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问setjmp / longjmp不会跳到我认为应该跳的地方。
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Stack Overflow用户

提问于 2021-12-12 14:02:05

回答 1查看 88关注 0票数 0

我想了解setjmp / longjmp是如何工作的，所以我创建了一个示例程序，其中routineA打印偶数，routineB打印奇数，然后它们跳到一起：

#include <setjmp.h>
#include <stdio.h>

#define COUNTER_BEGIN 0
#define COUNTER_END   6

void routineA( jmp_buf* pEnvA, jmp_buf* pEnvB );
void routineB( jmp_buf* pEnvA, jmp_buf* pEnvB );

int main() {
    const char message[] = "main      [ &envA=0x^%016lx &envB=0x^%016lx ]  -- %s\n";
    jmp_buf envA;
    jmp_buf envB;

    fprintf( stdout, message, &envA, &envB,
             "Started; Before calling routineA" );
    routineA( &envA, &envB );
    fprintf( stdout, message, &envA, &envB,
             "After routineA returned; Exiting" );

    return 0;
}

/* Prints even numbers from COUNTER_BEGIN to COUNTER_END */
void routineA( jmp_buf* pEnvA, jmp_buf* pEnvB ) {
    const char* message = "routineA: [ i=%d, sjr=%d ]  -- %s\n";
    static int i;
    static int sjr;
    static jmp_buf *s_pEnvA;
    static jmp_buf *s_pEnvB;

    s_pEnvA = pEnvA;
    s_pEnvB = pEnvB;
    fprintf( stdout, message, i, sjr, "After saving statics; Before starting the for loop" );
    for( i = COUNTER_BEGIN; i < COUNTER_END; i++ ) {
        if( i % 2 == COUNTER_BEGIN + 0 ) {
            fprintf( stdout, message, i, sjr, "Inside for and if; Before setjmp" );
            sjr = setjmp( *s_pEnvA );           /* Added */
            if( sjr == 0 ) {                    /* Added */
            /* if( ( sjr = setjmp( *s_pEnvA ) ) == 0 ) { */
                fprintf( stdout, message, i, sjr, "Got to this line directly - go to routineB somehow" );
                if( i == 0 ) {
                    fprintf( stdout, message, i, sjr, "This is the first iteration - call routineB as function" );
                    routineB( s_pEnvA, s_pEnvB );
                } else {
                    fprintf( stdout, message, i, sjr, "This is not the first iteration - longjmp to routineB" );
                    longjmp( *s_pEnvB, 12 );
                }
            } else {
                fprintf( stdout, message, i, sjr, "Got to this line via longjmp (in this program it must be from routineB) - continue" );
                ; /* Just continue */
            }
        }
    }
    fprintf( stdout, message, i, sjr, "After the for loop, Before leaving the function" );
}

/* Prints odd numbers from COUNTER_BEGIN to COUNTER_END */
void routineB( jmp_buf* pEnvA, jmp_buf* pEnvB ) {
    const char* message = "routineB: [ i=%d, sjr=%d ]  -- %s\n";
    static int i;
    static int sjr;
    static jmp_buf *s_pEnvA;
    static jmp_buf *s_pEnvB;

    s_pEnvA = pEnvA;
    s_pEnvB = pEnvB;
    fprintf( stdout, message, i, sjr, "After saving statics; Before starting the for loop" );
    for( i = COUNTER_BEGIN; i < COUNTER_END; i++ ) {
        if( i % 2 == 1 ) {
            fprintf( stdout, message, i, sjr, "Inside for and if; Before setjmp" );
            sjr = setjmp( *s_pEnvB );           /* Added */
            if( sjr == 0 ) {                    /* Added */
            /* if( ( sjr = setjmp( *s_pEnvB ) ) == 0 ) { */
                fprintf( stdout, message, i, sjr, "Got to this line directly - longjmp to routineA" );
                longjmp( *s_pEnvA, 21 );
            } else {
                fprintf( stdout, message, i, sjr, "Got to this line via longjmp (in this program it must be from routineA) - continue" );
                ; /* Just continue */
            }
        }
    }
    fprintf( stdout, message, i, sjr, "After the for loop, Before leaving the function" );
}

