Re: General method for dynamically allocating memory for a string

Randall wrote:
Frederick's code is hard to read and harder to learn from. Here is
some fully checked code with lots of comments.

///////////////////////////////////////////////////////////////////////////////
// file: substr.c
// Note: C++ style comments are allowed for C99 compliant compilers.
///////////////////////////////////////////////////////////////////////////////

Note: C99 compliant compilers are in very short supply. (Though
compilers that *claim* to be C99 compilers is a little higher and those
that accept // comments are even higher.)

#include <stdlib.h> // for malloc() and free()
#include <string.h> // for strncpy()
#include <sys/types.h> // for size_t
#include <stdio.h> // for printf()

[...]

/**
* Note: this function will return a newly allocated string. It
* is your responsibility to delete this memory to prevent a leak.
*
* param "string" - the string you want to extract a substring from.
* param "start" - the array index to begin your substring.

* param "start" - the array index to begin your substring.
* param "end" - the array index to terminate your substring.
*
* On Error: this function returns null;
*/
char * substr( char * string, size_t start, size_t end) {
// pointer to the substring on the heap
char *subString;

// calculate the total amount of memory needed
// to hold the substring.
// Algo: end - start + null terminator
size_t subStringSize = end - start + 1;

// request enough bytes to store the entire
// substring and null terminator.
subString = malloc( subStringSize );

// test to make sure we got the memory
// from malloc
if( subString != NULL ) {
// Note this copies one extra byte (the
// null terminator's spot) which is garbage.
// We have to terminate this string.
strncpy( subString, string + start, subStringSize );

This leads to UB, since you have not established that string + start is
a valid thing to be pointing at.

--
Paul Hsieh
http://www.pobox.com/~qed/
http://bstring.sf.net/

.

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