strlen segfaults on 2d dynamic array of strings - multidimensional-array

Why do I get a segfault when I try to print the strlen of a string, which is part of an array of strings? I can print each string - the printf works perfectly. But why does the strlen cause a segfault?
The below program first takes an input n, which is the number of strings I want to dynamically allocate. Then I allocate space for 32 bytes for each string
int main() {
int i;
int n;
char **nums;
scanf("%d", &n);
printf("n = %d\n", n);
nums = malloc(sizeof(char *) * n);
printf("allocated nums\n");
for(i=0; i<n; i++) {
nums[i] = malloc(sizeof(char) * 32);
memset(nums[i], '\0', sizeof(char) * 32);
}
for(i=0; i < n; i++) {
scanf("%s", &nums[i]);
}
for(i=0; i<n; i++) {
// THIS PRINTS FINE
printf("string = %s\n", &nums[i]);
// SEGFAULT HERE IMMEDIATELY
printf("length = %d\n", strlen(nums[i]));
}
Here is the console output. As a test, I entered in n=3, followed by the numbers 45, 46, and 47:
3
n = 3
allocated nums
45
46
47
string = 45
Segmentation fault (core dumped)
Additionally, I get a segfault when I try to access an individual character in each string. Again, the first printf in outer for loop prints the string, then I get a segfault accessing nums[i][k]:
int i, k=0;
for(i=0; i<n; i++) {
printf("Printiiing: %s\n", &nums[i]);
// WHY DOES THIS SEGFAULT???
//printf("first char = %c\n", nums[i][0]);
while(k<32 && nums[i][k] != '\0') {
// THIS CAUSES A SEG FAULT
printf("char = %c", (nums[i])[k]);
k++;
}
}

This is fine:
for(i=0; i<n; i++) {
nums[i] = malloc(sizeof(char) * 32);
memset(nums[i], '\0', sizeof(char) * 32);
}
After this loop, each nums[i] is a pointer to 32-byte buffer.
But this corrupts (overwrites) all the pointers, instead of reading strings into the allocated buffers:
for(i=0; i < n; i++) {
scanf("%s", &nums[i]);
}
To fix the bug, use: scanf("%s", nums[i]);.

There is the difference pointer between the &nums[i] and nums[i] address. You can check with the strlen((const char*)(&nums[i])) for get length.
The explanation of #Employee of Russia is exactly for reason.

Related

Implementing Rc4 algorithm

I need to implement a Rc4 algorithm with a seed: 1 2 3 6 and the plain text cryptology. I am following this guideline we were provided in class, but it's not initializing S correctly.
my output is
and needs to be
My code was previously printing negative values , not sure why but I managed to fix that error. Thought everything was good to go but it's not. Sorry for the pictures, I figured it was easier to explain what I was following for my code structure. I am mod 4 the seed since it contains 4 characters, could that possibly be my error?
#include <iostream>
#include <string>
#include <string.h>
using std::endl;
using std::string;
void swap(unsigned int *x, unsigned int *y);
int main()
{
string plaintext = "cryptology";
char cipherText[256] = { ' ' };
unsigned int S[256] = { 0 };
unsigned int t[256] = { 0 };
unsigned int seed[4] = { 1, 2, 3, 6 }; // seed used for test case 1
unsigned int temp = 0;
int runningTotal = 0;
unsigned int key = 0;
// inilializing s and t
for (int i = 0; i < 256; i++)
{
S[i] = i;
t[i] = seed[i % 4];
}
for (int i = 0; i < 256; i++)
{
runningTotal += S[i] + t[i];
runningTotal %= 256;
swap(&S[runningTotal], &S[i]);
std::cout << S[i] <<" ";
}
runningTotal = 0;
for (int i = 0; i < plaintext.size(); i++)
{
runningTotal %= 256;
swap(&S[i], &S[runningTotal]);
temp = (unsigned int)S[i] + (unsigned int)S[runningTotal];
temp %= 256;
key = S[temp];
std::cout << endl;
cipherText[i] = plaintext[i] ^ key;
}
std::cout << " this is cipher text " << endl;
std::cout << cipherText << endl;
system("pause");
return 0;
}
void swap(unsigned int *x, unsigned int *y)
{
unsigned int temp = 0;
temp = *x;
*x = *y;
*y = temp;
}
Actually I think you're generating S[] correctly. I can only assume you're supposed to do something different with the key. (Perhaps's its an ASCII string instead of four byte values? Check your assignment notes.)
There is a problem later on, however. In the stream generation loop, you're supposed to do the increment and swap operations before you fetch a byte from S[].
for (int k = 0; k < plaintext.size(); k++)
{
i = (i+1) % 256; // increment S[] index
runningTotal = (runningTotal + S[i]) % 256; // swap bytes
swap(&S[i], &S[runningTotal]);
temp = (S[i] + S[runningTotal]) % 256; // fetch byte from S and
cipherText[k] = plaintext[k] ^ S[temp]; // XOR with plaintext
}
NOTE: Although unrelated to your question, your code could be made a lot tidier by using unsigned char values instead of ints. That would eliminate the % 256 instructions that are littered all over the place. (But be careful during initialization, because i<256 will always be true if i is an unsigned char.)