我与gcc一起编译了它，并收到了以下输出(插入行号供以后参考)：

01 main      [ &envA=0x^00007ffce81b0280 &envB=0x^00007ffce81b01b0 ]  -- Started; Before calling routineA
02 routineA: [ i=0, sjr=0 ]  -- After saving statics; Before starting the for loop
03 routineA: [ i=0, sjr=0 ]  -- Inside for and if; Before setjmp
04 routineA: [ i=0, sjr=0 ]  -- Got to this line directly - go to routineB somehow
05 routineA: [ i=0, sjr=0 ]  -- This is the first iteration - call routineB as function
06 routineB: [ i=0, sjr=0 ]  -- After saving statics; Before starting the for loop
07 routineB: [ i=1, sjr=0 ]  -- Inside for and if; Before setjmp
08 routineB: [ i=1, sjr=0 ]  -- Got to this line directly - longjmp to routineA
09 routineA: [ i=0, sjr=21 ]  -- Got to this line via longjmp (in this program it must be from routineB) - continue
10 routineA: [ i=2, sjr=21 ]  -- Inside for and if; Before setjmp
11 routineA: [ i=2, sjr=0 ]  -- Got to this line directly - go to routineB somehow
12 routineA: [ i=2, sjr=0 ]  -- This is not the first iteration - longjmp to routineB
13 routineA: [ i=2, sjr=21 ]  -- Got to this line via longjmp (in this program it must be from routineB) - continue
14 routineA: [ i=4, sjr=21 ]  -- Inside for and if; Before setjmp
15 routineA: [ i=4, sjr=0 ]  -- Got to this line directly - go to routineB somehow
16 routineA: [ i=4, sjr=0 ]  -- This is not the first iteration - longjmp to routineB
17 routineA: [ i=4, sjr=21 ]  -- Got to this line via longjmp (in this program it must be from routineB) - continue
18 routineA: [ i=6, sjr=21 ]  -- After the for loop, Before leaving the function
19 main      [ &envA=0x^00007ffce81b0280 &envB=0x^00007ffce81b01b0 ]  -- After routineA returned; Exiting

在第12行之前，输出是正常的:但是，This is not the first iteration - longjmp to routineB之后，它应该转到routineB，并打印Got to this line via longjmp (in this program it must be from routineA) - continue，但是它停留在routineA中，并表示为Got to this line via longjmp (in this program it must be from routineB) - continue。

我的代码中是否有错误，或者我是否误解了setjmp / longjmp的某些内容？

更新

根据Eric的答案的第一部分修改了代码:我认为现在它适合最后一个要点，因为赋值语句就是表达式语句。我收到了相同的结果(除了不同的指针值)。

好的，我将sjr = setjmp( *s_pEnvA ); (以及routineB的相应部分)替换为

sjr = -1;
switch( setjmp( *s_pEnvA ) ) {
case  0: sjr =  0; break;
case  1: sjr =  1; break;
case 12: sjr = 12; break;
case 21: sjr = 21; break;
default: sjr = -2; break;
}

我认为这完全符合“选择或迭代语句的整个控制表达式”的要点。

结果仍然一样。

最终更新

埃里克和内特是对的，这是不确定的行为。但是，由于我想知道this answer中的代码为什么能工作，所以我逐渐地将代码转换为该代码，并观察它何时中断。最后我得到了这个：

#include <setjmp.h>
#include <stdio.h>

#if defined( SORTOFWORKS )
#define CHECKPOINTFORMAT "routineX:\n  -- %s\n\n"
#elif defined( DOESNOTWORK )
#define CHECKPOINTFORMAT message, pEnvA, pEnvB, s_pEnvA, s_pEnvB, i, sjr
#else
#error Version (SORTOFWORKS or DOESNOTWORK) is not defined
#endif

#define STRINGIFY( x ) STRINGIFY_IMPLEMENTATION( x )
#define                STRINGIFY_IMPLEMENTATION( x ) #x

