setjmp / longjmp 没有跳到我认为应该跳的地方

Question

setjmp / longjmp 没有跳到我认为应该跳的地方

我想了解 setjmp / longjmp 的工作原理，因此我创建了一个示例程序，其中例程 A 打印偶数，例程 B 打印奇数，并且它们使用 longjmp 相互跳转：

#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" );
}

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我用 gcc 编译它并收到以下输出（插入行号以供以后参考）：

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

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输出在第 12 行之前都是正常的：This is not the first iteration - longjmp to routineB但是，在此之后它应该转到例程 B 并打印，Got to this line via longjmp (in this program it must be from routineA) - continue但它保留在例程 A 中，并显示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 Postpischil 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，因为赋值语句是表达式语句。我收到了相同的结果（除了不同的指针值）。

好吧，我将sjr = setjmp( *s_pEnvA );（以及例程B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;
}

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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。

结果还是一样。

最终更新

埃里克和内特是对的，这是未定义的行为。但由于我想知道为什么这个答案中的代码有效，所以我逐渐将我的代码转换为那个代码，并观察它在哪一点崩溃。我最终得到了这个：

#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" );
}

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如果您使用编译上面的内容gcc -DSORTOFWORKS -o ./jump_sortofworks ./jump.c，那么它会打印标有注释的行/* SORTOFWORKS: printed, DOESNOTWORK: missing */，但如果您使用编译它gcc -DDOESNOTWORK -o ./jump_doesnotwork ./jump.c，则不会打印（编译器：GNU C11 (Debian 6.3.0-18+deb9u1) version 6.3.0 20170516 (x86_64 -linux-gnu））。

我认为这样一个小变化（打印的不同字符串格式）打破了它，清楚地表明 setjmp / longjmp 不能用于此目的。

Answer 1

Eri*_*hil 5

首先，您的调用行为setjmp不是由 C 标准定义的，因为它们违反了 C 2018 7.13.1.1 4 和 5 中的约束：

\n

\n
宏的调用setjmp只能出现在以下上下文之一中：
\n
\xe2\x80\x94 选择或迭代语句的整个控制表达式；
\n
\xe2\x80\x94 关系运算符或相等运算符的一个操作数，另一个操作数是整型常量表达式，结果表达式是选择或迭代语句的整个控制表达式；
\n
\xe2\x80\x94 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；或者
\n
\xe2\x80\x94 表达式语句的整个表达式（可能转换为void）。
\n
如果调用出现在任何其他上下文中，则行为未定义。
\n

\n

例如，在中if( ( sjr = setjmp( *s_pEnvA ) ) == 0 )，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。

\n

其次，longjmp只能向上跳转调用堆栈，跳转到仍在执行的函数。一旦对函数的调用停止执行（当它返回时），您就无法跳回该调用。您的代码将上下文保存在中routineB，然后跳转到routineA（结束执行routineB），然后尝试跳转到保存的上下文。但是 C 2018 7.13.2.1 2，关于longjmp说：

\n

\n
\xe2\x80\xa6 如果包含宏调用的函数setjmp在此期间终止执行，\xe2\x80\xa6 行为未定义。
\n

\n

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