CODEWITHSUNDEEP
cfreadfseek
user893569
user893569

Reputation: 1

Binary file reading

I am dealing with a code which reading data from a binary file. The code is given here. Would anyone please make clear to me the role of fseek and fread here.

fc = fopen(CLOUDS_FILE, "rb");
if (fc == NULL){ fputs("File open error.\n", stderr); exit(1); }
crs = aux[CLRS];
fpos = (int) (pixel[2]*crs*crs + pixel[1]*crs + pixel[0]);
flsz = sizeof(fd);
fseek(fc, fpos*flsz, 0);
rd = fread((void *) &fd, flsz, 1, fc);
if (rd != 1){ fputs("Read error.\n", stderr); exit(1); }
fclose(fc);

Upvotes: 0

Views: 448

Answers (4)

Another Code
Another Code

Reputation: 3151

The binary file reading is done with an internal 'pointer', just like text editors have a cursor position when editing something. When opening the file in reading mode (using fopen) the pointer will be at the beginning of the file. Read operations (like fread, which will read a specified number of bytes from the stream) start reading at the pointer position and usually advance the pointer when they're done. If it is only necessary to read a specific part of the file, it is possible to manually set the pointer to a certain (relative or absolute) position, this is what fseek is used for.

Upvotes: 1

janneb
janneb

Reputation: 37208

fseek() changes the file offset. fread() reads data starting from the current offset, incrementing the offset by the number of elements read.

(Or is the question something else entirely? I mean, the above is something one can trivially figure by reading the manpages)

Upvotes: 1

tbert
tbert

Reputation: 2097

Sure, fseek is forwarding the "read from" index in the file to a calculated offset in CLOUDS_FILE, while fread is reading one object of size sizeof(fd) (whatever fd is, as that's not in your pasted code) into fd.

Upvotes: 0

Zoozy
Zoozy

Reputation: 1325

   #include <stdio.h>

   int fseek(FILE *stream, long offset, int whence);

The fseek() function sets the file position indicator for the stream pointed to by stream. The new position, measured in bytes, is obtained by adding offset bytes to the position specified by whence. If whence is set to SEEK_SET, SEEK_CUR, or SEEK_END, the offset is relative to the start of the file, the current position indicator, or end-of-file, respectively. 

   #include <stdio.h>

   size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream);

The function fread() reads nmemb elements of data, each size bytes long, from the stream pointed to by stream, storing them at the loca‐ tion given by ptr.

Upvotes: 0

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