«

Do the functions fread and fwrite specifically deal with binary data and
not text? Can fgetc and fputc be used to deal with binary or are they only
for text streams. I know *nixs do not discriminate in texual or binary data
but the 'b' in fopen statements for opening streams is portabile across
platforms. But with a linux can I alter the data of an AVI? It is binary
isn't it and I would want to open a stream and use fprintf would I with the
%x conversion.

Bill

Don't make my brown eyes China Blue wrote:
> In article k1jr5h$aha$,

[snip]

Whether
> the resulting file is still an AVI depends on whether your changes
> conform to the AVI format.

Say the file signature of an AVI is RIFF which represents four bytes. I
have to find where that is in the stream with for example fseek. How would I
do that? Simple enough with a hex editor but lives streams is harder.

Bill

בתאריך יום רביעי, 29 באוגוסט 2012 03:27:46 UTC+1, מאת Bill Cunningham:
>
> Say the file signature of an AVI is RIFF which represents four bytes. I
> have to find where that is in the stream with for example fseek. How would I
> do that? Simple enough with a hex editor but lives streams is harder.
>
/*
find a tag / string in a stream.
Params: fp - pointer to open file
tag - return for tag.
Returns: 0 = no tag in file, 1 = seek to tag , -1 error or some sort
Notes: Not tag structure aware, so some tags may be be spurious.
Notes: untested
*/
int seektag(FILE *fp, char *tag)
{
int ch;
char *buff;
int N;
long pos;

N = strlen(tag);
buff = malloc(N);
if(!buff)
return -1;
while( (ch = fgetc(fp)) != EOF)
{
if(ch == buff[0])
{
ungetc(ch, fp);
pos = ftell(fp);
if( fread(buff, 1, N, fp) != N)
{
free(buff);
return -1;
}
if(!strncmp(buff, tag, N))
{
fseek(fp, pos, SEEK_SET);
free(buff);
return 1;
}
}
}
/* weve got to the end without finding the tag */
free(buff);
if(ferror(fp))
return -1;
return 0;
}

On Wed, 29 Aug 2012 04:27:43 -0700 (PDT), Malcolm McLean
<> wrote:

>?????? ??? ?????, 29 ??????? 2012 03:27:46 UTC+1, ??? Bill Cunningham:
>>
>> Say the file signature of an AVI is RIFF which represents four bytes. I
>> have to find where that is in the stream with for example fseek. How would I
>> do that? Simple enough with a hex editor but lives streams is harder.
>>
>/*
> find a tag / string in a stream.
> Params: fp - pointer to open file
> tag - return for tag.
> Returns: 0 = no tag in file, 1 = seek to tag , -1 error or some sort
> Notes: Not tag structure aware, so some tags may be be spurious.
> Notes: untested
>*/
>int seektag(FILE *fp, char *tag)
>{
> int ch;
> char *buff;
> int N;
> long pos;
>
> N = strlen(tag);
> buff = malloc(N);
> if(!buff)
> return -1;
> while( (ch = fgetc(fp)) != EOF)
> {
> if(ch == buff[0])

At this point, isn't buff[0] indeterminate during the first iteration
of the while loop? buff doesn't receive a value until the fread
below.

> {
> ungetc(ch, fp);
> pos = ftell(fp);
> if( fread(buff, 1, N, fp) != N)
> {
> free(buff);
> return -1;
> }
> if(!strncmp(buff, tag, N))
> {
> fseek(fp, pos, SEEK_SET);
> free(buff);
> return 1;
> }
> }
> }
> /* weve got to the end without finding the tag */
> free(buff);
> if(ferror(fp))
> return -1;
> return 0;
>}

--
Remove del for email

Malcolm McLean <> writes:

> בתאריך יום רביעי, 29 באוגוסט 2012 03:27:46 UTC+1, מאת Bill Cunningham:
>>
>> Say the file signature of an AVI is RIFF which represents four bytes. I
>> have to find where that is in the stream with for example fseek. How would I
>> do that? Simple enough with a hex editor but lives streams is harder.
>>
> /*
> find a tag / string in a stream.
> Params: fp - pointer to open file
> tag - return for tag.
> Returns: 0 = no tag in file, 1 = seek to tag , -1 error or some sort
> Notes: Not tag structure aware, so some tags may be be spurious.
> Notes: untested
> */
> int seektag(FILE *fp, char *tag)

A note the anyone learning C: Malcolm objects to both "const" and
"size_t" so this function won't work with const char * arguments
(without a cast) and it contains a lot of needless signed/unsigned int
conversions. I'd add const above and declare N below as size_t.

(Malcolm: I'm not trying to convert you, but I think your opinion is
singular enough to merit being pointed out.)

> {
> int ch;
> char *buff;
> int N;
> long pos;
>
> N = strlen(tag);
> buff = malloc(N);
> if(!buff)
> return -1;
> while( (ch = fgetc(fp)) != EOF)
> {
> if(ch == buff[0])

Typo: you mean tag[0] of course.

> {
> ungetc(ch, fp);
> pos = ftell(fp);
> if( fread(buff, 1, N, fp) != N)
> {
> free(buff);
> return -1;
> }
> if(!strncmp(buff, tag, N))
> {
> fseek(fp, pos, SEEK_SET);
> free(buff);
> return 1;
> }
> }
> }
> /* weve got to the end without finding the tag */
> free(buff);
> if(ferror(fp))
> return -1;
> return 0;
> }

I think this is working too hard. I'd match as we go, mainly because it
avoid using a buffer. I think the result is clearer:

int seektag(FILE *fp, const char *tag)
{
int ch;
size_t match = 0, tlen = strlen(tag);
long pos;

while (match < tlen && (ch = fgetc(fp)) != EOF)
if (ch == tag[match]) {
if (match == 0) {
ungetc(ch, fp);
pos = ftell(fp);
fgetc(fp);
}
match += 1;
}
else match = 0;

if (match == tlen) {
fseek(fp, pos, SEEK_SET);
return 1;
}
if (ferror(fp))
return -1;
return 0;
}

I'd be inclined protect against empty tag strings, but your code did not
so I copied that decision.

It's even simpler if you are prepared to rely on SEEK_CUR (i.e. a binary
stream):

int seektag(FILE *fp, const char *tag)
{
int ch;
size_t match = 0, tlen = strlen(tag);

while (match < tlen && (ch = fgetc(fp)) != EOF)
if (ch == tag[match])
match += 1;
else match = 0;

if (match == tlen) {
fseek(fp, -(long)tlen, SEEK_CUR);
return 1;
}
if (ferror(fp))
return -1;
return 0;
}

--
Ben.