gwyn@smoke.BRL.MIL (Doug Gwyn) (10/07/89)
In article <20065@mimsy.UUCP> lint@mimsy.umd.edu (The Lint Program) writes: >In article <14657@uhnix1.uh.edu> jsmng@sysmgr.cs.uh.edu writes: >[c code deleted] >t.c: >t.c(5): warning: argument argc unused in function main >lseek, arg. 1 used inconsistently llib-lc(90) :: t.c(19) >lseek, arg. 2 used inconsistently llib-lc(90) :: t.c(19) >lseek value used inconsistently llib-lc(90) :: t.c(19) >lseek value declared inconsistently llib-lc(90) :: t.c(19) >printf returns value which is always ignored "lint" missed some other problems: exit() misdeclared lseek() return comparison against NULL is wrong fgetc() return not compared against EOF <sys/file.h> L_SET should not be used in an application ... "What is wrong with this program?" reminded me of the "What is wrong with this picture?" puzzles in children's books. Often my reaction was, "What is RIGHT with this picture?".
jsmng@sysmgr.cs.uh.edu (10/08/89)
I have a small problem about lseek. I
tried to use the following program to
lseek a file ( with numbers 1 2 3 ).
I expect to get back 1, blank, and 1 again.
But my output was 1, blank and 2.
Initially I use fscanf(fp,"%f",&float_variable)
in place of fgetc and received 1 2 3 instead
of 1 2 1. Can anyone tell me what I did wrong?
Thanks. I think this is probably something
simple so please just send me a mail and
I will summarize it.
#include <stdio.h>
#include <sys/types.h>
#include <sys/file.h>
main(argc,argv)
int argc;
char *argv[];
{
FILE *fp;
char c1,c2,c3;
if ((fp = fopen(argv[1],"r")) == NULL) {
perror("cannot open file");
exit(1);
}
c1 = fgetc(fp);
printf("first number = %c\n",c1);
c2 = fgetc(fp);
printf("second number = %c\n",c2);
if (lseek(fp,0,L_SET) == NULL) {
perror("lseek error");
exit(1);
}
c3 = fgetc(fp);
printf("first number again = %c\n",c3);
}lint@mimsy.umd.edu (The Lint Program) (10/09/89)
In article <14657@uhnix1.uh.edu> jsmng@sysmgr.cs.uh.edu writes:
[c code deleted]
t.c:
t.c(5): warning: argument argc unused in function main
lseek, arg. 1 used inconsistently llib-lc(90) :: t.c(19)
lseek, arg. 2 used inconsistently llib-lc(90) :: t.c(19)
lseek value used inconsistently llib-lc(90) :: t.c(19)
lseek value declared inconsistently llib-lc(90) :: t.c(19)
printf returns value which is always ignoredcpcahil@virtech.UUCP (Conor P. Cahill) (10/10/89)
In article <14657@uhnix1.uh.edu>, jsmng@sysmgr.cs.uh.edu writes: > if (lseek(fp,0,L_SET) == NULL) { ^^ lseek requires a file descriptor, not a file pointer. Try fseek(). -- +-----------------------------------------------------------------------+ | Conor P. Cahill uunet!virtech!cpcahil 703-430-9247 ! | Virtual Technologies Inc., P. O. Box 876, Sterling, VA 22170 | +-----------------------------------------------------------------------+
jsmng@csuna.cs.uh.edu (10/10/89)
I posted a problem I encountered with lseek mainly due to my own defiency in C. The problem was I was aware of the difference in buffered I/O and low lever I/O. I have received a lot of constructive comments. They are all similar so I chose one which seems to be quite informative to post. Thanks for the contribution. From amdcad!cayman!tim@ames.arc.nasa.gov Mon Oct 9 09:08:10 1989 Return-Path: <amdcad!cayman!tim@ames.arc.nasa.gov@cs.uh.edu> Received: from ames.arc.nasa.gov by sun1.cs.uh.edu (4.0/SMI-3.2) id AA05545; Mon, 9 Oct 89 09:08:07 CDT Received: by ames.arc.nasa.gov (5.61/1.2); Mon, 9 Oct 89 07:06:51 -0700 Received: by amdcad.AMD.COM (5.51/2.6) id AA13558; Mon, 9 Oct 89 06:56:33 PDT Received: by cayman.AMD.COM (4.0/SMI-4.0) id AA13703; Mon, 9 Oct 89 08:59:11 CDT Date: Mon, 9 Oct 89 08:59:11 CDT From: amdcad!cayman!tim@ames.arc.nasa.gov (Tim Olson) Message-Id: <8910091359.AA13703@cayman.AMD.COM> To: