I tried using malloc on this table - struct that contains rows that contain cells. First I allocate the table, then succesfully add and initialize first row, but when trying to set cell count on second row it crashes - I have no idea why, probably some leftover from previous pointer? Or do I need to allocate memory even for props of the structs? This is my first time digging into malloc, so sorry if it is something trivial.
typedef struct cell_t
{
char* content;
int contentLength;
} cell_t;
typedef struct row_t
{
cell_t* cells[100];
int cellCount;
} row_t;
typedef struct table_t
{
row_t* rows[100];
int rowCount;
} table_t;
row_t* allocateRow()
{
row_t* allocatedRow;
allocatedRow = malloc(sizeof(row_t*));
if (allocatedRow)
{
printf("THIS PRINTS TWICE");
allocatedRow->cellCount = 0;
printf("THIS PRINTS ONCE");
}
else
{
printf("FAILED TO ALLOCATE ROW!");
}
return allocatedRow;
}
void addRow(table_t* tableToAddTo, int nToAllocate)
{
while (tableToAddTo->rowCount < nToAllocate)
{
tableToAddTo->rows[tableToAddTo->rowCount] = allocateRow();
tableToAddTo->rowCount++;
}
}
int main()
{
table_t* inputTable = malloc(sizeof(table_t));
if (inputTable)
{
inputTable->rowCount = 0;
}
else
{
printf("FAILED TO ALLOCATE TABLE!");
return 1;
}
addRow(inputTable, 5);
for (int i = 0; i < inputTable->rowCount; i++)
{
free(inputTable->rows[i]);
}
free(inputTable);
return 0;
}
In allocateRow() change
allocatedRow = malloc(sizeof(row_t*));
into
allocatedRow = malloc(sizeof(row_t));
And you will need to allocate cells or change
cell_t* cells[100];
into
cell_t cells[100];
Related
include "array.h"
int *
forloop_1_svc(input *argp, struct svc_req *rqstp)
{
int result[argp->a];
printf("Server started********\n");
for (int i = 1; i < (argp->a); ++i)
{
result[i]=i;
}
return result;
}
I try to read the name of a file using scanf but failed.
I am very bad at pointers and could not find the problem.
Is there a problem with the pointer to the array of string?
Here is my code:
int* Read_file(char* str[])
{
FILE* fp = fopen(str[1], "r");
if(fp == NULL)
{
printf("File cannot open\n");
return NULL;
}
int rows = 0;
while(!feof(fp))
{
if(fgetc(fp) == '\n')
{
rows ++;
}
}
rows ++;
int* keys = (int*)malloc(3 * rows * sizeof(int));
fseek(fp, 0L, 0);
while(!feof(fp))
{
for(int i = 0;i < rows;i ++)
{
for(int j = 0;j < 3;j ++)
{
fscanf(fp,"%d", &keys[(3 * i) + j]);
}
}
}
fclose(fp);
return keys;
}
int main()
{
char* str[20];
printf("Build_tree ");
scanf("%s",&str);
int* keys = Read_file(str);
return 0;
}
Okay, so the thing is:
You need a char array to store a string(file-name). So you should use a char array. Instead, you were using an array of char pointers.
An array is actually a series of memory blocks. The name of the array represents a pointer to the first element of the array(in this case the first char variable).
While reading a string, scanf needs a location to store it. So you need to give it the address of the first char variable of your char array, which is available in your char array itself. So you have to pass str only to scanf. In the case of normal int,float, and such fundamental data types, their names represent memory blocks and not pointers to memory blocks, and hence you had to use a &.
Then for fopen, fopen expects a char*(which points to the first character of the char array stoing the filename) and you have to provide it with a char* . So you should pass str.
I think your code should go like
int* Read_file(char str[])
{
FILE* fp = fopen(str, "r");
if(fp == NULL)
{
printf("File cannot open\n");
return NULL;
}
int rows = 0;
while(!feof(fp))
{
if(fgetc(fp) == '\n')
{
rows ++;
}
}
rows ++;
int* keys = (int*)malloc(3 * rows * sizeof(int));
fseek(fp, 0L, 0);
while(!feof(fp))
{
for(int i = 0;i < rows;i ++)
{
for(int j = 0;j < 3;j ++)
{
fscanf(fp,"%d", &keys[(3 * i) + j]);
}
}
}
fclose(fp);
return keys;
}
int main()
{
char str[20];
printf("Build_tree ");
scanf("%s",str);
int* keys = Read_file(str);
//Whatever you want to do with the keys
return 0;
}
Comment for any queries.
