c++ - Multiple threads reading from the same file

Question

My platform is windows vista 32, with visual c++ express 2008 .

for example:

if i have a file contains 4000 bytes, can i have 4 threads read from the file at same time? and each thread access a different section of the file.

thread 1 read 0-999, thread 2 read 1000 - 2999, etc.

please give a example in C language.

Answer 1

If you don't write to them, no need to take care of sync / race condition.

Just open the file with shared reading as different handles and everything would work. (i.e., you must open the file in the thread's context instead of sharing same file handle).

#include <stdio.h>
#include <windows.h>

DWORD WINAPI mythread(LPVOID param)
{
    int i = (int) param;
    BYTE buf[1000];
    DWORD numread;

    HANDLE h = CreateFile("c:\test.txt", GENERIC_READ, FILE_SHARE_READ,
        NULL, OPEN_EXISTING, 0, NULL);

    SetFilePointer(h, i * 1000, NULL, FILE_BEGIN);
    ReadFile(h, buf, sizeof(buf), &numread, NULL); 
    printf("buf[%d]: %02X %02X %02X
", i+1, buf[0], buf[1], buf[2]);

    return 0;
}

int main()
{
    int i;
    HANDLE h[4];

    for (i = 0; i < 4; i++)
        h[i] = CreateThread(NULL, 0, mythread, (LPVOID)i, 0, NULL);

    // for (i = 0; i < 4; i++) WaitForSingleObject(h[i], INFINITE);
    WaitForMultipleObjects(4, h, TRUE, INFINITE);

    return 0;
}