I tried to load a 10Gb .raw file using fopen() function in c. My compiler is gcc , but i cant load the file, always the fopen() function returns a NULL. Can anyone help me to solve this problem. I tried a lot, but i didn't get a solution.
Large file support with -D_FILE_OFFSET_BITS=64 puts you in a situation where fseek and ftell aren't very good, because you can now have file offsets that don't fit in a long.
You have to change your longs to off_ts, your fseeks to fseekos, and your ftells to ftellos. Those are less portable interfaces (POSIX, not STDC) but that is unavoidable.
Large file support, ILP32 ABI, or the ISO C seeking functions: you have to throw one away to have the other 2.
Related
I know fgetpos/fsetpos are used for returning to a file position.
But if I accessed that position with fseek to begin with, is it more efficient to use fgetpos/fsetpos to return later, or just the same fseek again?
Is fgetpos/fsetpos any faster than fseek?
For general file positioning, fseek()/ftell() are limited to files sizes about LONG_MAX. fsetpos()/fgetpos() are designed to handle the file system's file sizes.
For large files, fseek()/ftell() are not an option. #Thomas Padron-McCarthy
When coding C99 onward, robust code uses fsetpos()/fgetpos() in lieu of a minor optimization that may of may not be present using the more limited fseek()/ftell().
I'm using 64bit mingw to compile c code on windows x64.
I'm using fwrite to create binary files from memory array. I want to write ~20Gb calling this function but it just write until 1.4~1.5gb and then it stops writting (without crashing, just hangs there.... doing nothing).
Is there any solution? Right now I'm writing 20 files and then I merge them.
Opening the file as 'ab' works but I cant read the file properly if I use that mode.
Sample (pseudo)code:
short* dst= malloc(20GB);
*calculations to fill dst*
file=fopen("myfile",'wb');
fwrite(dst, sizeof(short), 20GB/sizeof(short), file);
fclose(file)
That program never ends and file size is never grater than 1.5GB
Write it in smaller chunks. For heaven's sake, don't try to malloc 20gb.
Depending on the environment (operating system, memory model, file system), it might not be possible to create a file greater than 2 GB. This is especially true with MSDOS file systems and of course could be true on any file system if there is insufficient disk space or allocation quota.
If you show your code, we could see if there is any intrinsic flaw in the algorithm and suggest alternatives.
Mingw is a 32 bit environment, there AFAIK does not exist a 64 bit variant.
It may be that fwrite() from mingw is unable to deal with more than 2 GB or 4GB unless mingw is large file aware.
If you can find something similar to truss(1), run your progran under this debugging tool. With the information you provided, it is not possible to give a better advise.
I've read posts that show how to use fseek and ftell to determine the size of a file.
FILE *fp;
long file_size;
char *buffer;
fp = fopen("foo.bin", "r");
if (NULL == fp) {
/* Handle Error */
}
if (fseek(fp, 0 , SEEK_END) != 0) {
/* Handle Error */
}
file_size = ftell(fp);
buffer = (char*)malloc(file_size);
if (NULL == buffer){
/* handle error */
}
I was about to use this technique but then I ran into this link that describes a potential vulnerability.
The link recommends using fstat instead. Can anyone comment on this?
The link is one of the many nonsensical pieces of C coding advice from CERT. Their justification is based on liberties the C standard allows an implementation to take, but which are not allowed by POSIX and thus irrelevant in all cases where you have fstat as an alternative.
POSIX requires:
that the "b" modifier for fopen have no effect, i.e. that text and binary mode behave identically. This means their concern about invoking UB on text files is nonsense.
that files have a byte-resolution size set by write operations and truncate operations. This means their concern about random numbers of null bytes at the end of the file is nonsense.
Sadly with all the nonsense like this they publish, it's hard to know which CERT publications to take seriously. Which is a shame, because lots of them are serious.
If your goal is to find the size of a file, definitely you should use fstat() or its friends. It's a much more direct and expressive method--you are literally asking the system to tell you the file's statistics, rather than the more roundabout fseek/ftell method.
A bonus tip: if you only want to know if the file is available, use access() rather than opening the file or even stat'ing it. This is an even simpler operation which many programmers aren't aware of.
The reason to not use fstat is that fstat is POSIX, but fopen, ftell and fseek are part of the C Standard.
There may be a system that implements the C Standard but not POSIX. On such a system fstat would not work at all.
I'd tend to agree with their basic conclusion that you generally shouldn't use the fseek/ftell code directly in the mainstream of your code -- but you probably shouldn't use fstat either. If you want the size of a file, most of your code should use something with a clear, direct name like filesize.
Now, it probably is better to implement that using fstat where available, and (for example) FindFirstFile on Windows (the most obvious platform where fstat usually won't be available).
