strange behavior with fread, fseek

[Joe Maimon <jmaimon at ttec dot com>]

Strange to me that is.

Why am I asking here? Perhaps I should make a fool of myself to the 
kernel or glibc people?

Well I know geniuses hang out here. Plus coreutils must have dealt with 
issues like this if it is indeed normal.


$ ls -la fread-fseek.c
-rw-r--r--  1 joe joe 1155 Jan 26 13:37 fread-fseek.c

$ cat fread-fseek.c | wc -l
56

$ cat fread-fseek.c | while ./fread-fseek 1156; do true ; done | wc -l
1

$ cat fread-fseek.c | while ./fread-fseek 1155; do true ; done | wc -l
1

$ cat fread-fseek.c | while ./fread-fseek 1; do true ; done | wc -l
1

$ while ./fread-fseek 1156; do true ; done < fread-fseek.c | wc -l
56

$ while ./fread-fseek 1155; do true ; done < fread-fseek.c | wc -l
1

$ while ./fread-fseek 1; do true ; done < fread-fseek.c | wc -l
1



where the numeric arg is the number of bytes that fread should read per 
call.

Strace shows reads are done in 4096 and _llseek calls look strange.

here is an example

read(0, "#include <stdio.h>\n#include <lib"..., 4096) = 1155
fstat64(1, {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 
0) = 0xb7f2c000
_llseek(0, 1155, [1155], SEEK_SET)      = 0
write(1, "#include <stdio.h>\n", 19#include <stdio.h>

and this is the one where things perform as expected

read(0, "#include <stdio.h>\n#include <lib"..., 4096) = 1155
read(0, "", 4096)                       = 0
fstat64(1, {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 
0) = 0xb7f6e000
_llseek(0, 0, [0], SEEK_SET)            = 0
read(0, "#include <stdio.h>\n", 19)     = 19
write(1, "#include <stdio.h>\n", 19#include <stdio.h>
)    = 19


Linux nameserver3 2.6.12.3686-smp-0.1.0 #1 SMP Wed Aug 3 21:18:13 EDT 
2005 i686 GNU/Linux

Debian testing




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

int main (int argc, char ** argv)
{
	char *buf;
	int len = BUFSIZ;
	int bytes_to_read = 32;
	int bytes_read = 0;
	char *line_end = NULL;
	long file_offset = ftell(stdin);
	
	if (argv[1])
		bytes_to_read = atoi(argv[1]);
	if (argv[1] && argv[2])
		len = atoi(argv[2]);

	if (!bytes_to_read || !len)
	{
		fprintf(stderr, "Not reading any bytes, bytes_to_read = %d , len = %d\n"
				, bytes_to_read, len);
		exit(1);
	}

	if (bytes_to_read > len)
		len = bytes_to_read + 1;
	
	buf = malloc(len);
	if (!buf)
	{
		fprintf(stderr, "No buf\n");
		exit(1);
	}
	*(buf+len) = '\0';

	do {
		bytes_read += fread(buf+bytes_read, 1, bytes_to_read, stdin); 
		line_end = memchr(buf, '\n', bytes_read);	
	} while (!feof(stdin) && !ferror(stdin) && !line_end && bytes_read < len);

	if (line_end && bytes_read < len)
		*(line_end) = '\0';
	else
		*(buf+bytes_read) = '\0';

	if (bytes_read)
		fprintf(stdout, "%s\n", buf);

	if (file_offset > -1 && line_end)
		fseek(stdin, file_offset + (line_end - buf) + 1, SEEK_SET);

	exit(bytes_read == 0 ? 1 : 0);
	return (bytes_read == 0 ? 1 : 0);
}