#define COUNTER_BEGIN 0
#define COUNTER_END   6

void routineA( jmp_buf* pEnvA, jmp_buf* pEnvB );
void routineB( jmp_buf* pEnvA, jmp_buf* pEnvB );

int main() {
    const char message[] = "main      [ &envA=0x^%016lx &envB=0x^%016lx ]\n  -- %s\n\n";
    jmp_buf envA;
    jmp_buf envB;

    fprintf( stdout, message, &envA, &envB,
             "Started; Before calling routineA the first time" );
    routineA( &envA, &envB );
    fprintf( stdout, message, &envA, &envB,
             "Before calling routineA the second time" );
    routineA( &envA, &envB );
    fprintf( stdout, message, &envA, &envB,
             "Exiting" );

    return 0;
}

/* Prints even numbers from COUNTER_BEGIN to COUNTER_END */
void routineA( jmp_buf* pEnvA, jmp_buf* pEnvB ) {
    const char* const message = "routineA: [ pEnvA=0x^%016lx, pEnvB=0x^%016lx, s_pEnvA=0x^%016lx, s_pEnvB=0x^%016lx, i=%d, sjr=%d ]\n"
                                "  -- %s\n\n";
    static int i;
    static jmp_buf *s_pEnvA;
    static jmp_buf *s_pEnvB;
    static int sjr;

    fprintf( stdout, message, pEnvA, pEnvB, s_pEnvA, s_pEnvB, i, sjr,
             "Called; Before saving statics" );
    s_pEnvA = pEnvA;
    s_pEnvB = pEnvB;
    fprintf( stdout, message, pEnvA, pEnvB, s_pEnvA, s_pEnvB, i, sjr,
             "After saving statics; Before starting the counting" );

    fprintf( stdout, CHECKPOINTFORMAT,
             "Checkpoint A0 at line " STRINGIFY( __LINE__ ) );

    i = COUNTER_BEGIN;
    fprintf( stdout, message, pEnvA, pEnvB, s_pEnvA, s_pEnvB, i, sjr,
             "Part of counting; Before setjmp" );
    sjr = -1;
    switch( setjmp( *s_pEnvA ) ) {
    case  0: sjr =  0; break;
    case  1: sjr =  1; break;
    case 12: sjr = 12; break;
    case 21: sjr = 21; break;
    default: sjr = -2; break;
    }
    if( sjr == 0 ) {
        fprintf( stdout, message, pEnvA, pEnvB, s_pEnvA, s_pEnvB, i, sjr,
                 "Got to this line directly - This is the first iteration - call routineB as function" );
        routineB( s_pEnvA, s_pEnvB );
    } else {
        fprintf( stdout, message, pEnvA, pEnvB, s_pEnvA, s_pEnvB, i, sjr,
                 "Got to this line via longjmp (in this program it must be from routineB) - continue" );
        ; /* Just continue */
    }

    fprintf( stdout, CHECKPOINTFORMAT,
             "Checkpoint A1 at line " STRINGIFY( __LINE__ ) );

    i += 2;
    fprintf( stdout, message, pEnvA, pEnvB, s_pEnvA, s_pEnvB, i, sjr,
             "Part of counting; Before setjmp" );
    sjr = -1;
    switch( setjmp( *s_pEnvA ) ) {
    case  0: sjr =  0; break;
    case  1: sjr =  1; break;
    case 12: sjr = 12; break;
    case 21: sjr = 21; break;
    default: sjr = -2; break;
    }
    if( sjr == 0 ) {
        fprintf( stdout, message, pEnvA, pEnvB, s_pEnvA, s_pEnvB, i, sjr,
                 "Got to this line directly - This is not the first iteration - longjmp to routineB" );
        longjmp( *s_pEnvB, 12 );
    } else {
        fprintf( stdout, message, pEnvA, pEnvB, s_pEnvA, s_pEnvB, i, sjr,
                 "Got to this line via longjmp (in this program it must be from routineB) - continue" );
        ; /* Just continue */
    }

    fprintf( stdout, CHECKPOINTFORMAT,
             "Checkpoint A2 at line " STRINGIFY( __LINE__ ) );

