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comp.lang.c FAQ list · Question 2.11

Q: How can I read/write structures from/to data files?

A: It is relatively straightforward to write a structure out using fwrite:

	fwrite(&somestruct, sizeof somestruct, 1, fp);
and a corresponding fread invocation can read it back in. What happens here is that fwrite receives a pointer to the structure, and writes (or fread correspondingly reads) the memory image of the structure as a stream of bytes. The sizeof operator determines how many bytes the structure occupies.

(The call to fwrite above is correct under an ANSI compiler as long as a prototype for fwrite is in scope, usually because <stdio.h> is #included.

However, data files written as memory images in this way will not be portable, particularly if they contain floating-point fields or pointers. The memory layout of structures is machine and compiler dependent. Different compilers may use different amounts of padding (see question 2.12), and the sizes and byte orders of fundamental types vary across machines. Therefore, structures written as memory images cannot necessarily be read back in by programs running on other machines (or even compiled by other compilers), and this is an important concern if the data files you're writing will ever be interchanged between machines. See also questions 2.12 and 20.5.

Also, if the structure contains any pointers (char * strings, or pointers to other data structures), only the pointer values will be written, and they are most unlikely to be valid when read back in. Finally, note that for widespread portability you must use the "b" flag when opening the files; see question 12.38.

A more portable solution, though it's a bit more work initially, is to write a pair of functions for writing and reading a structure, field-by-field, in a portable (perhaps even human-readable) way.

References: H&S Sec. 15.13 p. 381

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