fread returns more than i want
Franz Jeitler
I have some problems with fread.. first, let's see a part of the source
file:
FILE *fp;
char buf[512];
size_t nread;
.
.
while( nread = fread(buf,1,sizeof(buf),fp) )
fwrite(buf,1,nread,stdout);
The code above is working fine, it returns a desired file on the screen.
Now the problem:
------------------
For a file server I need to give back a structure like: <
strcpy(result.text, buf) >;
For easier understanding, temporary I can also write:
< fprintf(stdout, buf) > instead of < fwrite(buf,1,nread,stdout) >.
But now at the end of each part of the file I get some strange characters
and the last block continues again
with the first lines of the file.
So, why does this happen and how can I solve the problem?
Thank's
Franz
Ulrich Eckhardt
> For easier understanding, temporary I can also write:
> < fprintf(stdout, buf) > instead of < fwrite(buf,1,nread,stdout)
No. fprintf() takes a zero-terminated string, while fwrite takes a buffer
of raw bytes. Further, fprintf() also looks at the string and interprets
it, which might not be the right thing.
Uli
Bjørn Augestad
> Hello,
>
> I have some problems with fread.. first, let's see a part of the source
> file:
>
> FILE *fp;
> char buf[512];
> size_t nread;
> ..
> while( nread = fread(buf,1,sizeof(buf),fp) )
> fwrite(buf,1,nread,stdout);
>
> The code above is working fine, it returns a desired file on the screen.
>
> Now the problem:
> ------------------
> For a file server I need to give back a structure like: <
> strcpy(result.text, buf) >;
>
> For easier understanding, temporary I can also write:
> < fprintf(stdout, buf) > instead of < fwrite(buf,1,nread,stdout) >.
>
> But now at the end of each part of the file I get some strange characters
> and the last block continues again
> with the first lines of the file.
>
> So, why does this happen and how can I solve the problem?
I'm guessing here, as you didn't provide a complete program. fread()
does not terminate the buffer with a string terminator, so when you
strcpy() of fprintf(), you copy/write more characters than expected.
BTW, fprintf(stdout, buf) is a really bad idea,
fprintf(stdout, "%s", buf) is much better.
HTH
Bjørn
Eric Sosman
> [...]
> For a file server I need to give back a structure like: <
> strcpy(result.text, buf) >;
>
> For easier understanding, temporary I can also write:
> < fprintf(stdout, buf) > instead of < fwrite(buf,1,nread,stdout) >.
>
> But now at the end of each part of the file I get some strange characters
> and the last block continues again
> with the first lines of the file.
>
> So, why does this happen and how can I solve the problem?
You get a different interpretation if you regard buf as
a zero-terminated string than if you regard it as an array
of nread characters. In particular, if buf lacks a zero
terminator it is not a proper string, and passing it to
fprintf() invokes undefined behavior. (A likely result is
that fprintf() will "run off the end" of buf until it stumbles
upon a zero byte somewhere else in memory; you are probably
seeing the characters that precede that zero.)
Also, if '%' appears anywhere in buf, fprintf() will try
to interpret it as a formatting directive ...
Your best bet is to use fwrite(), as you apparently intend
to do eventually in any case.
--
Eric Sosman
eso...@acm-dot-org.invalid
Franz Jeitler
"Ulrich Eckhardt" <doom...@knuut.de> schrieb im Newsbeitrag
news:3vh2etF...@uni-berlin.de...
OK, I understand, so should I terminate the string manually (even if I
return a binary char-Array
in < result.text> ?
Franz
Franz Jeitler
"Eric Sosman" <eso...@acm-dot-org.invalid> schrieb im Newsbeitrag
news:2tSdnc4_jIL...@comcast.com...
> Your best bet is to use fwrite(), as you apparently intend
> to do eventually in any case.
>
> --
> Eric Sosman
> eso...@acm-dot-org.invalid
Sorry, but I have to write the block into a structure for returning it to a
client, so what do you suggest?
Franz
Ulrich Eckhardt
> "Ulrich Eckhardt" <doom...@knuut.de> schrieb im Newsbeitrag
> news:3vh2etF...@uni-berlin.de...
>> Franz Jeitler wrote:
>>> For easier understanding, temporary I can also write:
>>> < fprintf(stdout, buf) > instead of < fwrite(buf,1,nread,stdout)
>>
>> No. fprintf() takes a zero-terminated string, while fwrite takes a
>> buffer of raw bytes. Further, fprintf() also looks at the string
>> and interprets it, which might not be the right thing.
>
> return a binary char-Array
> in < result.text> ?
No, use the right functions for whatever you are doing. If you have a
buffer containing text, that text will usually be followed by a null
character, if there isn't one, you might add it. If your buffer contains
other data, it might be perfectly possible that there is a null character
in the middle - functions only reading a buffer up to the first null char
are then not the suitable tool to handle such data.
Uli
Eric Sosman
What does the client expect to receive from you? If
the struct contains the bytes, like this:
struct {
size_t count;
char bytes[512];
} client_data;
... then you can just read the data directly into the
struct, like this:
client_data.count = fread(client_data.buf, 1,
sizeof client_data.buf, fp);
--
Eric Sosman
eso...@acm-dot-org.invalid
Franz Jeitler
"Eric Sosman" <eso...@acm-dot-org.invalid> schrieb im Newsbeitrag
> char bytes[512];
> } client_data;
>
> ... then you can just read the data directly into the
> struct, like this:
>
> client_data.count = fread(client_data.buf, 1,
> sizeof client_data.buf, fp);
>
> --
> Eric Sosman
> eso...@acm-dot-org.invalid
Oh yes, I think that might work, because my client code has < printf ("%s",
result.text) instead of < fread...>.
Thank you
Franz Jeitler
Eric Sosman
Aha! The client expects you to supply a zero-terminated
string, not a counted array of bytes. You should be using a
string-oriented input function like fgets(), not the array-
oriented fread().
--
Eric Sosman
eso...@acm-dot-org.invalid