关于程序运行的堆栈结构

程序运行的堆栈结构并不是很好理解，今天碰到了这个问题，查了些资料，找到个比较好的文章理解，

 

首先：查看unix环境高级编程中P152页Stevens对address space of process 和a.out 文件各段的分析

 

链接原文如下：

          http://www.cnblogs.com/yanlingyin/archive/2011/11/28/RuningtimeDataStructures.html

 

wikipedia 网页内容附加：

setjmp.h

 

From Wikipedia, the free encyclopedia

C Standard Library

• Data types
• Character classification
• Strings
• Mathematics
• File input/output
• Date/time
• Localization
• Memory allocation
• Process control
• Signals
• Alternative tokens

Miscellaneous headers:

• <assert.h>
• <errno.h>
• <setjmp.h>
• <stdarg.h>

• v 
• t 
• e

setjmp.h is a header defined in the C standard library to provide "non-local jumps": control flow that deviates from the usual subroutine call and return sequence. The complementary functions setjmp and longjmp provide this functionality.

A typical use of setjmp/longjmp is implementation of an exception mechanismthat exploits the ability of longjmp to reestablish program or thread state, even across multiple levels of function calls. A less common use ofsetjmp is to create syntax similar to coroutines.

Contents

  [hide] 

• 1 Member functions
• 2 Member types
• 3 Caveats and limitations
• 4 Example usage
  • 4.1 Simple example
  • 4.2 Exception handling
  • 4.3 Cooperative multitasking
• 5 See also
• 6 References
• 7 External links

[edit]Member functions

int setjmp(jmp_buf env)Sets up the local jmp_buf buffer and initializes it for the jump. This routine^[1] saves the program's calling environment in the environment buffer specified by the env argument for later use by longjmp. If the return is from a direct invocation, setjmp returns 0. If the return is from a call to longjmp, setjmp returns a nonzero value.void longjmp(jmp_buf env, int value)Restores the context of the environment buffer env that was saved by invocation of the setjmproutine^[1] in the same invocation of the program. Invoking longjmp from a nested signal handler is undefined. The value specified by value is passed from longjmp to setjmp. Afterlongjmp is completed, program execution continues as if the corresponding invocation ofsetjmp had just returned. If the value passed to longjmp is 0, setjmp will behave as if it had returned 1; otherwise, it will behave as if it had returned value.

setjmp saves the current environment (i.e., the program state) at some point of program execution, into a platform-specific data structure (jmp_buf) which can be used, at some later point of program execution, by longjmpto restore the program state to that which was saved by setjmp into jmp_buf. This process can be imagined to be a "jump" back to the point of program execution where setjmp saved the environment. The (apparent) return value fromsetjmp indicates whether control reached that point normally or from a call to longjmp. This leads to a commonidiom: if( setjmp(x) ){/* handle longjmp(x) */}.

POSIX.1 does not specify whether setjmp and longjmp save or restore the current set of blocked signals — if a program employs signal handling it should use POSIX's sigsetjmp/siglongjmp.

[edit]Member types

jmp_bufAn array type, such as struct __jmp_buf_tag[1],^[2] suitable for holding the information needed to restore a calling environment.

The C99 Rationale describes jmp_buf as being an array type for backwards compatibility; existing code refers tojmp_buf storage locations by name (without the & address-of operator), which is only possible for array types.^[3]

[edit]Caveats and limitations

When a "non-local goto" is executed via setjmp/longjmp, normal "stack unwinding" does not occur and therefore, any required cleanup actions such as closing file descriptors, flushing buffers, freeing heap-allocated memory, etc., do not occur.

If the function in which setjmp was called returns, it is no longer possible to safely use longjmp with the corresponding jmp_buf object. This is because the stack frame is invalidated when the function returns. Callinglongjmp restores the stack pointer, which—because the function returned—would point to a non-existent and potentially overwritten/corrupted stack frame.^[4][5]

Similarly, C99 does not require that longjmp preserve the current stack frame. This means that jumping into a function which was exited via a call to longjmp is undefined.^[6] However, most implementations of longjmp leave the stack frame intact, allowing setjmp and longjmp to be used to jump back-and-forth between two or more functions—a feature exploited for multitasking.