Char conversion is wrong?

I have this:
Serial.write(charBuf[0]); //gives 5
String data(charBuf);
Serial.write(data); //gives nothing (space)
first is prints 5 , second prints nothing. What am i missing? is it has to do with NULL terminated stuff?
This is how the buffer is being created:
int len=1;
char charBuf[len];
for(int k=ACT_THRESH;k<ACT_THRESH+len;k++)
{
charBuf[k-ACT_THRESH]= EEPROM.read(k);
}
int len = 1;
int str_len = 0;
char charBuf[16];
for(int k = ACT_THRESH; k < ACT_THRESH + len && str_len < sizeof(charBuf); k++)
{
charBuf[str_len++]= EEPROM.read(k);
}
charBuf[str_len] = '\0';
But I don't get the goal of the loop if len = 1

OpenCL clEnqueueNDRangeKernel how to set work group size correctly

In OpenCL, if I want to add two N-dimension vectors, the global work group size (globalSize) should satisfy globalSize = ceil(N/localSize) * localSize, where localSize is the local work group size. Is this correct? If N = 1000, and localSize = 128, globalSize should be 1024? Can we always set globalSize some multiple of localSize and larger than needed?
I tried many times and it worked well for 1-dimension problems.
However, when it comes to 2d problems, for example, multiply two matrices of dimension m*n and n*p, the result matrix is of order m*p, things get more complicated.
The max work group size on my device is 128, so I set localSize [2] = {16,8} and
globalSize [2] = {ceil(m/16)*16,ceil(p/8)*8}.
It is similar to the 1-dimension case but the result is wrong!
If I set localSize [2] = {1,128} and change the globalSize accordingly, I can get the correct result. So where is the problem? Can anyone tell me why?
In addition, I find out the indices where the matrix element is wrong.
It seems that the result is wrong at (i,j) where i*p + j = n * some constant (n = 1,2,3...)
Why?
Here is my kernel function:
kernel void mmult(const int Mdim, const int Ndim, const int Pdim,
global float *A, global float *B, global float *C)
{
int i = get_global_id(1);
int j = get_global_id(0);
if(i < 0 || j < 0 || i > Mdim || j > Pdim) return;
else
{
float tmp = 0;
for(int k = 0; k < Ndim; k++)
tmp += A[i*Ndim+k] * B[k*Pdim+j];
C[i*Pdim + j] = tmp;
}
}
And then it is the host program:
#define __NO_STD_VECTOR // Use cl::vector instead of STL version
#define __CL_ENABLE_EXCEPTIONS
#include <CL/cl.hpp>
#include <utility>
#include <iostream>
#include <fstream>
#include <string>
#include <cmath>
using namespace cl;
int main()
{
// Create the two input matrices
int m = 1000;
int n = 1000;
int p = 1000;
float *A = new float[m*n];
float *B = new float[n*p];
for(int i = 0; i < m*n; i++)
{
A[i] = i;
}
for(int i = 0; i < n*p; i++)
{
B[i] = i;
}
try
{
// Get available platforms
vector<Platform> platforms;
Platform::get(&platforms);
// Select the default platform and create a context using this platform and the GPU
cl_context_properties cps[3] =
{
CL_CONTEXT_PLATFORM,
