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C Pitfalls
C is a powerful language that offers almost absolute control and maximum performance which necessarily comes with responsibility and danger of shooting oneself in the foot. Without knowledge of the pitfalls you may well find yourself fallen into one of them.
Unless specified otherwise, this article supposes the C99 standard of the C language.
Undefined/Unspecified Behavior
Undefined, unspecified and implementation-defined behaviors are kinds of unpredictable and sometimes non-intuitive behavior of certain operations that may differ between compilers, platforms or runs because they are not defined by the language specification; this is mostly done on purpose as to allow some implementation freedom which allows implementing the language in a way that is most efficient on given platform. This behavior may be completely random (unpredictable) or implementation-specified (consistent within each implementation but potentially different between implementations). In any case, one has to be very careful about letting such behavior influence computations. Note that tools such as cppcheck can help find undefined behavior in code. Description of some of these behaviors follow.
Data type sizes including int and char may not be the same on each platform. Even though we almost take it for granted than char is 8 bits wide, in theory it can be wider. The int (and unsigned int) type width should reflect the architectures native integer type, so nowadays mostly it's mostly 32 or 64 bits. To deal with this we can use the standard library limits.h and stdint.h headers.
No specific endianness is enforced. Nowadays little endian is what you'll encounter on most platforms, but e.g. PowerPC uses big endian.
Order of evaluation of operands and function arguments is not specified. I.e. in an expression or function call it is not defined which operands or arguments will be evaluated first, the order may be completely random and the order may differ even when evaluating the same expression at another time. This is demonstrated by the following code:
#include <stdio.h>
int x = 0;
int a(void)
{
x += 1;
return x;
}
int main(void)
{
printf("%d %d\n",x,a()); // may print 0 1 or 1 1
return 0;
}
Overflow behavior of signed type operations is not specified. Sometimes we suppose that e.g. addition of two signed integers that are past the data type's limit will produce two's complement overflow, but in fact this operation's behavior is undefined, C99 doesn't say what representation should be used for numbers. For portability, predictability and safety it is safer to use unsigned types.
Bit shifts by type width or more are undefined. Also bit shifts by negative values are undefined. So e.g. x >> 8 is undefined if width of the data type of x is 8 bits.
Char data type signedness is not defined. The signedness can be explicitly "forced" by specifying signed char or unsigned char.
Memory Unsafety
Besides being extra careful about writing memory safe code, one needs to also know that some functions of the standard library are memory unsafe. This is regarding mainly string functions such as strcpy or strlen which do not check the string boundaries (i.e. they rely on not being passed a string that's not zero terminated and so can potentially touch memory anywhere beyond); safer alternatives are available, they have an n added in the name (strncpy, strnlen, ...) and allow specifying a length limit.
Different Behavior Between C And C++
C is not a subset of C++, i.e. not every C program is a C++ program (for simple example imagine a C program in which we use the word class as an identifier). Furthermore a C program that is at the same time also a C++ program may behave differently when compiled as C vs C++. Of course, all of this may also apply between different standards of C, not just between C and C++.
For portability sake it is good to try to write C code that will also compile as C++ (and behave the same). For this we should know some basic differences in behavior between C and C++.
TODO: specific examples