Exactly what it sounds like, assuming you're used to the abbreviated way in which C and UNIX assigns words, it duplicates strings :-)
Keeping in mind it's actually not part of the ISO C standard itself(a) (it's a POSIX thing), it's effectively doing the same as the following code:
char *strdup(const char *src) {
char *dst = malloc(strlen (src) + 1); // Space for length plus nul
if (dst == NULL) return NULL; // No memory
strcpy(dst, src); // Copy the characters
return dst; // Return the new string
}
In other words:
It tries to allocate enough memory to hold the old string (plus a '\0' character to mark the end of the string).
If the allocation failed, it sets errno
to ENOMEM
and returns NULL
immediately. Setting of errno
to ENOMEM
is something malloc
does in POSIX so we don't need to explicitly do it in our strdup
. If you're not POSIX compliant, ISO C doesn't actually mandate the existence of ENOMEM
so I haven't included that here(b).
Otherwise the allocation worked so we copy the old string to the new string(c) and return the new address (which the caller is responsible for freeing at some point).
Keep in mind that's the conceptual definition. Any library writer worth their salary may have provided heavily optimised code targeting the particular processor being used.
(a) However, functions starting with str
and a lower case letter are reserved by the standard for future directions. From C11 7.1.3 Reserved identifiers
:
Each header declares or defines all identifiers listed in its associated sub-clause, and *optionally declares or defines identifiers listed in its associated future library directions sub-clause.**
The future directions for string.h
can be found in C11 7.31.13 String handling <string.h>
:
Function names that begin with str
, mem
, or wcs
and a lowercase letter may be added to the declarations in the <string.h>
header.
So you should probably call it something else if you want to be safe.
(b) The change would basically be replacing if (d == NULL) return NULL;
with:
if (d == NULL) {
errno = ENOMEM;
return NULL;
}
(c) Note that I use strcpy
for that since that clearly shows the intent. In some implementations, it may be faster (since you already know the length) to use memcpy
, as they may allow for transferring the data in larger chunks, or in parallel. Or it may not :-) Optimisation mantra #1: "measure, don't guess".
In any case, should you decide to go that route, you would do something like:
char *strdup(const char *src) {
size_t len = strlen(src) + 1; // String plus '\0'
char *dst = malloc(len); // Allocate space
if (dst == NULL) return NULL; // No memory
memcpy (dst, src, len); // Copy the block
return dst; // Return the new string
}
Microsoft decided to mark the name without underscore deprecated, because those names are reserved for the programmer to choose. Implementation specific extensions should use names starting with an underscore in the global namespace if they want to adhere to the C or C++ Standard - or they should mark themselves as a combined Standard compliant environment, such as POSIX/ANSI/ISO C, where such a function then corresponds to one of those Standards.
Read this answer about getcwd() too, for an explanation by P. J. Plauger, who knows stuff very well, of course.
If you are only interested to wait for some keys typed by the user, there really is no reason not to use getchar
. But sometimes it's just more practical and convenient to the user to use _getch
and friends. However, those are not specified by the C or C++ Standard and will thus limit the portability of your program. Keep that in mind.
Best Answer
strdup
is a perfectly correct POSIX function. Nevertheless, it doesn't belong to the standard, and the ANSI C standard reserves some (broad) classes of function names for further use. Among these, there aretherefore, the MS guys decided to replace
strdup
with_strdup
.I'd just continue using
strdup
. It's unlikely the C committee will definestrdup
to something else than POSIX. Either#define strdup _strdup
or silence the warning.BTW I hope you see this applies to your functions with names like
string_list
etc., too.