(cl_context_properties)(platforms[0])(),
0
};
Context context( CL_DEVICE_TYPE_GPU, cps);
// Get a list of devices on this platform
vector<Device> devices = context.getInfo<CL_CONTEXT_DEVICES>();
// Create a command queue and use the first device
CommandQueue queue = CommandQueue(context, devices[0]);
// Read source file
std::ifstream sourceFile("mmul.cl");
std::string sourceCode(
std::istreambuf_iterator<char>(sourceFile),
(std::istreambuf_iterator<char>()));
Program::Sources source(1, std::make_pair(sourceCode.c_str(), sourceCode.length()+1));
// Make program of the source code in the context
Program program = Program(context, source);
// Build program for these specific devices
program.build(devices);
// Make kernel
Kernel kernel(program, "mmult");
// Create memory buffers
Buffer bufferA = Buffer(context, CL_MEM_READ_ONLY, m*n * sizeof(float));
Buffer bufferB = Buffer(context, CL_MEM_READ_ONLY, p*n * sizeof(float));
Buffer bufferC = Buffer(context, CL_MEM_WRITE_ONLY, m*p * sizeof(float));
// Copy lists A and B to the memory buffers
queue.enqueueWriteBuffer(bufferA, CL_TRUE, 0, m * n * sizeof(float), A);
queue.enqueueWriteBuffer(bufferB, CL_TRUE, 0, p * n * sizeof(float), B);
// Set arguments to kernel
kernel.setArg(0, m);
kernel.setArg(1, n);
kernel.setArg(2, p);
kernel.setArg(3, bufferA);
kernel.setArg(4, bufferB);
kernel.setArg(5, bufferC);
// Run the kernel on specific ND range
NDRange global((ceil((float)(p)/16))*16,(ceil((float)(m)/8))*8);
NDRange local(16,8);
queue.enqueueNDRangeKernel(kernel, NullRange, global, local);
// Read buffer C into a local list
float *C = new float[m*p];
queue.enqueueReadBuffer(bufferC, CL_TRUE, 0, m*p * sizeof(float), C);
// check the correctness of the result
float *c = new float[m*p];
for(int i = 0; i < m; i++)
for(int j = 0; j < p; j++)
{
float z = 0.0;
for(int k = 0; k < n; k++)
{
z += A[i*n+k] * B[k*p+j];
}
c[i*p+j] = z;
}
for(int i = 0; i < m*p; i++)
{
if(fabs(c[i]-C[i])>0.001)
std::cout<<i<<" "<<c[i]<<" "<<C[i]<<std::endl;
}
delete []A;
delete []B;
delete []C;
}
catch(Error error)
{
std::cout << error.what() << "(" << error.err() << ")" << std::endl;
}
return 0;
}
Your bounds checking code inside your OpenCL kernel is incorrect. Instead of this:
if(i < 0 || j < 0 || i > Mdim || j > Pdim) return;
You should have this:
if(i < 0 || j < 0 || i >= Mdim || j >= Pdim) return;
Let's assume, that you have float matrix of size 1000x1000:
const int size = 1000;
// Whatever
float* myMatrix = (float*)calloc(size * size, sizeof(*myMatrix));
Determine size of Local Group first:
size_t localSize[] = {16, 8};
Then determine, how many Local Groups do you need:
size_t numLocalGroups[] = {ceil(size/localSize[0]), ceil(size/localSize[1])};
Finally, determine NDRange size:
size_t globalSize[] = {localSize[0] * numLocalGroups[0], localSize[1] * numLocalGroups[1]};
Don't forget to handle out-of-bounds access in right-most Local Groups.

