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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.
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The jmp_buf type is an array type suitable for storing information to restore a calling environment. The stored information is sufficient to restore execution at the correct block of the program and invocation of that block.
The setjmp() function saves a stack environment that can subsequently be restored by the longjmp() function. The setjmp() and longjmp() functions provide a way to perform a non-local goto. They are often used in signal handlers.
setjmp saves the current environment (the program state), at some point of program execution, into a platform-specific data structure ( jmp_buf ) that can be used at some later point of program execution by longjmp to restore the program state to that saved by setjmp into jmp_buf .
setjmp and longjmp are a pair of C function facilitating cross-procedure transfer of control. Typically they are used to allow resumption of execution at a known good point after an error. Both take as first argument a buffer, which is used to hold the machine state at the jump destination.
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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