File reading

[mjbauer95]

What is the best way to read a file in C?
I have heard of using fread but then how do you find the size of the file?

This is my example code but it seems to have odd errors.

char * read(char * filename){
    FILE * fo;
 
    fo = fopen(filename, "r");
    if(fo){
        char * buffer = NULL;
        size_t result;
        int length;
        length = file_length(fo);
        result = fread(buffer,1,length,fo);
        fclose(fo);
        return buffer;
    } else {
        fclose(fo);
        return NULL;
    }
}
 

[l034n]

Well, it does indeed have errors. Check out this one.

unsigned char* ReadFile(const char* szFileName, long& length);
 
int main(int argc, char* argv[])
{
    long len = 0;
    const char* filename = "c:\\test.txt";
    unsigned char* buffer = ReadFile(filename, len);
 
    if(buffer)
    {
        // Do something with the buffer
        //.....
 
        // And clean up after yourself
        delete[] buffer;
        buffer = NULL;
    }
 
    return 0;
}
 
unsigned char* ReadFile(const char* szFileName, long& length)
{
    length = 0;
    unsigned char* buffer = NULL;
    FILE* fo = fopen(szFileName, "r");
 
    if(fo)
    {
        fseek(fo, 0, SEEK_END);
        length = ftell(fo);
        fseek(fo, 0, SEEK_SET);
 
        if(length)
        {
            buffer = new unsigned char[length];
            fread(buffer, length, 1, fo);
        }
 
        fclose(fo);
    }
 
    return buffer;
}
 

You may also consider using the *_s fuctions (like fopen_s).

Cheers

[weaknessforcats]

You should be able to seek to the end of the file (SEEK_END) and then do a tell to get the offset from the beginning.

[Banfa]

As long as you opened the file in binary mode, if you opened it in text mode then that is not a reliable method of getting the file size.

[mjbauer95]

Does the buffer have to be unsigned char * ?
Can it be const char * ?

I am new to C, sorry.

[l034n]

Yes, it can be a const char* if you don't want to modify it. So, the line:

unsigned char* buffer = ReadFile(filename, len);
 

would be:

const unsigned char* buffer = ReadFile(filename, len);
 

And also change the return value of ReadFile(). I wrote it as unsigned char* again because it was how i did it in the first example. But if you want to use char* instead of unsigned char* it doesn't matter, since they are of the same size (1 byte), just interpreted differently. However, i don't know why do you want to use a char*. The manipulation of bytes is better with unsigned char, hence BYTE is defined as unsigned char.