Loading...
Searching...
No Matches
Better String library

by Paul Hsieh

The bstring library is an attempt to provide improved string processing functionality to the C language. At the heart of the bstring library (Bstrlib for short) is the management of bstrings which are a significant improvement over NULL terminated char buffers.

# Motivation

The standard C string library has serious problems:

1. Its use of NULL to denote the end of the string means knowing a string's length is O(n) when it could be O(1).
2. It imposes an interpretation for the character value NULL.
3. gets always exposes the application to a buffer overflow.
4. strtok modifies the string its parsing and thus may not be usable in programs which are re-entrant or multithreaded.
5. fgets has the unusual semantic of ignoring NULLs that occur before newline characters are consumed.
6. There is no memory management, and actions performed such as strcpy, strcat and sprintf are common places for buffer overflows.
7. strncpy doesn't NULL terminate the destination in some cases.
8. Passing NULL to C library string functions causes an undefined NULL pointer access.
9. Parameter aliasing (overlapping, or self-referencing parameters) within most C library functions has undefined behavior.
10. Many C library string function calls take integer parameters with restricted legal ranges. Parameters passed outside these ranges are not typically detected and cause undefined behavior.

So the desire is to create an alternative string library that does not suffer from the above problems and adds in the following functionality:

1. Incorporate string functionality seen from other languages.
   - MID from BASIC
   - split/join from Python
   - string/char x n from Perl
2. Implement analogs to functions that combine stream IO and char buffers without creating a dependency on stream IO functionality.
3. Implement the basic text editor-style functions insert, delete, find, and replace.
4. Implement reference based sub-string access (as a generalization of pointer arithmetic.)
5. Implement runtime write protection for strings.

There is also a desire to avoid "API-bloat." So functionality that can be implemented trivially in other functionality is omitted. So there is no left or right or reverse or anything like that as part of the core functionality.

# Explaining Bstrings

A bstring is basically a header which wraps a pointer to a char buffer. Let's start with the declaration of a struct tagbstring:

    struct tagbstring {
        int mlen;
        int slen;
        unsigned char * data;
    };

This definition is considered exposed, not opaque (though it is neither necessary nor recommended that low level maintenance of bstrings be performed whenever the abstract interfaces are sufficient). The mlen field (usually) describes a lower bound for the memory allocated for the data field. The slen field describes the exact length for the bstring. The data field is a single contiguous buffer of unsigned chars. Note that the existence of a NULL character in the unsigned char buffer pointed to by the data field does not necessarily denote the end of the bstring.

To be a well formed modifiable bstring the mlen field must be at least the length of the slen field, and slen must be non-negative. Furthermore, the data field must point to a valid buffer in which access to the first mlen characters has been acquired. So the minimal check for correctness is:

    (slen >= 0 && mlen >= slen && data != NULL)

Bstrings returned by bstring functions can be assumed to be either NULL or satisfy the above property. (When bstrings are only readable, the mlen >= slen restriction is not required; this is discussed later in this section.) A bstring itself is just a pointer to a struct tagbstring:

    typedef struct tagbstring *bstring;

Bstrlib basically manages bstrings allocated as a header and an associated data-buffer. Since the implementation is exposed, they can also be constructed manually. Functions which mutate bstrings assume that the header and data buffer have been malloced; the bstring library may perform free or realloc on both the header and data buffer of any bstring parameter. Functions which return bstring's create new bstrings. The string memory is freed by a bdestroy call (or using the bstrFree macro).

Since bstrings maintain interoperability with C library char-buffer style strings, all functions which modify, update or create bstrings also append a NULL character into the position slen + 1. This trailing NULL character is not required for bstrings input to the bstring functions; this is provided solely as a convenience for interoperability with standard C char-buffer functionality.

Analogs for the ANSI C string library functions have been created when they are necessary, but have also been left out when they are not. In particular there are no functions analogous to fwrite, or puts just for the purposes of bstring. The data member of any string is exposed, and therefore can be used just as easily as char buffers for C functions which read strings.

