Use GNU versions of basename() and dirname() in C source - c

Question

How do I use the GNU C Library version of basename() and dirname()?.
If you
#include <libgen.h>
for dirname
You're already getting the POSIX, not the GNU, version of basename(). (Even if you
#define _GNU_SOURCE
As far as I know there is no conditional importing in C. Is there a gcc specific trick?

Answer 1

Just write it yourself and give it a different name than basename. This GNU insistence on creating alternate non-conforming versions of standard functions that can be written in 1-3 lines is completely batty.
char *gnu_basename(char *path)
{
char *base = strrchr(path, '/');
return base ? base+1 : path;
}
This way, your program will also be more portable.

Answer 2

According to the man page you should do
#define _GNU_SOURCE
#include <string.h>
If you get the POSIX version, libgen.h is probably already included before that point. You may want to include -D_GNU_SOURCE in the CPPFLAGS for compilation:
gcc -D_GNU_SOURCE ....
Compare: POSIX Version vs GNU Version on Compiler Explorer.

Answer 3

After examining libgen.h, I'm pretty sure I have a warning-free and error-free solution:
/* my C program */
#define _GNU_SOURCE /* for GNU version of basename(3) */
#include <libgen.h> /* for dirname(3) */
#undef basename /* (snide comment about libgen.h removed) */
#include <string.h> /* for basename(3) (GNU version) and strcmp(3) */
/* rest of C program... */
With the #undef line, now my program includes dirname(3) from libgen.h and the GNU version of basename(3) from string.h.
No compiler warnings/errors from either gcc (version 4.5.2) or clang (version 3.3).

Answer 4

Make sure you're building with the GNU C library, rather than your system's (presumed) POSIX-compatible default.
This is often set in the GCC spec file. Use the -v option to show the current settings:
$ gcc -v
Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro 4.4.4-14ubuntu5' --with-bugurl=file:///usr/share/doc/gcc-4.4/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.4 --enable-shared --enable-multiarch --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.4 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc --disable-werror --with-arch-32=i686 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.4.5 (Ubuntu/Linaro 4.4.4-14ubuntu5)

Answer 5

It's crazy basename and dirname have two versions.
We worked at a big project, it looks like these two apis already caused
potentially bugs. So we marked "basename" "dirname" as deprecated for warning if
someone use it:
#ifdef basename
__attribute__ ((deprecated))
char *__xpg_basename(char *path);
#else
__attribute__ ((deprecated))
char *basename(const char *path);
#endif
__attribute__ ((deprecated))
char *dirname(char *path);
We also try to introduce a base c foundation library such as glib or libcork,
but it looks like too heavy. So we write a tiny library for this purpose, it
implementation like this:
#include <libgen.h> // for dirname
#include <linux/limits.h> // for PATH_MAX
#include <stdio.h> // for snprintf
#include <string.h> // for basename
#include <stdbool.h> // for bool
bool get_basename(const char *path, char *name, size_t name_size) {
char path_copy[PATH_MAX] = {'\0'};
strncpy(path_copy, path, sizeof(path_copy) - 1);
return snprintf(name, name_size, "%s", basename(path_copy)) < name_size;
}
bool get_dirname(const char *path, char *name, size_t name_size) {
char path_copy[PATH_MAX] = {'\0'};
strncpy(path_copy, path, sizeof(path_copy) - 1);
return snprintf(name, name_size, "%s", dirname(path_copy)) < name_size;
}
Then we replace all basename dirname call with get_basename get_dirname.