open binary files in c/c++

trixar · 12-14-2003, 06:16 PM

HI
I want to open a binary file and read from it using the read() function. if i use read(test, *buf, 1000)
buf will only read the first 2 or 3 bytes, why?

Tarts · 12-14-2003, 07:26 PM

Quote:

Originally posted by trixar
HI
I want to open a binary file and read from it using the read() function. if i use read(test, *buf, 1000)
buf will only read the first 2 or 3 bytes, why?

Can you show a little more code?

Tarts

trixar · 12-15-2003, 05:01 AM

....
char buf[1000];

if ((handle = open("binaryfile",O_RDONLY)) ==-1)
{
perror();
exit(1);
}

read (handle, buf, 900);

cout <<buf;

when executing:
./open

MZ

and done;

jim mcnamara · 12-15-2003, 05:44 AM

try:

Code:

if ((handle = open("binaryfile",O_RDONLY | O_BINARY)) ==-1)

vasudevadas · 12-16-2003, 06:34 AM

Don't use read(). That's meant for reading ASCII data, like you would from stdin - it reads until it gets a newline character and then returns.

If you're loading a binary file, use fread() instead:

Code:

  size_t fread(
          void *pointer,
          size_t *size,
          size_t *num_items,
          FILE *stream );

pointer points to your array, size should be set to 1, num_items should be set to the size of the file you are loading (in bytes), stream should be set to your file pointer.

If you don't know the size of your file, you can find it out using fstat(). Alternatively, read your file a chunk at a time: set num_items to some arbitrary value, and repeatedly call fread() until it returns a count less than num_items - this means the end of the file has been reached. You would probably need to realloc() your array as you go, and don't forget to increment the parameter you pass to the pointer argument by the number of bytes read so far!

kfank · 09-26-2012, 05:50 PM

trixar,

Your code is reading binary data correctly. Examine buf with a debugger and you should see all your data. The problem is you are sending a char * to cout and it only reads up to the first 0 byte (standard string).

BTW, O_BINARY is not a valid flag for open().

Alternatively, you could use fopen("binaryfile", "rb") to get a FILE pointer and follow the call presented by vasudevadas.

NevemTeve · 09-26-2012, 08:59 PM

1. Some platforms (unices) don't have O_BINARY. Solution:

Code:

#ifndef O_BINARY
#define O_BINARY 0
#endif

2. strlen doesn't work for binary data. Solution: don't use it; read and fread do return the length.

3. Which is better: read or fread? Bad question. They basically do the same thing, read works with file handle (low level); fread, with FILE-pointer (high level).

dwhitney67 · 09-27-2012, 06:00 AM

For C++, don't use fopen(), fread(), etc. Use std::ifstream and related functions.

Note that C and C++ are NOT the same languages.

Code:

#include <fstream>
#include <iostream>

int main()
{
    // attempt to open the file in the proper mode.
    std::ifstream binFile("binaryFile", std::ios::in | std::ios::binary);

    // if the file was successfully open
    if (binFile)
    {
        char buffer[1024];

        // read data from the file until EOF is reached
        while (binFile.read(buffer, sizeof(buffer)))
        {
            // do something with data in buffer
            // the number of bytes actually read can be obtained by
            // examining the value returned by gcount()

            std::cout << binFile.gcount() << " bytes were read from 'binFile'" << std::endl;

            // ...
        }
    }
    else
    {
        std::cerr << "Failed to open 'binaryFile'" << std::endl;
    }
}