Read file into memory?

Question

I can seem to only read file into memory if I explicitly declare the buffer size. This works

#include <stdio.h>
int main(){
  FILE *fp = fopen("test.log", "rb");
  char buffer[37];
  fread(buffer, 1, 36, fp);
  printf("%s", buffer);
}

This will add junk to the output

#include <stdio.h>
int main(){
  FILE *fp = fopen("test.log", "rb");
  fseek(fp, 0, SEEK_END);
  long siz = ftell(fp);
  rewind(fp);
  char buffer[siz + 1];
  fread(buffer, 1, siz, fp);
  printf("%s", buffer);
}

Conceptually wrong. In C, any declaration of array should indicate the size at COMPILE time. If you want to create an array dynamically, use `malloc` — SwiftMango, Aug 04 '12 at 17:43
You open the file in binary mode, but print it as a string. This only works if the data is really text. Also, you do not terminate the buffer. Remember that when creating an array it's not automatically zeroed out. — Some programmer dude, Aug 04 '12 at 17:47
Maybe irrelevant as it is not assured what the data is actually, you are not `NUL` terminating your string. Try adding `\0` to the end of char array & see what it prints out — another.anon.coward, Aug 04 '12 at 17:50
I . . . have absolutely no idea how that second one compiled. — geometrian, Aug 04 '12 at 17:51
@IanMallett Why not? Variable length arrays are standard well over 10 years. — Daniel Fischer, Aug 04 '12 at 17:51
@texasbruce, IanMallett Please see [Variable Length Arrays (VLA)](http://en.wikipedia.org/wiki/Variable-length_array) — Marlon, Aug 04 '12 at 17:52
@texasbruce While declaring a VLA the size of a file is questionable, since the file can easily be too large for your stack, I see no problems with VLAs in principle, they're mighty fine when you know the size is not too large. — Daniel Fischer, Aug 04 '12 at 17:54
@StackUnderflow Thanks for the info. Didn't know the standard updated.. — SwiftMango, Aug 04 '12 at 18:46

score 7 · Accepted Answer · answered Aug 04 '12 at 17:49

7

insert buffer[siz]='\0'; before printf("%s", buffer);

answered Aug 04 '12 at 17:49

BLUEPIXY

39,699
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The terminating of the array should really be done in both cases. The OP is just lucky that the first version worked. – Some programmer dude Aug 05 '12 at 06:48
@JoachimPileborg : I think so too. – BLUEPIXY Aug 05 '12 at 08:19
There may also the compiler rather than luck. – BLUEPIXY Aug 05 '12 at 09:12

score 0 · Answer 2 · answered Aug 04 '12 at 17:52

0

Try a different approach - use a "memory map". What it does is it allows you to access the file as if it was a memory block. This can dramatically improve performance while simplifying your code at the same time.

Read more about it at http://en.wikipedia.org/wiki/Mmap

answered Aug 04 '12 at 17:52

YePhIcK

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This doesn't answer the question asked, and there's no indication that a memory-mapped file is needed here. The question specifically asked how to "read a file into memory", which is not what your answer does. There's a difference between "reading a file into memory and "treating a file like it's in memory"." – Ken White Aug 04 '12 at 17:53

Read file into memory?

2 Answers2