I am learning to write kernel modules and in one of the examples I had to make sure that a thread executed 10 times and exits, so I wrote this according to what I have studied:
#include <linux/module.h>
#include <linux/kthread.h>
struct task_struct *ts;
int flag = 0;
int id = 10;
int function(void *data) {
int n = *(int*)data;
set_current_state(TASK_INTERRUPTIBLE);
schedule_timeout(n*HZ); // after doing this it executed infinitely and i had to reboot
while(!kthread_should_stop()) {
printk(KERN_EMERG "Ding");
}
flag = 1;
return 0;
}
int init_module (void) {
ts = kthread_run(function, (void *)&id, "spawn");
return 0;
}
void cleanup_module(void) {
if (flag==1) { return; }
else { if (ts != NULL) kthread_stop(ts);
}
return;
}
MODULE_LICENSE("GPL");
What I want to know is :
a) How to make thread execute 10 times like a loop
b) How does the control flows in these kind of processes that is if we make it to execute 10 times then does it go back and forth between function and cleanup_module or init_module or what exactly happens?
If you control kthread with kthread_stop, the kthread shouldn't exit until be ing stopped (see also that answer). So, after executing all operations, kthread should wait until stopped.
Kernel already implements kthread_worker mechanism, when kthread just executes works, added to it.
DEFINE_KTHREAD_WORKER(worker);
struct my_work
{
struct kthread_work *work; // 'Base' class
int n;
};
void do_work(struct kthread_work *work)
{
struct my_work* w = container_of(work, struct my_work, work);
printk(KERN_EMERG "Ding %d", w->n);
// And free work struct at the end
kfree(w);
}
int init_module (void) {
int i;
for(i = 0; i < 10; i++)
{
struct my_work* w = kmalloc(sizeof(struct my_work), GFP_KERNEL);
init_kthread_work(&w->work, &do_work);
w->n = i + 1;
queue_kthread_work(&worker, &w->work);
}
ts = kthread_run(&kthread_worker_fn, &worker, "spawn");
return 0;
}
void cleanup_module(void) {
kthread_stop(ts);
}
I want to create a 2 dimensional character array dynamically through pointers. Then input 10 strings in it and then take a string target from user and find it in array. if it is present then return its index. I have written code for it but it has errors. Please help me in correcting it. Thanks in advance.
#include<iostream>
#include<string>
using namespace std;
int strsearch(char [][50],char *);
int main()
{
char str[10][50];
char *target=new char [50];
int index;
for(int i=0; i<10; i++)
{
str = new char* [50];
str++;
}
for(int i=0; i<10; i++)
{
cout<<"Enter a sting";
cin>>str[i][50];
str++;
}
cout<<"Enter a string to find:";
cin>>target;
index=strsearch(str,target);
if(index<0)
{
cout<<"String not found";
}
else
{
cout<<"String exist at location "<<index<<endl;
}
return 0;
}
int strsearch(char string[10][50],char *fstr)
{
int slen;
for(int i=0;i<10;i++)
{
slen=strlen(**string);
if (strnicmp(*string[50],fstr,slen)== 0)
{
return i;
}
}
return -1;
}
Simply use:
std::vector<std::string> obj;
It will save you all the head & heart aches and guard you against easy to go wrong manual memory management issues. What you are trying to do is to solve the problem C way. With C++ the correct way to do it is using a vector of strings.
I think this is an error in any case:
for(int i=0;i<10;i++)
{
slen=strlen(**string);
if (strnicmp(*string[50],fstr,slen)== 0)
{
return i;
}
}
Must be something like:
for(int i=0;i<10;i++)
{
slen=strlen(string[i]);
if (strnicmp(string[i],fstr,slen)== 0)
{
return i;
}
}
I have done some correction, i think it can help you but i have not compiled to check for errors.
#include<iostream>
#include<string>
#define DIM_1 10 // Avoid to use "magic numbers" in your code
#define DIM_2 50
using namespace std;
int strsearch(char **string,char *fstr);
int main()
{
char **str = new char*[DIM_1]; //char str[10][50]; dynamically allocated array.
char *target=new char [DIM_2];
int index;
for(int i=0; i<DIM_1; i++)
{
str[i] = new char[DIM_2]; //Do not lost the original pointer
//str++;
}
for(int i=0; i<DIM_1; i++)
{
cout<<"Enter a sting";
cin>>str[i][DIM_2];
//str++; Do not lost the original pointer
}
cout<<"Enter a string to find:";
cin>>target;
index=strsearch(str,target);
if(index<0)
{
cout<<"String not found";
}
else
{
cout<<"String exist at location "<<index<<endl;
}
// Free memory!!
for (int i=0; i<DIM_1;i++) delete[] str[i];
delete[] str;
delete[] target;
return 0;
}
int strsearch(char **string,char *fstr) //its dinamicly allocated array
{
int slen;
int result=-1; //Only one return-> structured programming
for(int i=0;i<DIM_1;i++)
{
slen=strlen(**string);
//strlen and strnicmp is C, not C++, check string class.
if (strnicmp(string[i],fstr,DIM_2)== 0) //Find in the string[i]
{
result= i;
}
}
return result;
}
Modified the below circular queue code for my app.