For those that wish to hand construct bstrings, the following should be kept in mind:

1. While bstrlib can accept constructed bstrings without terminating NULL characters, the rest of the C language string library will not function properly on such non-terminated strings. This is obvious but must be kept in mind.
2. If it is intended that a constructed bstring be written to by the bstring library functions then the data portion should be allocated by the malloc function and the slen and mlen fields should be entered properly. The struct tagbstring header is not reallocated, and only freed by bdestroy.
3. Writing arbitrary NULL characters at various places in the string will not modify its length as perceived by the bstring library functions. In fact, NULL is a legitimate non-terminating character for a bstring to contain.
4. For read only parameters bstring functions do not check the mlen, i.e., the minimal correctness requirements are reduced to (slen >= 0 && data != NULL).

# Better Pointer Arithmetic

One built-in feature of NULL terminated char * strings, is that its very easy and fast to obtain a reference to the tail of any string using pointer arithmetic. Bstrlib does one better by providing a way to get a reference to any substring of a bstring (or any other length delimited block of memory). So rather than just having pointer arithmetic, with bstrlib one essentially has segment arithmetic. This is achieved using the macro blk2tbstr which builds a reference to a block of memory and the macro bmid2tbstr which builds a reference to a segment of a bstring. Bstrlib also includes functions for direct consumption of memory blocks into bstrings, namely bcatblk and blk2bstr.

One scenario where this can be extremely useful is when string contains many substrings which one would like to pass as read-only reference parameters to some string consuming function without the need to allocate entire new containers for the string data. More concretely, imagine parsing a command line string whose parameters are space delimited. This can only be done for tails of the string with NULL terminated char * strings.

# Improved NULL Semantics and Error Handling

Unless otherwise noted, if a NULL pointer is passed as a bstring or any other detectably illegal parameter, the called function will return with an error indicator (either NULL or BSTR_ERR) rather than simply performing a NULL pointer access, or having undefined behavior.

To illustrate the value of this, consider the following example:

    strcpy(p = malloc (13 * sizeof(char)), "Hello,");
    strcat(p, " World");

This is not correct because malloc may return NULL (due to an out of memory condition), and the behaviour of strcpy is undefined if either of its parameters are NULL. The following however, is well defined:

    bstrcat(p = bfromcstr("Hello,"), q = bfromcstr(" World"));
    bdestroy(q);

If either p or q are assigned NULL (indicating a failure to allocate memory) both bstrcat and bdestroy will recognize it and perform no detrimental action.

Note that it is not necessary to check any of the members of a returned bstring for internal correctness (in particular the data member does not need to be checked against NULL when the header is non-NULL), since this is assured by the bstring library itself.

# bStreams

In addition to the bgets and bread functions, bstrlib can abstract streams with a high performance read only stream called a bStream. In general, the idea is to open a core stream (with something like fopen) then pass its handle as well as a bNread function pointer (like fread) to the bsopen function which will return a handle to an open bStream. Then the functions bsread, bsreadln or bsreadlns can be called to read portions of the stream. Finally, the bsclose function is called to close the bStream – it will return a handle to the original (core) stream. So bStreams, essentially, wrap other streams.

The bStreams have two main advantages over the bgets and bread (as well as fgets/ungetc) paradigms:

1. Improved functionality via the bunread function (which allows a stream to unread characters) and giving the bStream stack-like functionality if so desired.
2. A very high performance bsreadln function. The C library function fgets (and the bgets function) can typically be written as a loop on top of fgetc, thus paying all of the overhead costs of calling fgetc on a per character basis. bsreadln will read blocks at a time, thus amortizing the overhead of fread calls over many characters at once.

However, clearly bStreams are suboptimal or unusable for certain kinds of streams, e.g. stdin, or certain usage patterns (a few spotty, or non-sequential reads from a slow stream). For those situations, using bgets is appropriate.

The semantics of bStreams allows practical construction of layerable data streams. What this means is that by writing a bNread compatible function on top of a bStream, one can construct a new bStream on top of it. This can be useful for writing multi-pass parsers that don't actually read the entire input more than once and don't require the use of intermediate storage.

