Glibc - error in ucontext.h, but only with -std=c11

问题

I have this minimal helloworld, extended with an include of ucontext.h:

#include <ucontext.h>
#include <stdio.h>

int main(int argc, char** argv) {
  printf ("hello world!\n");
  return 0;
}

It compiles without warning with gcc-4.9 (gcc -c hw.c -Wall).

But if I switch to the c11 standard (gcc -std=c11 -c hw.c -Wall), I get the following error:

$ gcc -std=c11 -c hw.c -Wall
In file included from /usr/include/ucontext.h:26:0,
                 from hw.c:1:
/usr/include/x86_64-linux-gnu/sys/ucontext.h:137:5: error: unknown type name ‘stack_t’
     stack_t uc_stack;
     ^

My first idea is that glibc doesn't support c11. Googling for that didn't reveal usable information. What is the case?

(I use glibc-2.19 with gcc-4.9. It is a debian jessie, amd64.)

回答1:

-std=c11 is C11 standard compliant mode. <ucontext.h> isn't strictly part of C11 (see Stas's answer).

To use these headers either use extension mode -std=gnu11 or define appropriate macro depending on which platform do you intend to support (_POSIX_C_SOURCE, _BSD_SOURCE, _XOPEN_SOURCE, _GNU_SOURCE or maybe others).

See this page for more info about feature-enabling macros.

回答2:

It seems <ucontext.h> functions are deprecated, because they use deprecated C feature. So they can't be used in standard compliant C code. See the rationale:

With the incorporation of the ISO/IEC 9899:1999 standard into this specification it was found that the ISO C standard (Subclause 6.11.6) specifies that the use of function declarators with empty parentheses is an obsolescent feature. Therefore, using the function prototype:

void makecontext(ucontext_t *ucp, void (*func)(), int argc, ...);

is making use of an obsolescent feature of the ISO C standard. Therefore, a strictly conforming POSIX application cannot use this form. Therefore, use of getcontext(), makecontext(), and swapcontext() is marked obsolescent.

So, it is not directly related to C11. For example, I can't compile your example with clang on Mac OS X at all.

It was deprecated in C99 standard:

6.11.6 Function declarators

The use of function declarators with empty parentheses (not prototype-format parameter type declarators) is an obsolescent feature.

来源：https://stackoverflow.com/questions/32426835/glibc-error-in-ucontext-h-but-only-with-std-c11