    i += 2;
    fprintf( stdout, message, pEnvA, pEnvB, s_pEnvA, s_pEnvB, i, sjr,
             "Part of counting; Before setjmp" );
    sjr = -1;
    switch( setjmp( *s_pEnvA ) ) {
    case  0: sjr =  0; break;
    case  1: sjr =  1; break;
    case 12: sjr = 12; break;
    case 21: sjr = 21; break;
    default: sjr = -2; break;
    }
    if( sjr == 0 ) {
        fprintf( stdout, message, pEnvA, pEnvB, s_pEnvA, s_pEnvB, i, sjr,
                 "Got to this line directly - This is not the first iteration - longjmp to routineB" );
        longjmp( *s_pEnvB, 12 );
    } else {
        fprintf( stdout, message, pEnvA, pEnvB, s_pEnvA, s_pEnvB, i, sjr,
                 "Got to this line via longjmp (in this program it must be from routineB) - continue" );
        ; /* Just continue */
    }

    fprintf( stdout, CHECKPOINTFORMAT,
             "Checkpoint A3 at line " STRINGIFY( __LINE__ ) );

    /* Should be able to do this but it causes segfault: longjmp( *s_pEnvB, 12 ); */

    fprintf( stdout, message, pEnvA, pEnvB, s_pEnvA, s_pEnvB, i, sjr,
             "After the counting, Before leaving the function" );
}

/* Prints odd numbers from COUNTER_BEGIN to COUNTER_END */
void routineB( jmp_buf* pEnvA, jmp_buf* pEnvB ) {
    const char* const message = "routineB: [ pEnvA=0x^%016lx, pEnvB=0x^%016lx, s_pEnvA=0x^%016lx, s_pEnvB=0x^%016lx, i=%d, sjr=%d ]\n"
                                "  -- %s\n\n";
    static int i;
    static jmp_buf *s_pEnvA;
    static jmp_buf *s_pEnvB;
    static int sjr;

    fprintf( stdout, message, pEnvA, pEnvB, s_pEnvA, s_pEnvB, i, sjr,
             "Called; Before saving statics" );
    s_pEnvA = pEnvA;
    s_pEnvB = pEnvB;
    fprintf( stdout, message, pEnvA, pEnvB, s_pEnvA, s_pEnvB, i, sjr,
             "After saving statics; Before starting the for loop" );

    fprintf( stdout, CHECKPOINTFORMAT,
             "Checkpoint B0 at line " STRINGIFY( __LINE__ ) );

    i = 1 + COUNTER_BEGIN;
    fprintf( stdout, message, pEnvA, pEnvB, s_pEnvA, s_pEnvB, i, sjr,
             "Part of counting; Before setjmp" );
    sjr = -1;
    switch( setjmp( *s_pEnvB ) ) {
    case  0: sjr =  0; break;
    case  1: sjr =  1; break;
    case 12: sjr = 12; break;
    case 21: sjr = 21; break;
    default: sjr = -2; break;
    }
    if( sjr == 0 ) {
        fprintf( stdout, message, pEnvA, pEnvB, s_pEnvA, s_pEnvB, i, sjr,
                 "Got to this line directly - longjmp to routineA" );
        longjmp( *s_pEnvA, 21 );
    } else {
        fprintf( stdout, message, pEnvA, pEnvB, s_pEnvA, s_pEnvB, i, sjr,
                 "Got to this line via longjmp (in this program it must be from routineA) - continue" );
        ; /* Just continue */
    }
    
    fprintf( stdout, CHECKPOINTFORMAT,
             "Checkpoint B1 at line " STRINGIFY( __LINE__ ) ); /* SORTOFWORKS: printed, DOESNOTWORK: missing */

