1]);
// randomly populate arrA and arrB
for(i = 0; i < n; i++)
{
printf("%d\n", i);
for(j = 0; j < n; j++)
{
printf("%4d", j);
arrA[i][j] = (double)rand()/(double)RAND_MAX;
arrB[i][j] = (double)rand()/(double)RAND_MAX;
}
}
printf("Exiting Matrix randomization");
// multiply the matrices and write them to the file
for(i = 0; i < n; i++)
{
for(j = 0; j < n; j++)
{
sumArr[i][j] = arrA[i][j] * arrB[i][j];
printf("Writing matrix ");
fprintf(matrixFile, "%0.3lf\n", sumArr[i][j]);
}
}
if(matrixFile)
{
fclose(matrixFile);
}
matrixFile = NULL;
return 0;
}
This error is going to come down to writing off the end of your array or failure to open your file. I would suggest running gdb to check out your program when it is running, but from a quick glance I wonder if you don't mean to have
"/home/acolwell/Documents/CPE631_HW2_Number1/results.txt"
as the file to write instead of
"home/acolwell/Documents/CPE631_HW2_Number1/results.txt"
I would suggest checking the result of your fopen call before calling fprintf.
If n is large enough, you'll generate a stack overflow using VLAs. I've verified this experimentally with your code (e.g. use n of 5000).
So, you'll need to use malloc to allocate from heap. But, that would require a bit of a rewrite.
Here's a way to use heap allocation and get the benefit of a VLA [using some slight trickery]:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define C(_arr) (double (*)[(size_t)(n)]) _arr
void
docalc(FILE *fout,int n,double arrA[n][n],double arrB[n][n],double sumArr[n][n])
{
// must declare outside of for loop due to resolve C99 mode error
int i,
j;
// randomly populate arrA and arrB
for (i = 0; i < n; i++) {
printf("%d\n", i);
for (j = 0; j < n; j++) {
printf("%4d", j);
arrA[i][j] = (double) rand() / (double) RAND_MAX;
arrB[i][j] = (double) rand() / (double) RAND_MAX;
}
}
printf("Exiting Matrix randomization");
// multiply the matrices and write them to the file
for (i = 0; i < n; i++) {
for (j = 0; j < n; j++) {
sumArr[i][j] = arrA[i][j] * arrB[i][j];
printf("Writing matrix\n");
fprintf(fout, "%0.3lf\n", sumArr[i][j]);
}
}
}
int
main(int argc, char *argv[])
{
FILE *matrixFile;
int n = atoi(argv[1]); // the number of matrices
printf("Usage: %s <size of nxn matrices>\n", argv[1]);
matrixFile = fopen("/tmp/results.txt", "w+");
if (matrixFile == NULL) {
perror("fopen");
exit(1);
}
double *arrA = malloc(sizeof(double) * n * n);
double *arrB = malloc(sizeof(double) * n * n);
double *sumArr = malloc(sizeof(double) * n * n);
docalc(matrixFile,n,C(arrA),C(arrB),C(sumArr));
if (matrixFile)
fclose(matrixFile);
matrixFile = NULL;
return 0;
}
I just compiled and tested your code. The file name you are giving is incorrect; you need a "/" in front of "home".
Not sure what the requirements are, but write your matrixFile like a matrix: add a new line after each row of the matrix is "multiplied", not after every element:
for(i = 0; i < n; i++) {
for(j = 0; j < n; j++) {
sumArr[i][j] = arrA[i][j] * arrB[i][j];
printf("Writing matrix ");
fprintf(matrixFile, "%0.3lf ", sumArr[i][j]);
}
fprintf(matrixFile, "\n");
}
Also, take Craig Easley's comment seriously. Stack Overflow can happen, even off the premises this website ;) Consider allocating your matrix dynamically on the heap.
I followed few examples on this forum, but it seems like my program still keeps crashing at some point.
All i want to do is just use a void function for memory allocation.
void alloc(int ***matrix, int n)
{
int i = 0;
for( ; i < n; i++)
{
(*matrix)[i] = (int*)malloc(n * sizeof(int));
}
i = 0;
for( ; i < n; i++)
{
int j = 0;
for( ; j < n; j++)
{
(*matrix)[i][j] = i * j;
}
}
}
//-------------------------------------------------------------------
int main()
{
int n;
int **matrix_pp;
printf("Enter n: ");
scanf("%d", &n);
alloc(&matrix_pp, n);
free(matrix_pp);
return 0;
}
You try to use (*matrix)[i] before it's been allocated. Add:
(*matrix) = malloc(n * sizeof(**matrix));
before your for loop.
Note two things here:
1) Don't cast the result of malloc,
2) use sizeof(*pointer) instead of explicitly writing out the type; this way, if you decide to change the type later, it will still work.
Further, you will need to free all of the allocations you have in a loop as a loop as well; otherwise, you have a memory leak.
I am struggling with an algorithm to print numbers between 1 and a dynamic variable n into an int.
int n = // dynamic value
int i = 0;
int output[n];
for(i = 0; i < n; i++) {
output[i] = i;
}
However, as n is dynamic, the code won't compile.
Any help would be much appreciated - thanks in advance.
You need to allocate a buffer, or dynamic-sized array, with malloc:
int n = // whatever
int i = 0;
int* output = NULL;
// Allocate the buffer
output = malloc(n * sizeof(int));
if (!output) {
fprintf(stderr, "Failed to allocate.\n");
exit(1);
}
// Do the work with the array
for(i = 0; i < n; i++) {
output[i] = i;
}
// Finished with the array
free(output);
output is a pointer to the beginning of the buffer you allocated, and you can treat it as an array of n ints.
When you're finished with the array, you need to de-allocate the memory with free.
This should work:
int n = // whatever
int i = 0;
int* output = (int*)malloc(sizeof(int)*n);
for(i = 0; i < n; i++) {
output[i] = i;
}
Don't forget to free(output); when you don't need it anymore.
EDIT: Made it C.
If 'n' is changing during runtime, then you could use malloc like suggested in the comments. Then check if you need more space, then automatically realloc more space should it be needed