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,siz eof(buf),fp) )
fwrite(buf,1,nr ead,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.t ext, buf) >;
For easier understanding, temporary I can also write:
< fprintf(stdout, buf) > instead of < fwrite(buf,1,nr ead,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 9 2367
Franz Jeitler wrote: For easier understanding, temporary I can also write: < fprintf(stdout, buf) > instead of < fwrite(buf,1,nr ead,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
Franz Jeitler wrote: 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,siz eof(buf),fp) ) fwrite(buf,1,nr ead,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.t ext, buf) >;
For easier understanding, temporary I can also write: < fprintf(stdout, buf) > instead of < fwrite(buf,1,nr ead,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
Franz Jeitler wrote: [...] For a file server I need to give back a structure like: < strcpy(result.t ext, buf) >;
For easier understanding, temporary I can also write: < fprintf(stdout, buf) > instead of < fwrite(buf,1,nr ead,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 es*****@acm-dot-org.invalid
"Ulrich Eckhardt" <do******@knuut .de> schrieb im Newsbeitrag
news:3v******** *****@uni-berlin.de... Franz Jeitler wrote: For easier understanding, temporary I can also write: < fprintf(stdout, buf) > instead of < fwrite(buf,1,nr ead,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
OK, I understand, so should I terminate the string manually (even if I
return a binary char-Array
in < result.text> ?
Franz
"Eric Sosman" <es*****@acm-dot-org.invalid> schrieb im Newsbeitrag
news:2t******** ************@co mcast.com... Your best bet is to use fwrite(), as you apparently intend to do eventually in any case.
-- Eric Sosman es*****@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
Franz Jeitler wrote: "Ulrich Eckhardt" <do******@knuut .de> schrieb im Newsbeitrag news:3v******** *****@uni-berlin.de... Franz Jeitler wrote: For easier understanding, temporary I can also write: < fprintf(stdout, buf) > instead of < fwrite(buf,1,nr ead,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.
OK, I understand, so should I terminate the string manually (even if I 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
Franz Jeitler wrote: "Eric Sosman" <es*****@acm-dot-org.invalid> schrieb im Newsbeitrag news:2t******** ************@co mcast.com...
Your best bet is to use fwrite(), as you apparently intend to do eventually in any case.
-- Eric Sosman es*****@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?
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.cou nt = fread(client_da ta.buf, 1,
sizeof client_data.buf , fp);
--
Eric Sosman es*****@acm-dot-org.invalid
"Eric Sosman" <es*****@acm-dot-org.invalid> schrieb im Newsbeitrag
news:kJ******** ************@co mcast.com... char bytes[512]; } client_data;
... then you can just read the data directly into the struct, like this:
client_data.cou nt = fread(client_da ta.buf, 1, sizeof client_data.buf , fp);
-- Eric Sosman es*****@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
Franz Jeitler wrote: "Eric Sosman" <es*****@acm-dot-org.invalid> schrieb im Newsbeitrag news:kJ******** ************@co mcast.com...
char bytes[512]; } client_data;
... then you can just read the data directly into the struct, like this:
client_data.c ount = fread(client_da ta.buf, 1, sizeof client_data.buf , fp);
-- Eric Sosman es*****@acm-dot-org.invalid
Oh yes, I think that might work, because my client code has < printf ("%s", result.text) instead of < fread...>.
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 es*****@acm-dot-org.invalid This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics |
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