Compared to mechanisms in higher-level programming languages such as Python, Java, C++, C#, and even pre-C languages such as Algol 60, the technique of using setjmp/longjmp to implement an exception mechanism is cumbersome. These languages provide more powerful exception handling techniques, while languages such as Scheme,Smalltalk, and Haskell provide even more general continuation-handling constructs.

[edit]Example usage

[edit]Simple example

This example shows the basic idea of setjmp. Main calls first, which in turn calls second. The "second" function jumps back into main, skipping "first"'s print statement.

#include <stdio.h>#include <setjmp.h> static jmp_buf buf; void second(void) {    printf("second\n");         // prints    longjmp(buf,1);             // jumps back to where setjmp was called - making setjmp now return 1} void first(void) {    second();    printf("first\n");          // does not print} int main() {       if ( ! setjmp(buf) ) {        first();                // when executed, setjmp returns 0    } else {                    // when longjmp jumps back, setjmp returns 1        printf("main\n");       // prints    }     return 0;}

When executed, the above program will output:

secondmain

Notice that although the first() subroutine gets called, "first" is never printed. "main" gets printed as the conditional statement if ( ! setjmp(buf) ) is executed a second time.

[edit]Exception handling

In this example, setjmp is used to bracket exception handling, like try in some other languages. The call tolongjmp is analogous to a throw statement, allowing an exception to return an error status directly to the setjmp. The following code adheres to the 1999 ISO C standard and Single UNIX Specification by invoking setjmp in a limited range of contexts:^[7]

• As the condition to an if, switch or iteration statement
• As above in conjunction with a single ! or comparison with an integer constant
• As a statement (with the return value unused)

Following these rules can make it easier for the implementation to create the environment buffer, which can be a sensitive operation.^[3] More general use of setjmp can cause undefined behaviour, such as corruption of local variables; conforming compilers and environments are not required to protect or even warn against such usage. However, slightly more sophisticated idioms such as switch ((exception_type = setjmp(env))) { } are common in literature and practice, and remain relatively portable. A simple conforming methodology is presented below, where an additional variable is maintained along with the state buffer. This variable could be elaborated into a structure incorporating the buffer itself.

#include <stdio.h>#include <stdlib.h>#include <string.h>#include <setjmp.h> void first(void);void second(void); /* This program's output is: calling firstcalling secondentering secondsecond failed with type 3 exception; remapping to type 1.first failed, exception type 1 */ /* Use a file scoped static variable for the exception stack so we can access * it anywhere within this translation unit. */static jmp_buf exception_env;static int exception_type; int main() {    void *volatile mem_buffer;     mem_buffer = NULL;    if (setjmp(exception_env)) {        /* if we get here there was an exception */        printf("first failed, exception type %d\n", exception_type);    } else {        /* Run code that may signal failure via longjmp. */        printf("calling first\n");        first();        mem_buffer = malloc(300); /* allocate a resource */        printf("%s",strcpy((char*) mem_buffer, "first succeeded!")); /* ... this will not happen */    }    if (mem_buffer)        free((void*) mem_buffer); /* carefully deallocate resource */    return 0;} void first(void) {    jmp_buf my_env;     printf("calling second\n");    memcpy(my_env, exception_env, sizeof(jmp_buf));    switch (setjmp(exception_env)) {        case 3:            /* if we get here there was an exception. */            printf("second failed with type 3 exception; remapping to type 1.\n");            exception_type = 1;         default: /* fall through */            memcpy(exception_env, my_env, sizeof(jmp_buf)); /* restore exception stack */            longjmp(exception_env, exception_type); /* continue handling the exception */         case 0:            /* normal, desired operation */            second();            printf("second succeeded\n");  /* not reached */    }    memcpy(exception_env, my_env, sizeof(jmp_buf)); /* restore exception stack */} void second(void) {    printf("entering second\n" ); /* reached */    exception_type = 3;    longjmp(exception_env, exception_type); /* declare that the program has failed */    printf("leaving second\n"); /* not reached */}

