Autoconf, Automake, and Libtool: C Data Type Sizes

| ### 15.1.2 C Data Type Sizes The C language defines data types in terms of a minimum size, rather than an exact size. As of this writing, this mainly matters for the types `int` and `long`. A variable of type `int` must be at least 16 bits, and is often 32 bits. A variable of type `long` must be at least 32 bits, and is sometimes 64 bits. The range of a 16 bit number is -32768 to 32767 for a signed number, or 0 to 65535 for an unsigned number. If a variable may hold numbers larger than 16 bits, use `long` rather than `int`. Never assume that `int` or `long` have a specific size, or that they will overflow at a particular point. When appropriate, use variables of system defined types rather than `int` or `long`: `size_t` Use this to hold the size of an object, as returned by `sizeof`. `ptrdiff_t` Use this to hold the difference between two pointers into the same array. `time_t` Use this to hold a time value as returned by the `time` function. `off_t` On a Unix system, use this to hold a file position as returned by `lseek`. `ssize_t` Use this to hold the result of the Unix `read` or `write` functions. Some books on C recommend using typedefs to specify types of particular sizes, and then adjusting those typedefs on specific systems. GNU Autotools supports this using the ``AC_CHECK_SIZEOF'` macro. However, while we agree with using typedefs for clarity, we do not recommend using them purely for portability. It is safest to rely only on the minimum size assumptions made by the C language, rather than to assume that a type of a specific size will always be available. Also, most C compilers will define `int` to be the most efficient type for the system, so it is normally best to simply use `int` when possible.

This document was generated by Gary V. Vaughan on February, 8 2006 using texi2html