jsmng@sysmgr.cs.uh.edu Subject: Re: lseek problem Newsgroups: comp.lang.c In-Reply-To: <14657@uhnix1.uh.edu> Organization: Advanced Micro Devices, Austin, TX Cc: Status: RO In article <14657@uhnix1.uh.edu> you write: | I have a small problem about lseek. I | tried to use the following program to | lseek a file ( with numbers 1 2 3 ). | I expect to get back 1, blank, and 1 again. | But my output was 1, blank and 2. | Initially I use fscanf(fp,"%f",&float_variable) | in place of fgetc and received 1 2 3 instead | of 1 2 1. Can anyone tell me what I did wrong? | Thanks. I think this is probably something | simple so please just send me a mail and | I will summarize it. You can't mix lseek and f{scan,get}f. The later functions use the buffered I/O provided in the stdio package. What is happening is that on your first fscanf, an attempt is made to read BUFSIZE bytes from the file and place them into the _iob structure. Once this is done, the fscanf reads out of this buffer. Lseeks do not affect this buffer at all; they change the actual file pointer (affecting reads and writes). What you want to use is fseek, which works with the other "f" functions. -- -- Tim Olson Advanced Micro Devices (tim@amd.com)
rns@se-sd.NCR.COM (Rick Schubert ) (10/13/89)
In article <14779@uhnix1.uh.edu> jsmng@csuna.cs.uh.edu () writes: >I posted a problem I encountered with lseek mainly due to my own >defiency in C. The problem was I was aware of the difference in >buffered I/O and low lever I/O. I have received a lot of constructive >comments. They are all similar so I chose one which seems to be quite >informative to post. Thanks for the contribution. >> -- Tim Olson >>You can't mix lseek and f{scan,get}f. The later functions use the >>buffered I/O provided in the stdio package. What is happening is that >>on your first fscanf, an attempt is made to read BUFSIZE bytes from >>the file and place them into the _iob structure. Once this is done, >>the fscanf reads out of this buffer. Lseeks do not affect this buffer >>at all; they change the actual file pointer (affecting reads and >>writes). What you want to use is fseek, which works with the other >>"f" functions. Unfortunately, you (jsmng@csuna.cs.uh.edu) chose the wrong response to post as the right answer. While much of what Tim said is correct (or at least correct with traditional Unix implementations of |lseek|/|fseek| -- |lseek| is not an ANSI library functions; POSIX owns |lseek|, but I don't know what it says about the relationship between |lseek| and |fseek|), his explanation doesn't explain why your example failed. I'm pretty sure there were correct responses posted, but they have expired on our system. The immediate problem with your program is that (as I believe others have said) |lseek| expects an |int| as an argument, and you passed it a |FILE *| argument. |lseek| will try to interpret the bits of the |FILE *| argument as an integer to determine what file to manipulate. It will almost assuredly find garbage and either do nothing or do something potentially destructive to your address space. It is very unlikely that it will do anything constructive regarding the file you were trying to manipulate. Now, if, instead of FILE *fp; /*...*/ lseek(fp,/*...*/); you had, lseek(fileno(fp),/*...*/); then Tim's explanation would have applied. -- Rick Schubert (rns@se-sd.sandiego.NCR.COM) Although correct explanations to the original problem were posted and I make it a policy to not contribute to the problem of multiple correct solutions being posted, the article I am following up to was posted as the final say in this matter, and in the three days since the article appeared on my machine, no articles have appeared here that have corrected this "final say".