    i += 2;
    fprintf( stdout, message, pEnvA, pEnvB, s_pEnvA, s_pEnvB, i, sjr,
             "Part of counting; Before setjmp" );
    sjr = -1;
    switch( setjmp( *s_pEnvB ) ) {
    case  0: sjr =  0; break;
    case  1: sjr =  1; break;
    case 12: sjr = 12; break;
    case 21: sjr = 21; break;
    default: sjr = -2; break;
    }
    if( sjr == 0 ) {
        fprintf( stdout, message, pEnvA, pEnvB, s_pEnvA, s_pEnvB, i, sjr,
                 "Got to this line directly - longjmp to routineA" );
        longjmp( *s_pEnvA, 21 );
    } else {
        fprintf( stdout, message, pEnvA, pEnvB, s_pEnvA, s_pEnvB, i, sjr,
                 "Got to this line via longjmp (in this program it must be from routineA) - continue" );
        ; /* Just continue */
    }
    
    fprintf( stdout, CHECKPOINTFORMAT,
             "Checkpoint B2 at line " STRINGIFY( __LINE__ ) ); /* SORTOFWORKS: printed, DOESNOTWORK: missing */

    i += 2;
    fprintf( stdout, message, pEnvA, pEnvB, s_pEnvA, s_pEnvB, i, sjr,
             "Part of counting; Before setjmp" );
    sjr = -1;
    switch( setjmp( *s_pEnvB ) ) {
    case  0: sjr =  0; break;
    case  1: sjr =  1; break;
    case 12: sjr = 12; break;
    case 21: sjr = 21; break;
    default: sjr = -2; break;
    }
    if( sjr == 0 ) {
        fprintf( stdout, message, pEnvA, pEnvB, s_pEnvA, s_pEnvB, i, sjr,
                 "Got to this line directly - longjmp to routineA" );
        longjmp( *s_pEnvA, 21 );
    } else {
        fprintf( stdout, message, pEnvA, pEnvB, s_pEnvA, s_pEnvB, i, sjr,
                 "Got to this line via longjmp (in this program it must be from routineA) - continue" );
        ; /* Just continue */
    }
    
    fprintf( stdout, message, pEnvA, pEnvB, s_pEnvA, s_pEnvB, i, sjr,
             "After the for loop, Before leaving the function" );
}

如果用gcc -DSORTOFWORKS -o ./jump_sortofworks ./jump.c编译上面的代码，那么它会打印用注释/* SORTOFWORKS: printed, DOESNOTWORK: missing */标记的行，但是如果用gcc -DDOESNOTWORK -o ./jump_doesnotwork ./jump.c编译，则不会(编译器: GNU C11 (Debian6.3.0-18+deb9u1)版本6.3.0 20170516 (x86_64-linux-gnu))。

我认为这样一个小小的更改(不同的字符串格式来打印)破坏了它，这清楚地说明了setjmp / longjmp不能用于这个目的。

setjmp

回答 1

Stack Overflow用户

回答已采纳

发布于 2021-12-12 14:32:55

首先，C标准没有定义调用setjmp的行为，因为它们违反了C 2018 7.13.1.1 4和5中的约束：

对setjmp宏的调用只能出现在以下上下文中之一：

-选择或迭代语句的整个控制表达；

-一个关系运算符或相等运算符的一个操作数与另一个操作数的整数常量表达式，其结果表达式是选择或迭代语句的整个控制表达式；

-一元!运算符的操作数，其结果表达式是选择或迭代语句的整个控制表达式；或

-表达式语句的整个表达式(可能转换为void)。

如果调用出现在任何其他上下文中，则行为是未定义的。

例如，在if( ( sjr = setjmp( *s_pEnvA ) ) == 0 )中，setjmp调用不是整个控制表达式，它不是关系运算符或相等运算符(<、<=、>、>=、==或!=)的操作数，它不是!的操作数，也不是表达式语句的整个表达式。

第二，longjmp只能跳过调用堆栈，跳到仍在执行的函数。一旦对函数的调用停止执行(就像它返回时一样)，您就不能跳回该调用。您的代码将上下文保存在routineB中，然后跳转到routineA (这将结束routineB的执行)，然后尝试跳转到保存的上下文。但是C 2018年7.13.2.1 2关于longjmp说：

…如果包含setjmp宏调用的函数在临时期间终止了执行，则…该行为未定义。

票数 5

页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持

原文链接：

https://stackoverflow.com/questions/70324244

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