OpenCL: Expected identifier in kernel

I am running the following kernel on windows 7, 64 bit, with Intel CPU and HD graphics.
I get very strange error reporting by clGetProgramBuildInfo for the following code:
#define BLOCK_SIZE 256
__kernel void reduce4(__global uint* input, __global uint* output, __local uint* sdata)
{
unsigned int tid = get_local_id(0);
unsigned int bid = get_group_id(0);
unsigned int gid = get_global_id(0);
unsigned int blockSize = get_local_size(0);
unsigned int index = bid*(BLOCK_SIZE*2) + tid;
sdata[tid] = input[index] + input[index+BLOCK_SIZE];
barrier(CLK_LOCAL_MEM_FENCE);
for(unsigned int s = BLOCK_SIZE/2; s > 64 ; s >>= 1) {
// Unrolling the last wavefront and we cut 7 iterations of this
// for-loop while we practice wavefront-programming
if(tid < s)
{
sdata[tid] += sdata[tid + s];
}
barrier(CLK_LOCAL_MEM_FENCE);
}
if (tid < 64) {
if (blockSize >= 128) sdata[tid] += sdata[tid + 64];
if (blockSize >= 64) sdata[tid] += sdata[tid + 32];
if (blockSize >= 32) sdata[tid] += sdata[tid + 16];
if (blockSize >= 16) sdata[tid] += sdata[tid + 8];
if (blockSize >= 8) sdata[tid] += sdata[tid + 4];
if (blockSize >= 4) sdata[tid] += sdata[tid + 2];
if (blockSize >= 2) sdata[tid] += sdata[tid + 1];
}
// write result for this block to global mem
if(tid == 0)
{
output[bid] = sdata[0];
}
}
It always says:
Compilation started
:38:2: error: expected identifier or '('
Compilation failed
this is for the last line, where I have put }. What is wrong here?
Update:
This is how I am reading the kernel file:
int offset = 0;
for(int i = 0; i < numOfDevices; ++i, ++offset ) {
/* Load the two source files into temporary datastores */
const char *file_names[] = {"SimpleOptimizations.cl"};
const int NUMBER_OF_FILES = 1;
char* buffer[NUMBER_OF_FILES];
size_t sizes[NUMBER_OF_FILES];
loadProgramSource(file_names, NUMBER_OF_FILES, buffer, sizes);
/* Create the OpenCL program object */
program = clCreateProgramWithSource(context, NUMBER_OF_FILES, (const char**)buffer, sizes, &error);
if(error != CL_SUCCESS) {
perror("Can't create the OpenCL program object");
exit(1);
}
Definition of loadProgramSource
void loadProgramSource(const char** files,
size_t length,
char** buffer,
size_t* sizes) {
/* Read each source file (*.cl) and store the contents into a temporary datastore */
for(size_t i=0; i < length; i++) {
FILE* file = fopen(files[i], "r");
if(file == NULL) {
perror("Couldn't read the program file");
exit(1);
}
fseek(file, 0, SEEK_END);
sizes[i] = ftell(file);
rewind(file); // reset the file pointer so that 'fread' reads from the front
buffer[i] = (char*)malloc(sizes[i]+1);
buffer[i][sizes[i]] = '\0';
fread(buffer[i], sizeof(char), sizes[i], file);
fclose(file);
}
}
I believe this is an issue with the way the Windows deals with text files opened with fopen(). If you take a look at the MSDN page for fopen(), it indicates that if you open a file with just "r" as the mode string, some translations will happen with regards to line-endings. This means that the size of the file you query may not match the amount of data read by fread().
To solve this, simply change the mode string to indicate that you wish to read the file as binary data (i.e. without any pesky translations):
FILE* file = fopen(files[i], "rb");

