Add support for the GNU extensions 'program_invocation_name' and
'program_invocation_short_name'. These are useful to print error
messages, which by convention include the program name.

As these are global variables which take up memory even if not used,
similar to 'errno', gate them behind NOLIBC_IGNORE_ERRNO.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
---
 tools/include/nolibc/crt.h                   | 26 ++++++++++++++++++++++
 tools/include/nolibc/errno.h                 |  2 ++
 tools/testing/selftests/nolibc/nolibc-test.c | 33 ++++++++++++++++++++++++++++
 3 files changed, 61 insertions(+)

diff --git a/tools/include/nolibc/crt.h b/tools/include/nolibc/crt.h
index d9262998dae9..842f86e41f2f 100644
--- a/tools/include/nolibc/crt.h
+++ b/tools/include/nolibc/crt.h
@@ -17,6 +17,7 @@ const unsigned long *_auxv __attribute__((weak));
 void _start(void);
 static void __stack_chk_init(void);
 static void exit(int);
+static char *strrchr(const char *s, int c);
 
 extern void (*const __preinit_array_start[])(int, char **, char**) __attribute__((weak));
 extern void (*const __preinit_array_end[])(int, char **, char**) __attribute__((weak));
@@ -27,6 +28,24 @@ extern void (*const __init_array_end[])(int, char **, char**) __attribute__((wea
 extern void (*const __fini_array_start[])(void) __attribute__((weak));
 extern void (*const __fini_array_end[])(void) __attribute__((weak));
 
+extern char *program_invocation_name __attribute__((weak));
+extern char *program_invocation_short_name __attribute__((weak));
+
+static __inline__
+char *__nolibc_program_invocation_short_name(char *long_name)
+{
+	char *short_name;
+
+	if (!long_name)
+		return NULL;
+
+	short_name = strrchr(long_name, '/');
+	if (!short_name || !short_name[0])
+		return NULL;
+
+	return short_name + 1;
+}
+
 void _start_c(long *sp);
 __attribute__((weak,used))
 #if __nolibc_has_feature(undefined_behavior_sanitizer)
@@ -76,6 +95,13 @@ void _start_c(long *sp)
 		;
 	_auxv = auxv;
 
+#ifndef NOLIBC_IGNORE_ERRNO
+	if (argc > 0) {
+		program_invocation_name = argv[0];
+		program_invocation_short_name = __nolibc_program_invocation_short_name(argv[0]);
+	}
+#endif /* NOLIBC_IGNORE_ERRNO */
+
 	for (ctor_func = __preinit_array_start; ctor_func < __preinit_array_end; ctor_func++)
 		(*ctor_func)(argc, argv, envp);
 	for (ctor_func = __init_array_start; ctor_func < __init_array_end; ctor_func++)
diff --git a/tools/include/nolibc/errno.h b/tools/include/nolibc/errno.h
index 08a33c40ec0c..64d9f9bc6539 100644
--- a/tools/include/nolibc/errno.h
+++ b/tools/include/nolibc/errno.h
@@ -15,6 +15,8 @@
 #ifndef NOLIBC_IGNORE_ERRNO
 #define SET_ERRNO(v) do { errno = (v); } while (0)
 int errno __attribute__((weak));
+char *program_invocation_name __attribute__((weak));
+char *program_invocation_short_name __attribute__((weak));
 #else
 #define SET_ERRNO(v) do { } while (0)
 #endif
diff --git a/tools/testing/selftests/nolibc/nolibc-test.c b/tools/testing/selftests/nolibc/nolibc-test.c
index 180611aabbfb..07af23833879 100644
--- a/tools/testing/selftests/nolibc/nolibc-test.c
+++ b/tools/testing/selftests/nolibc/nolibc-test.c
@@ -702,6 +702,37 @@ static void constructor2(int argc, char **argv, char **envp)
 		constructor_test_value |= 1 << 1;
 }
 