# Aliasing

Aliasing occurs when a function is given two parameters which point to data structures which overlap in the memory they occupy. While this does not disturb read only functions, for many libraries this can make functions that write to these memory locations malfunction. This is a common problem of the C standard library and especially the string functions in the C standard library.

The C standard string library is entirely chararacter by character oriented (as is bstring) which makes conforming implementations alias safe for some scenarios. However no actual detection of aliasing is typically performed, so it is easy to find cases where the aliasing will cause anomolous or undesirable behaviour (consider: strcat(p, p)). The C99 standard includes the restrict pointer modifier which allows the compiler to document and assume a no-alias condition on usage. However, only the most trivial cases can be caught (if at all) by the compiler at compile time, and thus there is no actual enforcement of non-aliasing.

Bstrlib, by contrast, permits aliasing and is completely aliasing safe, in the C99 sense of aliasing. That is to say, under the assumption that pointers of incompatible types from distinct objects can never alias, bstrlib is completely aliasing safe. (In practice this means that the data buffer portion of any bstring and header of any bstring are assumed to never alias). With the exception of the reference building macros, the library behaves as if all read-only parameters are first copied and replaced by temporary non-aliased parameters before any writing to any output bstring is performed (though actual copying is extremely rarely ever done).

Besides being a useful safety feature, bstring searching/comparison functions can improve to O(1) execution when aliasing is detected.

Note that aliasing detection and handling code in Bstrlib is generally extremely cheap. There is almost never any appreciable performance penalty for using aliased parameters.

# Reenterancy

Nearly every function in Bstrlib is a leaf function, and is completely reenterable with the exception of writing to common bstrings. The split functions which use a callback mechanism requires only that the source string not be destroyed by the callback function unless the callback function returns with an error status (note that Bstrlib functions which return an error do not modify the string in any way.) The string can in fact be modified by the callback and the behaviour is deterministic. See the documentation of the various split functions for more details.

# Undefined Scenarios

One of the basic important premises for Bstrlib is to not to increase the propogation of undefined situations from parameters that are otherwise legal in of themselves. In particular, except for extremely marginal cases, usages of bstrings that use the bstring library functions alone cannot lead to any undefined action. But due to C language and library limitations, there is no way to define a non-trivial library that is completely without undefined operations. All such possible undefined operations are described below:

1. Bstrings or struct tagbstrings that are not explicitely initialized cannot be passed as a parameter to any bstring function.
2. The members of the NULL bstring cannot be accessed directly. (Though all APIs and macros detect the NULL bstring.)
3. A bstring whose data member has not been obtained from a malloc or compatible call and which is write accessible passed as a writable parameter will lead to undefined results, i.e., do not writeAllow any constructed bstrings unless the data portion has been obtained from the heap.
4. If the headers of two strings alias but are not identical (which can only happen via a defective manual construction) then passing them to a bstring function in which one is writable is not defined.
5. If the mlen member is larger than the actual accessible length of the data member for a writable bstring, or if the slen member is larger than the readable length of the data member for a readable bstring, then the corresponding bstring operations are undefined.
6. Any bstring definition whose header or accessible data portion has been assigned to inaccessible or otherwise illegal memory clearly cannot be acted upon by the bstring library in any way.
7. Destroying the source of an incremental split from within the callback and not returning with a negative value (indicating that it should abort) will lead to undefined behaviour, although modifying or adjusting the state of the source data (even if those modifications fail within the bstrlib API) has well defined behavior.
8. Modifying a bstring which is write protected by direct access has undefined behavior.

While this may seem like a long list (with the exception of invalid uses of the writeAllow macro) and source destruction during an iterative split without an accompanying abort, no usage of the bstring API alone can cause any undefined scenario to occur. The policy of restricting usage of bstrings to the bstring API can significantly reduce the risk of runtime errors (in practice it should eliminate them) related to string manipulation due to undefined action.

