Question

I am running Ubuntu 9.10 (Karmic Koala), and I took a look at the jmp_buf structure which is simply an array of 12 ints. When I use setjmp, and pass in a jmp_buf structure—4 out of 12 entries are saved off. These 4 entries are the stack pointer, frame pointer, program counter and return address. What are the other 8 entries for? Are they machine-dependent? Is another entry the segment table base register? What else is needed to properly restore a thread/process s environment? I looked through the man page, other sources, but I couldn t find the assembly code for setjmp.

Answer 1

On MacOS X 10.6.2, the header <setjmp.h> ends up using <i386/setjmp.h>, and in there it says:

#if defined(__x86_64__)
/*
 * _JBLEN is number of ints required to save the following:
 * rflags, rip, rbp, rsp, rbx, r12, r13, r14, r15... these are 8 bytes each
 * mxcsr, fp control word, sigmask... these are 4 bytes each
 * add 16 ints for future expansion needs...
 */
#define _JBLEN ((9 * 2) + 3 + 16)
typedef int jmp_buf[_JBLEN];
typedef int sigjmp_buf[_JBLEN + 1];

#else

/*
 * _JBLEN is number of ints required to save the following:
 * eax, ebx, ecx, edx, edi, esi, ebp, esp, ss, eflags, eip,
 * cs, de, es, fs, gs == 16 ints
 * onstack, mask = 2 ints
 */

#define _JBLEN (18)
typedef int jmp_buf[_JBLEN];
typedef int sigjmp_buf[_JBLEN + 1];

#endif

You would probably find similar requirements on Linux - the jmp_buf contains enough information to store the necessary state. And, to use it, you really don t need to know what it contains; all you need to do is trust that the implementers got it correct. If you want to alter the implementation, then you do need to understand it, of course.

Note that setjmp and longjmp are very machine specific. Read Plauger s "The Standard C Library" for a discussion of some of the issues involved in implementing them. More modern chips make it harder to implement really well.

Answer 2

setjmp/longjmp/sigsetjmp are highly dependent on the CPU architecture, operating system, and threading model. The first two functions famously (or infamously—depending on your POV) appeared in the original Unix kernel as a "structured" way to unwind out of a failed system call, as from an i/o error or other nasty situations.

The structure s comments in /usr/include/setjmp.h (Linux Fedora) say Calling environment, plus possibly a saved signal mask. It includes /usr/include/bits/setjmp.h to declare jmp_buf to have an array of six 32-bit ints, apparently specific to the x86 family.

While I couldn t find source other than a PPC implementation, the comments there reasonably hint that FPU settings should be saved. That makes sense since failing to restore the rounding mode, default operand size, exception handling, etc. would be surprising.

It s typical of system engineers to reserve a little more space than actually needed in such a structure. A few extra bytes are hardly anything to sweat—especially considering the rarity of actual uses of setjmp/longjmp. Having too little space definitely is a hazard. The most salient reason I can think of is having extra—as opposed to being spot on—is that if the runtime library version is changed to need more space in jmp_buf, by having extra room already reserved, there s no need to recompile programs referring to it.

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