[edit]Cooperative multitasking

C99 provides that longjmp is guaranteed to work only when the destination is a calling function, i.e., that the destination scope is guaranteed to be intact. Jumping to a function that has already terminated by return orlongjmp is undefined.^[6] However, most implementations of longjmp do not specifically destroy local variables when performing the jump. Since the context survives until its local variables are erased, it could actually be restored by setjmp. In many environments (such as Really Simple Threads and TinyTimbers), idioms such asif(!setjmp(child_env)) longjmp(caller_env); can allow a called function to effectively pause-and-resume at asetjmp.

This is exploited by thread libraries to provide cooperative multitasking facilities without using setcontext or other fiber facilities. Whereas setcontext is a library service which can create an execution context in heap-allocated memory and can support other services such as buffer overflow protection^{[citation needed]}, abuse of setjmpis implemented by the programmer, who may reserve memory on the stack and fail to notify the library or operating system of the new operating context. On the other hand, a library's implementation of setcontext may internally use setjmp in a fashion similar to this example to save and restore a context, after it has been initialised somehow.

Considering that setjmp to a child function will generally work unless sabotaged, and setcontext, as part ofPOSIX, is not required to be provided by C implementations, this mechanism may be portable where the setcontextalternative fails.

Since no exception will be generated upon overflow of one of the multiple stacks in such a mechanism, it is essential to overestimate the space required for each context, including the one containing main() and including space for any signal handlers that might interrupt regular execution. Exceeding the allocated space will corrupt the other contexts, usually with the outermost functions first. Unfortunately, systems requiring this kind of programming strategy are often also small ones with limited resources.

#include <setjmp.h>#include <stdio.h> jmp_buf mainTask, childTask; void call_with_cushion(void);void child(void); int main(void) {    if (!setjmp(mainTask)) {        call_with_cushion(); /* child never returns */ /* yield */    } /* execution resumes after this "}" after first time that child yields */    for (;;) {        printf("Parent\n");        if (!setjmp(mainTask)) {            longjmp(childTask, 1); /* yield - note that this is undefined under C99 */        }    }} void call_with_cushion (void) {    char space[1000]; /* Reserve enough space for main to run */    space[999] = 1; /* Do not optimize array out of existence */    child();} void child (void) {    for (;;) {        printf("Child loop begin\n");        if (!setjmp(childTask)) longjmp(mainTask, 1); /* yield - invalidates childTask in C99 */         printf("Child loop end\n");        if (!setjmp(childTask)) longjmp(mainTask, 1); /* yield - invalidates childTask in C99 */    }    /* Don't return. Instead we should set a flag to indicate that main()       should stop yielding to us and then longjmp(mainTask, 1) */}

[edit]See also

• setcontext
• continuation

[edit]References

1. ^ ^a b ISO C states that setjmp must be implemented as a macro, but POSIX explicitly states that it is undefined whether setjmpis a macro or a function.
2. ^ This is the type used by the GNU C Library, version 2.7
3. ^ ^a b C99 Rationale, version 5.10, April 2003, section 7.13
4. ^ CS360 Lecture Notes — Setjmp and Longjmp
5. ^ setjmp(3)
6. ^ ^a b ISO/IEC 9899:1999, 2005, 7.13.2.1:2 and footnote 211
7. ^ setjmp: set jump point for a non-local goto – System Interfaces Reference, The Single UNIX® Specification, Issue 7 fromThe Open Group

[edit]External links

• setjmp(3): save stack context for non-local goto – Linux Library Functions Manual
• Exceptions in C with Longjmp and Setjmp
• is there sigsetjmp/siglongjmp (again) (about this functions in mingw/MSYS)

View page ratings

Rate this page

What's this?

Trustworthy

Objective

Complete

Well-written

I am highly knowledgeable about this topic (optional)

Submit ratings

Categories: 

• C standard library headers
• Control flow