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This week, we're updating our course layouts and descriptions. Presentation and materials always gently change over time, but just occasionally there's a need to make a step change to clear out some of the old and roll in the new. That's now happening - but over a long and complex site it's not instant and you'll see sections of the site changing up to and including 19th September.

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Reading and writing files in C

In C you can use either open or fopen to open a file. open is what you would use for binary files, and you have plenty of low level controls; use fopen for text files.

When you fopen a file, you'll get back a pointer to a structure of type FILE which you then pass into function such as fgets and fprintf.

Each call to fgets reads the next line from the incoming file (up to a certain number of characters) and puts it into a character array. There's no need to move any pointers on to say where in the file you're going to read from - that's automatic as fgets is what we call an "iterator".

fgets returns the number of characters it has read; if you have an empty line in the incoming file you still get "1" back as there's a new line character (which is also included in the string you get). "0" indicates that there is no more data - i.e. you have reached the end of the file - and so that's how you test to know when to stop.

Remember that (in C) a character string has a null on the end of it, so if you're telling your program to read up to (say) 128 characters in a line, you have to dimension you char array to at least 129 ... otherwise you get undefined (and potentially nasty) results if the file you're reading has any lines longer than your maximum.

Finally, you should close your file(s) with fclose. For a file you're reading / files in programs that run quickly, you won't notice any difference if you forget. If you're writing to files in long-running programs, you may have more of an issue, as information that you think you have output may not reach the file until the program ends.

Example showing all of this ... [HERE] from the Learning to program in C course just concluded!

(written 2010-01-12, updated 2010-01-13)

Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
C210 - C and C based languages - File Handling
  [3386] Adding the pieces together to make a complete language - C - (2011-08-11)
  [3122] When is a program complete? - (2011-01-06)
  [2572] The what and why of C pointers - (2010-01-13)
  [2002] New C Examples - pointers, realloc, structs and more - (2009-01-20)

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Function Prototypes in C
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