Help with the read() function in C

smoothdogg00 · 09-25-2006, 07:09 PM

I need help with the read() function in C. I am trying to read a file one character at a time using the open() and read() functions, but I am getting an infinite loop. The open() function I have below returns a "3", meaning the file is opened successfully. However, the read() function returns -1 all the time, sending it into an endless loop when it shoudl be returning the number of bytes read at any given moment until the EOF.

Code:

#include <stdio.h>
#include <fcntl.h>

main(int argc, char * argv[])
{
  char *buf;
  int bytesRead;
  int fp;
  fp = open("test", O_RDWR, "rb");
  do{
    bytesRead = read(fp, buf, 1);
    printf("%d", bytesRead);
    }
  while(bytesRead != 0);
  close(fp);

  exit(0);
}

paulsm4 · 09-25-2006, 07:19 PM

"-1" indicates an error condition, or EOF.

You should definitely check for "-1" in your read call, and you should definitely #include <errno.h> and check the value of "errno". You might also wish to call "perror()" for bona fide error conditions (but not, of course, for run-of-the-mill EOF conditions).

exvor · 09-25-2006, 07:21 PM

buff is a pointer to a char but you dont point it to anything. Remeber in c you cannot have varible length strings.

you should inilize it like this

buff = malloc(sizeof(char));

but thats just silly defining it as
char buff;
would make more sense if your only trying to get one byte of information.

smoothdogg00 · 09-25-2006, 07:41 PM

The description of read():

Code:

read()
	#include  <fcntl.h>
	int  read(  int  handle,  void  *buffer,  int  nbyte );

Can someone please explain the arguments for the read function? What should *buffer be declared as and what is the int handle (I do understand the use of nbyte.)?

paulsm4 · 09-25-2006, 08:44 PM

Exvor's right (as was I). You need to:

1. Allocate a buffer: either "malloc()", or declare some "char myarray[BUFSIZE]" on the stack.

2. Open the file.
Make sure the file is opened successfully.
Print error status (errno and/or perror) if it fails.

3. Read from the file; again checking the return value and again checking error status.
This time, however, you need to distinguish between
a) a successful read (#/bytes read == #/bytes expected)
b) a partial read (#/bytes read < #/bytes expected)
c) EOF (#/bytes == -1, but errno == 0)
d) An I/O error (#/bytes == -1, errno is nonzero)

4. Close the file

STYLE POINT:
I would use the variable name "fd" (integer "file descriptor") for open/read/close, vs the name "fp" (FILE "file pointer"), which I would use for "fopen/fread/fclose".

As far as the arguments: it should be pretty self-explanatory:

- fd: file descriptor (of some file you opened)
- buffer: (array of bytes, or a struct, you allocated)
- nbytes: (the #/bytes you'd like to read (there may or may not be that many bytes available)

'Hope that helps .. PSM