# Multithreading

A mutable bstring is kind of analogous to a small (two entry) linked list allocated by malloc, with all aliasing completely under programmer control. Manipulation of one bstring will never affect any other distinct bstring unless explicitely constructed to do so by the programmer via hand construction or via building a reference. Bstrlib also does not use any static or global storage, so there are no hidden unremovable race conditions. Bstrings are also clearly not inherently thread local. So just like char *, bstrings can be passed around from thread to thread and shared and so on, so long as modifications to a bstring correspond to some kind of exclusive access lock as should be expected (or if the bstring is read-only, which can be enforced by bstring write protection) for any sort of shared object in a multithreaded environment.

# Problems Not Solved

Bstrlib is written for the C language, which has inherent weaknesses that cannot be easily solved:

1. Memory leaks: Forgetting to call bdestroy on a bstring that is about to be unreferenced, just as forgetting to call free on a heap buffer that is about to be dereferenced. Though bstrlib itself is leak free.
2. Read before write usage: In C, declaring an auto bstring does not automatically fill it with legal/valid contents. (The bstrDeclare and bstrFree macros from bstraux can be used to help mitigate this problem).

Other problems not addressed:

1. Built-in mutex usage to automatically avoid all bstring internal race conditions in multitasking environments: The problem with trying to implement such things at this low a level is that it is typically more efficient to use locks in higher level primitives. There is also no platform independent way to implement locks or mutexes.
2. Unicode/wide character support.

> Note: except for spotty support of wide characters, the default C standard library does not address any of these problems either.

# Unicode functions

The two modules utf8util.c and buniutil.c implement basic functions for parsing and collecting Unicode data in the UTF8 format. Unicode is described by a sequence of "code points" which are values between 0 and 1114111 inclusive mapped to symbol content corresponding to nearly all the standardized scripts of the world.

The semantics of Unicode code points is varied and complicated. The base support of the better string library does not attempt to perform any interpretation of these code points. The better string library solely provides support for iterating through unicode code points, appending and extracting code points to and from bstrings, and parsing UTF8 and UTF16 from raw data.

The types cpUcs4 and cpUcs2 respectively are defined as 4 byte and 2 byte encoding formats corresponding to UCS4 and UCS2 respectively. To test if a raw code point is valid, the macro isLegalUnicodeCodePoint() has been defined. The utf8 iterator is defined by struct utf8Iterator. To test if the iterator has more code points to walk through the macro utf8IteratorNoMore() has been defined.

# The bstest Module

The bstest module is just a unit test for the bstrlib module. For correct implementations of bstrlib, it should execute with 0 failures being reported. This test should be utilized if modifications/customizations to bstrlib have been performed. It tests each core bstrlib function with bstrings of every mode (read-only, NULL, static and mutable) and ensures that the expected semantics are observed (including results that should indicate an error). It also tests for aliasing support. Passing bstest is a necessary but not a sufficient condition for ensuring the correctness of the bstrlib module.

# Using Bstring an Alternative to the C Library

First let us give a table of C library functions and the alternative bstring functions that should be used instead of them.

C-library | Bstring Alternative
--- | ---
gets | bgets
strcpy | bassign
strncpy | bassignmidstr
strcat | bconcat
strncat | bconcat+btrunc
strtok | bsplit,bsplits
sprintf | bformat
snprintf | bformat + btrunc
vsprintf | bvformata
vsnprintf | bvformata + btrunc
vfprintf | bvformata + fputs
strcmp | biseq, bstrcmp
strncmp | bstrncmp, memcmp
strlen | slen, blength
strdup | bstrcpy
strset | bpattern
strstr | binstr
strpbrk | binchr
stricmp | bstricmp
strlwr | btolower
strupr | btoupper
strrev | bReverse (aux module)
strchr | bstrchr
strspnp | usestrspn
ungetc | bsunread

The top 9 C functions listed here are troublesome in that they impose memory management in the calling function. The Bstring and CBstring interfaces have built-in memory management, so there is far less code with far less potential for buffer overrun problems. strtok can only be reliably called as a "leaf" calculation, since it (quite bizarrely) maintains hidden internal state. And gets is well known to be broken no matter what. The Bstrlib alternatives do not suffer from those sorts of problems.

The substitute for strncat can be performed with higher performance by using the blk2tbstr macro to create a presized second operand for bconcat.