+int test_program_invocation_name(void)
+{
+	char buf[100];
+	char *dirsep;
+	ssize_t r;
+	int fd;
+
+	fd = open("/proc/self/cmdline", O_RDONLY);
+	if (fd == -1)
+		return 1;
+
+	r = read(fd, buf, sizeof(buf));
+	close(fd);
+	if (r < 1 || r == sizeof(buf))
+		return 1;
+
+	buf[r - 1] = '\0';
+
+	if (strcmp(program_invocation_name, buf) != 0)
+		return 1;
+
+	dirsep = strrchr(buf, '/');
+	if (!dirsep || dirsep[1] == '\0')
+		return 1;
+
+	if (strcmp(program_invocation_short_name, dirsep + 1) != 0)
+		return 1;
+
+	return 0;
+}
+
 int run_startup(int min, int max)
 {
 	int test;
@@ -716,6 +747,7 @@ int run_startup(int min, int max)
 #ifdef NOLIBC
 	test_auxv = _auxv;
 #endif
+	bool proc = access("/proc", R_OK) == 0;
 
 	for (test = min; test >= 0 && test <= max; test++) {
 		int llen = 0; /* line length */
@@ -741,6 +773,7 @@ int run_startup(int min, int max)
 		CASE_TEST(constructor);      EXPECT_EQ(is_nolibc, constructor_test_value, 0x3); break;
 		CASE_TEST(linkage_errno);    EXPECT_PTREQ(1, linkage_test_errno_addr(), &errno); break;
 		CASE_TEST(linkage_constr);   EXPECT_EQ(1, linkage_test_constructor_test_value, 0x3); break;
+		CASE_TEST(prog_name);        EXPECT_ZR(proc, test_program_invocation_name()); break;
 		case __LINE__:
 			return ret; /* must be last */
 		/* note: do not set any defaults so as to permit holes above */

-- 
2.53.0

Hi Thomas,

On Fri, Mar 13, 2026 at 09:26:28PM +0100, Thomas Weißschuh wrote:
> Add support for the GNU extensions 'program_invocation_name' and
> 'program_invocation_short_name'. These are useful to print error
> messages, which by convention include the program name.
> 
> As these are global variables which take up memory even if not used,
> similar to 'errno', gate them behind NOLIBC_IGNORE_ERRNO.
> 
> Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
> ---
>  tools/include/nolibc/crt.h                   | 26 ++++++++++++++++++++++
>  tools/include/nolibc/errno.h                 |  2 ++
>  tools/testing/selftests/nolibc/nolibc-test.c | 33 ++++++++++++++++++++++++++++
>  3 files changed, 61 insertions(+)
> 
> diff --git a/tools/include/nolibc/crt.h b/tools/include/nolibc/crt.h
> index d9262998dae9..842f86e41f2f 100644
> --- a/tools/include/nolibc/crt.h
> +++ b/tools/include/nolibc/crt.h
> @@ -17,6 +17,7 @@ const unsigned long *_auxv __attribute__((weak));
>  void _start(void);
>  static void __stack_chk_init(void);
>  static void exit(int);
> +static char *strrchr(const char *s, int c);
>  
>  extern void (*const __preinit_array_start[])(int, char **, char**) __attribute__((weak));
>  extern void (*const __preinit_array_end[])(int, char **, char**) __attribute__((weak));
> @@ -27,6 +28,24 @@ extern void (*const __init_array_end[])(int, char **, char**) __attribute__((wea
>  extern void (*const __fini_array_start[])(void) __attribute__((weak));
>  extern void (*const __fini_array_end[])(void) __attribute__((weak));
>  
> +extern char *program_invocation_name __attribute__((weak));
> +extern char *program_invocation_short_name __attribute__((weak));
> +
> +static __inline__
> +char *__nolibc_program_invocation_short_name(char *long_name)
> +{
> +	char *short_name;
> +
> +	if (!long_name)
> +		return NULL;
> +
> +	short_name = strrchr(long_name, '/');
> +	if (!short_name || !short_name[0])
> +		return NULL;

Here it should return long_name, not NULL, since you want a valid name
to use later. I'm seeing it passed to strcmp() for example.

Also because of this, I'm wondering for the first test about !long_name.
Either we consider that it's not possible to have a NULL long_name and
we don't need to test for it, or we consider it is valid, and we should
return a non-null string (e.g. "") so that the rest of the program
survives it. Both approaches are fine to me, but IMHO testing for NULL
to return a NULL that will crash the program later instead of just now
is not very useful. Maybe returning the empty string is still slightly
better because it allows well-designed programs that check their argc
before using argv[0] to survive.

Cheers,
Willy