switching between descriptor and file pointer?

Thinking · 12-13-2005, 06:22 AM

hiho@ll

i suddenly noticed that i can't do such code:

Code:

/*
 int descr for my problem it was a fifo O_RDONLY descriptor
*/
int test(int descr){
 char buff[1024]={0};
 FILE *fp=fdopen(descr,"r");
 fgets(buff,1023,fp);            // P1
 int len=read(descr,buff,1023);  // P2
 return len;
}

P2 will return 0
if i replace P2 with fread(buff,sizeof(char),1023,fp); it works

adding the line
descr=fileno(fp); between P1 and P2 doesn't work too (btw: descr == fileno(fp))

Q1: why can't i use the descriptor if i did a fdopen?
Q2: is it possible to use the descriptor after using fdopen? if yes, how?
Q2.2: is it possible to mix the usage of descriptor and file pointer?

thx@ll

jim mcnamara · 12-13-2005, 08:21 AM

use fileno(descr)

Thinking · 12-13-2005, 10:08 AM

hmm
i didn't tested it but i'm not sure if it can work as you described
have a look at the filno prototype

int fileno(FILE *stream);
defined in stdio.h

it wants a FILE* and not an int

so fileno(descr) can't work, not?

jim mcnamara · 12-13-2005, 10:36 AM

My bad - FILE * is a struct pointer.
fileno(fp) is what you should use in your code fragment above.

try man open

Thinking · 12-13-2005, 11:07 AM

i tried with fileno

i ask differently and more noobish ;-)

why does this code don't work?

Code:

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>

int main(int argc,char **argv){

 FILE *fp=fopen("readtest.cpp","r");
 int descr=fileno(fp);

 char buffer[100]={0};
 fread(buffer,sizeof(char),99,fp);
 printf("%s",buffer);

 memset(buffer,0,100);

 int i=read(descr,buffer,99);  // LX
 printf("\nREAD: %d %s\n",i,buffer);
 exit(0);
}

on line X (LX) read returns 0 and buffer isn't changed in any way
why?

thx@ll

Thinking · 12-13-2005, 11:13 AM

i just found this

http://www.delorie.com/gnu/docs/glibc/libc_242.html

but i'm not sure if i understand right

if i read the article i think that the only difference between FILE pointer and descriptor is their different "file position" handling
file pointer and descriptor use different offsets
right?
if yes, why read(); returns 0 in the above example and doesn't return the same as fread returns?

thx@ll

primo · 12-13-2005, 06:42 PM

Try using setbuf(fp, NULL) right after fopen() or fdopen(). You need to turn off any buffering and I guess that fread() is filling one buffer and making read() return 0 (EOF), in the case your file is large enough...

Thinking · 12-14-2005, 12:46 AM

@primo
damn, youre absolutly right
using this code

Code:

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>

int main(int argc,char **argv){

 FILE *fp=fopen("readtest.cpp","r");
 setbuf(fp,NULL);

 int descr=fileno(fp);

 char buffer[100]={0};
 fread(buffer,sizeof(char),99,fp);
 printf("%s",buffer);

 memset(buffer,0,100);

 int i=read(descr,buffer,99);
 printf("\nREAD: %d %s\n",i,buffer);
 exit(0);
}

it worked perfectly and i only added setbuf(fp,NULL);

just for understanding:
fread seems to buffer the input data
the problem was that my readtest.cpp was too small in size, so it was read completely by the fread function and the read thought it was at the end of the file
using a bigger file "read" continues reading where the buffer of fread ends (it's not necessaraliy the fread buffer parameter)

to mix both fread and read you have to set the file pointer fp to be unbuffered using as primo said
setbuf(fp,NULL);

thx