C-library | Bstring alternative
--- | ---
strspn | strspn acceptable
strcspn | strcspn acceptable
strnset | strnset acceptable
printf | printf acceptable
puts | puts acceptable
fprintf | fprintf acceptable
fputs | fputs acceptable
memcmp | memcmp acceptable

Remember that Bstring functions will automatically append the NULL character to the character data buffer. So by simply accessing the data buffer directly, ordinary C string library functions can be called directly on them. Note that bstrcmp is not the same as memcmp in exactly the same way that strcmp is not the same as memcmp.

C-library | Bstring alternative
--- | ---
fread | balloc + fread
fgets | balloc + fgets

These are odd ones because of the exact sizing of the buffer required. The Bstring alternatives requires that the buffers are forced to hold at least the prescribed length, then just use fread or fgets directly. However, typically the automatic memory management of Bstring will make the typical use of fgets and fread to read specifically sized strings unnecessary.

# Implementation Choices

## Overhead

The bstring library has more overhead versus straight char buffers for most functions. This overhead is essentially just the memory management and string header allocation. This overhead usually only shows up for small string manipulations. The performance loss has to be considered in light of the following:

1. What would be the performance loss of trying to write this management code in one's own application?
2. Since the bstring library source code is given, a sufficiently powerful modern inlining globally optimizing compiler can remove function call overhead.

Since the data type is exposed, a developer can replace any unsatisfactory function with their own inline implementation. And that is besides the main point of what the better string library is mainly meant to provide. Any overhead lost has to be compared against the value of the safe abstraction for coupling memory management and string functionality.

## Performance of the C Interface

The algorithms used have performance advantages versus the analogous C library functions. For example:

1. bfromcstr/blk2str/bstrcpy versus strcpy/strdup

   By using memmove instead of strcpy, the break condition of the copy loop is based on an independent counter (that should be allocated in a register) rather than having to check the results of the load. Modern out-of-order executing CPUs can parallelize the final branch mis-predict penality with the loading of the source string. Some CPUs will also tend to have better built-in hardware support for counted memory moves than load-compare-store. This is a minor, but non-zero gain.

2. biseq versus strcmp

   If the strings are unequal in length, bsiseq will return in O(1) time. If the strings are aliased, or have aliased data buffers, biseq will return in O(1) time. strcmp will always be O(k), where k is the length of the common prefix or the whole string if they are identical.

3. slen versus strlen

   slen is obviously always O(1), while strlen is always O(n) where n is the length of the string.

4. bconcat versus strcat

   Both rely on precomputing the length of the destination string argument, which will favor the bstring library. On iterated concatenations the performance difference can be enormous.

5. bsreadln versus fgets

   The bsreadln function reads large blocks at a time from the given stream, then parses out lines from the buffers directly. Some C libraries will implement fgets as a loop over single fgetc calls. Testing indicates that the bsreadln approach can be several times faster for fast stream devices (such as a file that has been entirely cached.)

6. bsplits/bsplitscb versus strspn

   Accelerators for the set of match characters are generated only once.

7. binstr versus strstr

   The binstr implementation unrolls the loops to help reduce loop overhead. This will matter if the target string is long and source string is not found very early in the target string. With strstr, while it is possible to unroll the source contents, it is not possible to do so with the destination contents in a way that is effective because every destination character must be tested against NULL before proceeding to the next character.

8. bReverse versus strrev

   The C function must find the end of the string first before swaping character pairs.

9. bstrrchr versus no comparable C function

   Its not hard to write some C code to search for a character from the end going backwards. But there is no way to do this without computing the length of the string with strlen.

Practical testing indicates that in general Bstrlib is never signifcantly slower than the C library for common operations, while very often having a performance advantage that ranges from significant to massive. Even for functions like bninchr versus strspn (where, in theory, there is no advantage for the Bstrlib architecture) the performance of Bstrlib is vastly superior to most tested C library implementations.

