What does SYNOPSIS part in perror man page mean? - c

Question

SYNOPSIS section in perror's man page is:
#include <stdio.h>
void perror(const char *s);
#include <errno.h>
const char * const sys_errlist[];
int sys_nerr;
int errno; /* Not really declared this way; see errno(3) */
according to man page specification, SYNOPSIS section indicates that
For functions, it shows any required data declarations or #include directives, followed by the function declaration.
The following code:
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
int main()
{
char *ls_args[2] = {"nonsense", NULL};
execv(ls_args[0], ls_args); // no return
perror("execve failed");
return 2;
}
outputs error message execve failed: No such file or directory, with corresponding errno being 2
since errno is a global variable (actually a macro) defined in errno.h, and errno.h header is not included, how does this code trigger errno modification?
What does #include <errno.h> ... int errno; mean in the SYNOPSIS section? seems like perror() can be called without this portion of code, thanks!

Answer 1

According to the C standard, the macro errno is declared in errno.h and you must include errno.h explicitly if you want to write a portable program which uses errno. The manpage synopsis is telling you that. (It is not saying that you need to include errno.h in order to use perror. Sometimes the Synopsis section tells you about other related library facilities.)
Nothing in the standard specifies what the definition of the errno macro is, or exactly where in the implementation you can find any objects referenced by the expansion of that definition. The implementation of perror obviously needs to be able to access whatever object errno refers to, but since it doesn't need to be portable, it is completely unspecified how that works.
In particular, recent C standards require that the object errno refers to be thread-local, so that every thread has its own errno object. (If this were not the case, the mechanism would be essentially unusable in multithreaded code.) The precise implementation of thread-local storage is also not specified by the standard, and on a particular implementation it might be mapped onto some facility provided by the underlying operating system.