Spellcheck program using MPI

So, my assignment is to write a spell check program and then parallelize it using openMPI. My take was to load the words from a text file into my array called dict[] and this is used as my dictionary. Next, I get input from the user and then it's supposed go through the dictionary array and check whether the current word is within the threshold percentage, if it is, print it out. But I'm only supposed to print out a certain amount of words. My problem is, is that, my suggestions[] array, doesn't seem to fill up the way I need it to, and it gets a lot of blank spots in it, whereas, I thought at least, is that the way I wrote it, is to just fill it when a word is within threshold. So it shouldn't get any blanks in it until there are no more words being added. I think it's close to being finished but I can't seem to figure this part out. Any help is appreciated.
#include <stdio.h>
#include <mpi.h>
#include <string.h>
#include <stdlib.h>
#define SIZE 30
#define max(x,y) (((x) > (y)) ? (x) : (y))
char *dict[50000];
char *suggestions[50000];
char enterWord[50];
char *myWord;
int wordsToPrint = 20;
int threshold = 40;
int i;
int words_added = 0;
int levenshtein(const char *word1, int len1, const char *word2, int len2){
int matrix[len1 + 1][len2 + 1];
int a;
for(a=0; a<= len1; a++){
matrix[a][0] = a;
}
for(a=0;a<=len2;a++){
matrix[0][a] = a;
}
for(a = 1; a <= len1; a++){
int j;
char c1;
c1 = word1[a-1];
for(j = 1; j <= len2; j++){
char c2;
c2 = word2[j-1];
if(c1 == c2){
matrix[a][j] = matrix[a-1][j-1];
}
else{
int delete, insert, substitute, minimum;
delete = matrix[a-1][j] +1;
insert = matrix[a][j-1] +1;
substitute = matrix[a-1][j-1] +1;
minimum = delete;
if(insert < minimum){
minimum = insert;
}
if(substitute < minimum){
minimum = substitute;
}
matrix[a][j] = minimum;
}//else
}//for
}//for
return matrix[len1][len2];
}//levenshtein
void prompt(){
printf("Enter word to search for: \n");
scanf("%s", &enterWord);
}
int p0_compute_output(int num_processes, char *word1){
int totalNumber = 0;
int k = 0;
int chunk = 50000 / num_processes;
for(i = 0; i < chunk; i++){
int minedits = levenshtein(word1, strlen(word1), dict[i], strlen(dict[i]));
int thresholdPercentage = (100 * minedits) / max(strlen(word1), strlen(dict[i]));
if(thresholdPercentage < threshold){
suggestions[totalNumber] = dict[i];
totalNumber = totalNumber + 1;
}
}//for
return totalNumber;
}//p0_compute_output
void p0_receive_output(int next_addition){
int num_to_add;
MPI_Comm comm;
MPI_Status status;
MPI_Recv(&num_to_add,1,MPI_INT,MPI_ANY_SOURCE, MPI_ANY_TAG,MPI_COMM_WORLD, MPI_STATUS_IGNORE);
printf("--%d\n", num_to_add);
suggestions[next_addition] = dict[num_to_add];
next_addition = next_addition + 1;
}
void compute_output(int num_processes, int me, char *word1){
int chunk = 0;
int last_chunk = 0;
MPI_Comm comm;
if(50000 % num_processes == 0){
chunk = 50000 / num_processes;
last_chunk = chunk;
int start = me * chunk;
int end = me * chunk + chunk;
for(i = start; i < end;i++){
int minedits = levenshtein(word1, strlen(word1), dict[i], strlen(dict[i]));
int thresholdPercentage = (100 * minedits) / max(strlen(word1), strlen(dict[i]));
if(thresholdPercentage < threshold){
int number_to_send = i;
MPI_Send(&number_to_send, 1, MPI_INT, 0, 1, MPI_COMM_WORLD);
}
}
}
else{
chunk = 50000 / num_processes;
last_chunk = 50000 - ((num_processes - 1) * chunk);
if(me != num_processes){
int start = me * chunk;
int end = me * chunk + chunk;