Some of Bstrlib's extra functionality also lead to inevitable performance advantages over typical C solutions. For example, using the blk2tbstr macro, one can (in O(1) time) generate an internal substring by reference while not disturbing the original string. If disturbing the original string is not an option, typically, a comparable char * solution would have to make a copy of the substring to provide similar functionality. Another example is reverse character set scanning – the strspn functions only scan in a forward direction which can complicate some parsing algorithms.

Where high performance char * based algorithms are available, Bstrlib can still leverage them by accessing the data field on bstrings. So realistically Bstrlib can never be significantly slower than any standard NULL terminated char * based solutions.

## Memory Management

The bstring functions which write and modify bstrings will automatically reallocate the backing memory for the char buffer whenever it is required to grow. The algorithm for resizing chosen is to snap up to sizes that are a power of two which are sufficient to hold the intended new size. Memory reallocation is not performed when the required size of the buffer is decreased. This behavior can be relied on, and is necessary to make the behaviour of balloc deterministic. This trades off additional memory usage for decreasing the frequency for required reallocations:

1. For any bstring whose size never exceeds n, its buffer is not ever reallocated more than log2(n) times for its lifetime.
2. For any bstring whose size never exceeds n, its buffer is never more than 2 * (n + 1) in length. (The extra characters beyond 2 * n are to allow for the implicit NULL which is always added by the bstring modifying functions).

Decreasing the buffer size when the string decreases in size would violate 1. above and in real world case lead to pathological heap thrashing. Similarly, allocating more tightly than "least power of 2 greater than necessary" would lead to a violation of 1. and have the same potential for heap thrashing.

Property 2. needs emphasizing. Although the memory allocated is always a power of 2, for a bstring that grows linearly in size, its buffer memory also grows linearly, not exponentially. The reason is that the amount of extra space increases with each reallocation, which decreases the frequency of future reallocations.

Obviously, given that bstring writing functions may reallocate the data buffer backing the target bstring, one should not attempt to cache the data buffer address and use it after such bstring functions have been called. This includes making reference struct tagbstrings which alias to a writable bstring.

balloc or bfromcstralloc can be used to preallocate the minimum amount of space used for a given bstring. This will reduce even further the number of times the data portion is reallocated. If the length of the string is never more than one less than the memory length then there will be no further reallocations.

Note that invoking the bwriteallow macro may increase the number of reallocs by one more than necessary for every call to bwriteallow interleaved with any bstring API which writes to this bstring.

The library does not use any mechanism for automatic clean up for the C API. Thus explicit clean up via calls to bdestroy are required to avoid memory leaks.

## Constant and Static tagbstrings

A struct tagbstring can be write protected from any bstrlib function using the bwriteprotect macro. A write protected struct tagbstring can then be reset to being writable via the bwriteallow macro. There is, of course, no protection from attempts to directly access the bstring members. Modifying a bstring which is write protected by direct access has undefined behavior.

static struct tagbstrings can be declared via the bsStatic macro. They are considered permanently unwritable. Such struct tagbstrings's are declared such that attempts to write to it are not well defined. Invoking either bwriteallow or bwriteprotect on static struct tagbstrings has no effect.

struct tagbstrings initialized via btfromcstr or blk2tbstr are protected by default but can be made writeable via the bwriteallow macro. If bwriteallow is called on such struct tagbstrings, it is the programmer's responsibility to ensure that:

1. The buffer supplied was allocated from the heap.
2. bdestroy is not called on this tagbstring unless the header itself has also been allocated from the heap.
3. free is called on the buffer to reclaim its memory.

bwriteallow and bwriteprotect can be invoked on ordinary bstrings (they have to be dereferenced with the * operator to get the levels of indirection correct) to give them write protection.

## Buffer Declaration

The memory buffer is actually declared unsigned char * instead of char *. The reason for this is to trigger compiler warnings whenever uncasted char buffers are assigned to the data portion of a bstring. This will draw more diligent programmers into taking a second look at the code where they have carelessly left off the typically required cast. (Research from AT&T/Lucent indicates that additional programmer eyeballs is one of the most effective mechanisms at ferreting out bugs).

