[C & Linux] Read line from text-file

Y_Haarman · 11-06-2002, 09:28 AM

The following code is supposed to read a text file and prints it in the console:

==============================

char *myfile = "test.txt";
int fd;
char line[300];
char ch1;

if(fd = open(myfile, O_RDONLY )== -1 )
{
printf("File open error.\n");
exit(-1);
}

do
{
ch1 = read(fd, &line, 300);
printf("%c\n", line);

} while (ch1 > 0);

close(fd);

==============================

When I execute it, it looks like the "Read" function reads a line from the console. It asks for input but doesn't show anything. Is their a problem in the code maybe?

[BTW: I don't want to (may not) use stdio.h for file operations]

llama_meme · 11-06-2002, 10:16 AM

Firstly, read(...) doesn't read a line, it reads however many characters you ask it to (in your case 300). Presumably your file contains less than 300 characters, and so read never returns.

Secondly, the variable line in your program is a character string, so you need to use %s to print it, not %c. Also, you haven't terminated line with a 0 ('\0'), so printf will segfault. read does not put a 0 on the end for you

Hope that all helps
Alex

Hko · 11-06-2002, 10:25 AM

... and the line:

ch1 = read(fd, &line, 300);

should read:

ch1 = read(fd, line, 300);

without the '&' for the reason Alex mentioned.

Y_Haarman · 11-06-2002, 10:58 AM

Thanks for the replys.

So read doesn't read a line. Is it still smart to use it? Is their a systemcall similar to fgets() in stdio.h?

llama_meme · 11-06-2002, 12:33 PM

Quote:

So read doesn't read a line. Is it still smart to use it? Is their a systemcall similar to fgets() in stdio.h?

I'm not sure, but I doubt it. Remember, these functions are supposed to be low level functions for reading and writing streams of bytes (be they from a binary or ASCII file, TCP/IP packets, whatever...) Reading a line from an ASCII file is actually quite a specialised operation in this context, and I doubt anybody would want to bloat the kernel by writing a system call to do it.

Why can't you use stdio.h? Unless you're writing a kernel module or something I don't see why you shouldn't...

Alex

M3xican · 11-08-2002, 04:44 PM

Quote:

Originally posted by llama_meme
Firstly, read(...) doesn't read a line, it reads however many characters you ask it to (in your case 300). Presumably your file contains less than 300 characters, and so read never returns.

Read doesn't return???????????

from the man page of read()

Quote:

On success, the number of bytes read is returned (zero indicates end of file), and the file position is advanced by this number.

Now u know that read returns

-----------------------------------------------------------

The best way to use I/O files is using buffers, there are a lot of technical reasons, but
I think that now u have to know only that this is the fast way.
A typical code that could be used to manage files looks like this:

Code:

#include .....
#include .....
......

#define DIM_BUF 4096

int main()
{
        ......
        char buf[DIM_BUF];
        int lung;

        fd=open(file,O_RDONLY);

        while(lung=read(fd,buf,DIM_BUF))
        {
                ....
                code that manages the data
                (in example: write(STDOU_FILENO,buf,lung); )
                ....
        }

        ......

        close(fd);

        return 0;
}

I have omitted an error management because this is only an example.