Question

I’m trying to read a file into a buffer in blocks of size BLOCK_SIZE (currently equal to 1000 unsigned chars). My code initially finds the number of blocks it will have to read in order to read the entire file (usually 2-4), then iterates through a for loop reading the file (ignore the “+17+filenamesize” stuff, that is all needed for later in the program.

However, only on the first time, when j=1, does it actually put data into the buf array. In other cases, when j != 1, strlen(buf) returns 0.

I think the problem is either with the use of fseek() to seek to the second part of a file before reading it or a memory allocation issue.

Any help would be appreciated for getting it to read the 1000-1999^th chars of the file into the buf array.

Attached is the relevant part of the code:

unsigned char *buf;
source = fopen(localpath,"r");
temp = filesize / BLOCK_SIZE + 1;

for (j=1; j <= temp; j++) {
  if (j == 1) {
     buf = (unsigned char *) malloc((sizeof(unsigned char)) * (BLOCK_SIZE + 17 + filenamesize));
     fread(buf+17+filenamesize, sizeof(unsigned char), BLOCK_SIZE, source);
   } else if (j == temp) {
     buf = (unsigned char *) malloc((sizeof(unsigned char)) * (filesize + 5 - BLOCK_SIZE*(j-1)));
     fseek(source, BLOCK_SIZE*(j-1), SEEK_SET); // off by one warning
     fread(buf+5, sizeof(unsigned char), filesize - BLOCK_SIZE*(j-1), source);
   } else {
     buf = (unsigned char *) malloc((sizeof(unsigned char)) * (5+BLOCK_SIZE*(j-1)));
     fseek(source, BLOCK_SIZE*(j-1), SEEK_SET); // off by one warning
     fread(buf+5, sizeof(unsigned char), BLOCK_SIZE, source);
   }
   // do stuff with buf here

   buf = "";
   free(buf);
}

Answer 1

I would recommend checking the results of fseek and fread. In particular, make sure fseek is returning 0 – if it’s not, this may be the entire problem.

Provided that fseek is succeeding, fread should tell you the total number of bytes read.

Also, strlen is not necessarily a valid thing to use, since it’s going to assume that this is a null terminated string. If the first character you read is a 0 byte, strlen will return 0. You’re not treating this as a null terminated string (you aren’t allocating enough space for teh null terminator – just exactly what’s needed to fit your binary data), so strlen is probably inappropriate.