This queue can hold 32 elements max and I have declared the elements as a structure array inside the class. For adding an element to the queue you have to call CreateElement() functions, which checks for a free element and returns an index. When I reuse an element after processing the following line in the CreateElement functions crashes
boost::shared_array<char> tData(new char[bufferSize]);
m_QueueStructure[queueElems].data = tData;
As per documentation, the assignment operator is supposed to destroy the earlier object and assign the new one. Why is it crashing? Can someone tell me where am I screwing?
#include "boost/thread/condition.hpp"
#include "boost/smart_ptr/shared_array.hpp"
#include <queue>
#define MAX_QUEUE_ELEMENTS 32
typedef struct queue_elem
{
bool inUse;
int index;
int packetType;
unsigned long compressedLength;
unsigned long uncompressedLength;
boost::shared_array<char> data;
}Data;
class CQueue
{
private:
int m_CurrentElementsOfQueue;
std::queue<Data> the_queue;
mutable boost::mutex the_mutex;
boost::condition_variable the_condition_variable;
Data m_QueueStructure[MAX_QUEUE_ELEMENTS];
public:
CQueue()
{
m_CurrentElementsOfQueue = 0;
for(int i = 0; i < MAX_QUEUE_ELEMENTS; i++)
{
m_QueueStructure[i].inUse = false;
m_QueueStructure[i].index = i;
}
}
~CQueue()
{
for(int i = 0; i < m_CurrentElementsOfQueue; i++)
{
int index = wait_and_pop();
Data& popped_value = m_QueueStructure[index];
popped_value.inUse = false;
}
m_CurrentElementsOfQueue = 0;
}
void push(Data const& data)
{
boost::mutex::scoped_lock lock(the_mutex);
the_queue.push(data);
lock.unlock();
the_condition_variable.notify_one();
}
bool empty() const
{
boost::mutex::scoped_lock lock(the_mutex);
return the_queue.empty();
}
bool try_pop(Data& popped_value)
{
boost::mutex::scoped_lock lock(the_mutex);
if(the_queue.empty())
{
return false;
}
popped_value=the_queue.front();
the_queue.pop();
return true;
}
int wait_and_pop()
{
boost::mutex::scoped_lock lock(the_mutex);
while(the_queue.empty())
{
the_condition_variable.wait(lock);
}
Data& popped_value=the_queue.front();
the_queue.pop();
return popped_value.index;
}
int CreateElement(int bufferSize, unsigned long _compressedLength,
unsigned long _uncompressedLength, int _packetType) /* Send data length for this function */
{
int queueElems = 0;
if(m_CurrentElementsOfQueue == 32)
{
CCommonException ex(QERROR, QUEUE_FULL, "Circular Buffer Queue is full");
throw ex;
}
for(queueElems = 0; queueElems < MAX_QUEUE_ELEMENTS; queueElems++)
{
if(m_QueueStructure[queueElems].inUse == false)
break;
}
boost::shared_array<char> tData(new char[bufferSize]);
m_QueueStructure[queueElems].data = tData;
m_QueueStructure[queueElems].inUse = true;
m_QueueStructure[queueElems].compressedLength = _compressedLength;
m_QueueStructure[queueElems].uncompressedLength = _uncompressedLength;
m_QueueStructure[queueElems].packetType = _packetType;
m_CurrentElementsOfQueue++;
return queueElems;
}
Data& GetElement(int index)
{
Data& DataElement = m_QueueStructure[index];
return DataElement;
}
void ClearElementIndex(Data& delValue)
{
m_CurrentElementsOfQueue--;
delValue.inUse = false;
}
};
for(queueElems = 0; queueElems < MAX_QUEUE_ELEMENTS; queueElems++) after looping queueElems has value 32 but in your m_QueueStructure only 32 elements so you trying to access m_QueueStructure[queueElems].data to 33rd element. That the problem.
EDIT: try use m_QueueStructure[queueElems].data.reset(new char[bufferSize]);
Solved the problem. Two changes I did. In the wait_and_pop function, I was returning an index rather than a Data&. When I returned Data&, that solved the assignment problem. Another crash was happening due to a memset of a shared_array.get(). Lesson learnt, never memset a shared_array or a shared_ptr.