The other side of the story is that many (most?) of the limitations on fseek with respect to binary files actually originated with CP/M, which didn't explicitly store the size of a file anywhere. The end of a text file was signaled by a control-Z. For a binary file, however, all you really knew was what sectors were used to store the file. In the last sector, you had some amount of unused data that was often (but not always) zero-filled. Unfortunately, there might be zeros that were significant, and/or non-zero values that weren't significant.
If the entire C standard had been written just before being approved (e.g., if it had been started in 1988 and finished in 1989) they'd probably have ignored CP/M completely. For better or worse, however, they started work on the C standard in something like 1982 or so, when CP/M was still in wide enough use that it couldn't be ignored. By the time CP/M was gone, many of the decisions had already been made and I doubt anybody wanted to revisit them.
For most people today, however, there's just no point -- most code won't port to CP/M without massive work; this is one of the relatively minor problems to deal with. Making a modern program run in only 48K (or so) of memory for both the code and data is a much more serious problem (having a maximum of a megabyte or so for mass storage would be another serious problem).
CERT does have one good point though: you probably should not (as is often done) find the size of a file, allocate that much space, and then assume the contents of the file will fit there. Even though the fseek/ftell will give you the correct size with modern systems, that data could be stale by the time you actually read the data, so you could overrun your buffer anyway.
According to C standard, §7.21.3:
Setting the file position indicator to end-of-file, as with fseek(file,
0, SEEK_END), has undefined behavior for a binary stream (because of
possible trailing null characters) or for any stream with
state-dependent encoding that does not assuredly end in the initial
shift state.
A letter-of-the-law kind of guy might think this UB can be avoided by calculating file size with:
fseek(file, -1, SEEK_END);
size = ftell(file) + 1;
But the C standard also says this:
A binary stream need not meaningfully support fseek calls with a
whence value of SEEK_END.
As a result, there is nothing we can do to fix this with regard to fseek / SEEK_END. Still, I would prefer fseek / ftell instead of OS-specific API calls.
I have to analyze a 16 GB file. I am reading through the file sequentially using fread() and fseek(). Is it feasible? Will fread() work for such a large file?
You don't mention a language, so I'm going to assume C.
I don't see any problems with fread, but fseek and ftell may have issues.
Those functions use long int as the data type to hold the file position, rather than something intelligent like fpos_t or even size_t. This means that they can fail to work on a file over 2 GB, and can certainly fail on a 16 GB file.
You need to see how big long int is on your platform. If it's 64 bits, you're fine. If it's 32, you are likely to have problems when using ftell to measure distance from the start of the file.
Consider using fgetpos and fsetpos instead.
Thanks for the response. I figured out where I was going wrong. fseek() and ftell() do not work for files larger than 4GB. I used _fseeki64() and _ftelli64() and it is working fine now.
If implemented correctly this shouldn't be a problem. I assume by sequentially you mean you're looking at the file in discrete chunks and advancing your file pointer.
Check out http://www.computing.net/answers/programming/using-fread-with-a-large-file-/10254.html
It sounds like he was doing nearly the same thing as you.
It depends on what you want to do. If you want to read the whole 16GB of data in memory, then chances are that you'll run out of memory or application heap space.
Rather read the data chunk by chunk and do processing on those chunks (and free resources when done).
But, besides all this, decide which approach you want to do (using fread() or istream, etc.) and do some test cases to see which works better for you.
If you're on a POSIX-ish system, you'll need to make sure you've built your program with 64-bit file offset support. POSIX mandates (or at least allows, and most systems enforce this) the implementation to deny IO operations on files whose size don't fit in off_t, even if the only IO being performed is sequential with no seeking.
On Linux, this means you need to use -D_FILE_OFFSET_BITS=64 on the gcc command line.
I have a huge binary file which is 2148181087 bytes (> 2gb)
I am trying to do fopen (file, "r") and it failed with
Can not open: xyz file (Value too
large to be stored in data type)
I read on the man page EOVERFLOW error is received when the file size > 2gb.
The weird thing is, I use a different input file which is also "almost" as big as the first file 2142884400 bytes (also >2gb), fopen works fine with this.
Is there any cutoff on the file size for fopen or is there any alternate way to solve this?
The cutoff is 2GB which, contrary to what you may think, is not 2,000,000,000 (2x10003).
It's 2,147,483,648 (2x10243). So your second file, which works, is actually less than 2GB in size).
2GB, in the computer world, is only 2,000,000,000 in the minds of hard drive manufacturers so they can say their disks are bigger than they really are :-) - it lets them say their disks are actually 2.1GB.
The "alternative way to solve this" depends on which operating system/library you are using.
For the GNU C library, you can use fopen64 as a replacement for fopen; it uses 64-bit file handles (there's also a macro to have fopen use 64-bit file handles).
For Windows, you'll probably have to switch to the Win32 file management API, with which you can use CreateFile.