## Function Pointers

The bgets, bread and bStream functions use function pointers to obtain strings from data streams. The function pointer declarations have been specifically chosen to be compatible with the fgetc and fread functions. While this may seem to be a convoluted way of implementing fgets and fread style functionality, it has been specifically designed this way to ensure that there is no dependency on a single narrowly defined set of device interfaces, such as just stream I/O. In the embedded world, its quite possible to have environments where such interfaces may not exist in the standard C library form. Furthermore, the generalization that this opens up allows for more sophisticated uses for these functions (performing an fgets like function on a socket, for example.) By using function pointers, it also allows such abstract stream interfaces to be created using the bstring library itself while not creating a circular dependency.

## Use of int For Sizes

This is just a recognition that 16bit platforms with requirements for strings that are larger than 64K and 32bit+ platforms with requirements for strings that are larger than 4GB are pretty marginal. The main focus is for 32bit platforms, and emerging 64bit platforms with reasonable < 4GB string requirements. Using ints allows for negative values which has meaning internally to bstrlib.

## Semantic Consideration

Certain care needs to be taken when copying and aliasing bstrings. A bstring is essentially a pointer type which points to a multipart abstract data structure. Thus usage, and lifetime of bstrings have semantics that follow these considerations. For example:

    bstring a, b;
    struct tagbstring t;

    a = bfromcstr("Hello"); /* Create new bstring and copy "Hello" into it. */
    b = a;                  /* Alias b to the contents of a. */
    t = *a;                 /* Create a current instance pseudo-alias of a. */
    bconcat(a, b);          /* Double a and b, t is now undefined. */
    bdestroy(a);            /* Destroy the contents of both a and b. */

Variables of type bstring are really just references that point to real bstring objects. The equal operator creates aliases, and the asterisk dereference operator creates a kind of alias to the current instance (which is generally not useful for any purpose.) Using bstrcpy is the correct way of creating duplicate instances. The ampersand operator is useful for creating aliases to struct tagbstrings (remembering that constructed struct tagbstrings are not writable by default).

# Security

Bstrlib does not come with explicit security features outside of its fairly comprehensive error detection, coupled with its strict semantic support. That is to say that certain common security problems, such as buffer overrun, constant overwrite, arbitrary truncation etc, are far less likely to happen inadvertently. Where it does help, Bstrlib maximizes its advantage by providing developers a simple adoption path that lets them leave less secure string mechanisms behind. The library will not leave developers wanting, so they will be less likely to add new code using a less secure string library to add functionality that might be missing from Bstrlib.

That said there are a number of security ideas not addressed by Bstrlib:

1. Race condition exploitation (i.e., verifying a string's contents, then raising the privilege level and execute it as a shell command as two non-atomic steps) is well beyond the scope of what Bstrlib can provide. It should be noted that MFC's built-in string mutex actually does not solve this problem either – it just removes immediate data corruption as a possible outcome of such exploit attempts (it can be argued that this is worse, since it will leave no trace of the exploitation). In general race conditions have to be dealt with by careful design and implementation; it cannot be assisted by a string library.
2. Any kind of access control or security attributes to prevent usage in dangerous interfaces such as system. Perl includes a "trust" attribute which can be endowed upon strings that are intended to be passed to such dangerous interfaces. However, Perl's solution reflects its own limitations – notably that it is not a strongly typed language. In the example code for Bstrlib, there is a module called taint.cpp. It demonstrates how to write a simple wrapper class for managing "untainted" or trusted strings using the type system to prevent questionable mixing of ordinary untrusted strings with untainted ones then passing them to dangerous interfaces. In this way the security correctness of the code reduces to auditing the direct usages of dangerous interfaces or promotions of tainted strings to untainted ones.
3. Encryption of string contents is way beyond the scope of Bstrlib. Maintaining encrypted string contents in the futile hopes of thwarting things like using system-level debuggers to examine sensitive string data is likely to be a wasted effort (imagine a debugger that runs at a higher level than a virtual processor where the application runs). For more standard encryption usages, since the bstring contents are simply binary blocks of data, this should pose no problem for usage with other standard encryption libraries.