for(i = start; i < end; i++){
int minedits = levenshtein(word1, strlen(word1), dict[i], strlen(dict[i]));
int thresholdPercentage = (100 * minedits) / max(strlen(word1), strlen(dict[i]));
if(thresholdPercentage < threshold){
int number_to_send = i;
MPI_Send(&number_to_send, 1, MPI_INT, 0, 1, MPI_COMM_WORLD);
}//if
}//for
}//if me != num_processes
else{
int start = me * chunk;
int end = 50000 - start;
for(i = start; i < end; i++){
int minedits = levenshtein(word1, strlen(word1), dict[i], strlen(dict[i]));
int thresholdPercentage = (100 * minedits) / max(strlen(word1), strlen(dict[i]));
if(thresholdPercentage < threshold){
int number_to_send = i;
MPI_Send(&number_to_send, 1, MPI_INT, 0, 1, MPI_COMM_WORLD);
}
}
}//me == num_processes
}//BIG else
return;
}//COMPUTE OUTPUT
void set_data(){
prompt();
MPI_Bcast(&enterWord,20 ,MPI_CHAR, 0, MPI_COMM_WORLD);
}//p0_send_inpui
//--------------------------MAIN-----------------------------//
main(int argc, char **argv){
int ierr, num_procs, my_id, loop;
FILE *myFile;
loop = 0;
for(i=0;i<50000;i++){
suggestions[i] = calloc(SIZE, sizeof(char));
}
ierr = MPI_Init(NULL, NULL);
ierr = MPI_Comm_rank(MPI_COMM_WORLD, &my_id);
ierr = MPI_Comm_size(MPI_COMM_WORLD, &num_procs);
printf("Check in from %d of %d processors\n", my_id, num_procs);
set_data();
myWord = enterWord;
myFile = fopen("words", "r");
if(myFile != NULL){
for(i=0;i<50000;i++){
dict[i] = calloc(SIZE, sizeof(char));
fscanf(myFile, "%s", dict[i]);
}//for
fclose(myFile);
}//read word list into dictionary
else printf("File not found");
if(my_id == 0){
words_added = p0_compute_output(num_procs, enterWord);
printf("words added so far: %d\n", words_added);
p0_receive_output(words_added);
printf("Threshold: %d\nWords To print: %d\n%s\n", threshold, wordsToPrint, myWord);
ierr = MPI_Finalize();
}
else{
printf("my word %s*\n", enterWord);
compute_output(num_procs, my_id, enterWord);
// printf("Process %d terminating...\n", my_id);
ierr = MPI_Finalize();
}
for(i=0;i<wordsToPrint;i++){
printf("*%s\n", suggestions[i]);
}//print suggestions
return (0);
}//END MAIN
Here are a few problems I see with what you're doing:
prompt() should only be called by rank 0.
The dictionary file should be read only by rank 0, then broadcast the array out to the other ranks
Alternatively, have rank 1 read the file while rank 0 is waiting for input, broadcast input and dictionary afterwards.
You're making the compute_output step overly complex. You can merge p0_compute_output and compute_output into one routine.
Store an array of indices into dict in each rank
This array will not be the same size in every rank, so the simplest way to do this would be to send from each rank a single integer indicating the size of the array, then send the array with this size. (The receiving rank must know how much data to expect). You could also use the sizes for MPI_Gatherv, but I expect this is more than you're wanting to do right now.
Once you have a single array of indices in rank 0, then use this to fill suggestions.
Save the MPI_Finalize call until immediately before the return call
For the final printf call, only rank 0 should be printing that. I suspect this is causing a large part of the "incorrect" result. As you have it, all ranks are printing suggestions, but it is only filled in rank 0. So the others will all be printing blank entries.
Try some of these changes, especially the last one, and see if that helps.

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