How does fread really work?

by

There may or may not be any difference in performance. There is a difference in semantics.

fread(a, 1, 1000, stdin);

attempts to read 1000 data elements, each of which is 1 byte long.

fread(a, 1000, 1, stdin);

attempts to read 1 data element which is 1000 bytes long.

They’re different because fread() returns the number of data elements it was able to read, not the number of bytes. If it reaches end-of-file (or an error condition) before reading the full 1000 bytes, the first version has to indicate exactly how many bytes it read; the second just fails and returns 0.

In practice, it’s probably just going to call a lower-level function that attempts to read 1000 bytes and indicates how many bytes it actually read. For larger reads, it might make multiple lower-level calls. The computation of the value to be returned by fread() is different, but the expense of the calculation is trivial.

There may be a difference if the implementation can tell, before attempting to read the data, that there isn’t enough data to read. For example, if you’re reading from a 900-byte file, the first version will read all 900 bytes and return 900, while the second might not bother to read anything. In both cases, the file position indicator is advanced by the number of characters successfully read, i.e., 900.

But in general, you should probably choose how to call it based on what information you need from it. Read a single data element if a partial read is no better than not reading anything at all. Read in smaller chunks if partial reads are useful.

More Related Contents:

  1. Usage of uint_8, uint_16 and uint_32
  2. C: Reading strings from binary file
  3. Unsuccessful fread() of int stored in binary file, segmentation fault [closed]
  4. Writing and reading (fwrite – fread) structures with pointers
  5. Does fread move the file pointer?
  6. How to pass arguments in __VA_ARGS to a 2d character array? [closed]
  7. C – list all files in current directory then move in directory above, list files, and so on until root directory is reached [closed]
  8. C/SDL – Segmentation fault when program run multiple times in succession [closed]
  9. Why does clang produce inefficient asm with -O0 (for this simple floating point sum)?
  10. What is the difference between ++i and i++?
  11. Casting a function pointer to another type
  12. C: Multiple scanf’s, when I enter in a value for one scanf it skips the second scanf [duplicate]
  13. C error: undefined reference to function, but it IS defined
  14. Get the current time in C [duplicate]
  15. How to detect a TCP socket disconnection (with C Berkeley socket)
  16. Going through a text file line by line in C
  17. Project Euler Question 14 (Collatz Problem)
  18. How to implement a timeout in read() function call?
  19. EOF in Windows command prompt doesn’t terminate input stream
  20. Calling a FORTRAN subroutine from C
  21. What are the useful GCC flags for C?
  22. Anonymous functions using GCC statement expressions
  23. Compound condition in C: if (0.0 < a < 1.0)
  24. Do threads have a distinct heap?
  25. Linking a C program directly with ld fails with undefined reference to `__libc_csu_fini`
  26. How is a CRC32 checksum calculated?
  27. Why does my compiler not accept fork(), despite my inclusion of ?
  28. How do you use offsetof() on a struct?
  29. How to concatenate string and int in C?
  30. Conditional tests in primality by trial division

Leave a Comment