# Compatibility

The Better String Library is known to compile and function correctly with the following compilers:

- GNU C/C++
- Microsoft Visual C++
- Watcom C/C++
- Intel's C/C++ compiler (Windows)
- The GNU C/C++ compiler (cygwin and Linux on PPC64)
- Borland C
- Turbo C

Setting of configuration options should be unnecessary for these compilers (unless exceptions are being disabled or STLport has been added to WATCOM C/C++). Bstrlib has been developed with an emphasis on portability. As such porting it to other compilers should be straight forward. This package includes a porting guide (called porting.txt) which explains what issues may exist for porting Bstrlib to different compilers and environments.

# ANSI issues

1. The function pointer types bNgetc and bNread have prototypes which are very similar to, but not exactly the same as fgetc and fread respectively.

   Basically the FILE * parameter is replaced by void *. The purpose of this was to allow one to create other functions with fgetc and fread like semantics without being tied to ANSI C's file streaming mechanism, i.e., one could very easily adapt it to sockets, or simply reading a block of memory, or procedurally generated strings (for fractal generation, for example.)

   The problem is that invoking the functions bNgetc, fgetc, bNread, and fread is not technically legal in ANSI C. The reason being that the compiler is only able to coerce the function pointers themselves into the target type, however are unable to perform any cast (implicit or otherwise) on the parameters passed once invoked, i.e., if internally void * and FILE * need some kind of mechanical coercion, the compiler will not properly perform this conversion and thus lead to undefined behavior.

   Apparently a platform from Data General called "Eclipse" and another from Tandem called "NonStop" have a different representation for pointers to bytes and pointers to words, for example, where coercion via casting is necessary. (Actual confirmation of the existence of such machines is hard to come by, so it is prudent to be skeptical about this information). However, this is not an issue for any known contemporary platforms. One may conclude that such platforms are effectively apocryphal even if they do exist.

   To correctly work around this problem to the satisfaction of the ANSI limitations, one needs to create wrapper functions for fgets and/or fread with the prototypes of bNgetc and/or bNread respectively which performs no other action other than to explicitely cast the void * parameter to a FILE *, and simply pass the remaining parameters straight to the function pointer call.

   The wrappers themselves are trivial:

       size_t freadWrap(void *buff, size_t esz, size_t eqty, void *parm) {
           return fread(buff, esz, eqty, (FILE *) parm);
       }

       int fgetcWrap(void *parm) {
           return fgetc((FILE *)parm);
       }

   These have not been supplied in bstrlib or bstraux to prevent unnecessary linking with file I/O functions.

2. The bstrlib function names are not unique in the first 6 characters.

   This is only an issue for older C compiler environments which do not store more than 6 characters for function names.

# Examples

Dumping a line numbered file:

    FILE *fp;
    int i, ret;
    struct bstrList * lines;
    struct tagbstring prefix = bsStatic("-> ");

    if (NULL != (fp = fopen("bstrlib.txt", "rb"))) {
        bstring b = bread((bNread) fread, fp);
        fclose (fp);
        if (NULL != (lines = bsplit(b, '\n'))) {
            for (i=0; i < lines->qty; i++) {
                binsert(lines->entry[i], 0, &prefix, '?');
                printf("%04d: %s\n", i, bdatae(lines->entry[i], "NULL"));
            }
            bstrListDestroy(lines);
        }
        bdestroy(b);
    }

For numerous other examples, see bstraux.c, bstraux.h and the example archive.

# License

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
3. Neither the name of bstrlib nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

Alternatively, the contents of this file may be used under the terms of GNU General Public License Version 2 (the "GPL").

# Acknowledgements

The following individuals have made significant contributions to the design and testing of the Better String Library:

- Bjorn Augestad
- Clint Olsen
- Darryl Bleau
- Fabian Cenedese
- Graham Wideman
- Ignacio Burgueno
- International Business Machines Corporation
- Ira Mica
- John Kortink
- Manuel Woelker
- Marcel van Kervinck
- Michael Hsieh
- Richard A. Smith
- Simon Ekstrom
- Wayne Scott
- Zed A. Shaw