setjmp / longjmp 没有跳到我认为应该的地方
setjmp / longjmp does not jump where I think it should
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，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" );
}
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
第 12 行之前的输出都是正常的：This is not the first iteration - longjmp to routineB，但是，在此之后它应该转到 routineB，并打印 Got to this line via longjmp (in this program it must be from routineA) - continue 但它停留在 routineA，并显示 Got to this line via longjmp (in this program it must be from routineB) - continue。
我的代码有错误还是我对 setjmp / longjmp 有什么误解？
更新
根据 Eric Postpischil 的回答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）。
好的，我把sjr = setjmp( *s_pEnvA );（和routineB的对应部分）换成了
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;
}
我认为这完全符合要点“选择或迭代语句的整个控制表达式”。
结果还是一样
最后更新
Eric 和 Nate 是对的，这是未定义的行为。但由于我想知道为什么 this answer 中的代码有效，我逐渐将我的代码转换为那个代码，并观察它在哪一点中断。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" );
}
如果你用 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，但是如果你用 gcc -DDOESNOTWORK -o ./jump_doesnotwork ./jump.c 编译它，那么它不会（编译器: GNU C11 (Debian 6.3.0-18+deb9u1) 版本 6.3.0 20170516 (x86_64-linux-gnu)).
我认为这么小的变化（不同的字符串格式来打印）破坏了它，清楚地表明 setjmp / longjmp 不能用于此目的。
首先，您调用 setjmp 的行为未由 C 标准定义，因为它们违反了 C 2018 7.13.1.1 4 和 5 中的约束：
An invocation of the setjmp macro shall appear only in one of the following contexts:
— the entire controlling expression of a selection or iteration statement;
— one operand of a relational or equality operator with the other operand an integer constant expression, with the resulting expression being the entire controlling expression of a selection or iteration statement;
— the operand of a unary ! operator with the resulting expression being the entire controlling expression of a selection or iteration statement; or
— the entire expression of an expression statement (possibly cast to void).
If the invocation appears in any other context, the behavior is undefined.
例如，在if( ( sjr = setjmp( *s_pEnvA ) ) == 0 )中，setjmp调用不是整个控制表达式，它不是关系或相等运算符的操作数（<，<=, >, >=, ==, 或 !=), 它不是 ! 的操作数, 也不是整个表达式表达式语句。
其次，longjmp只能将调用堆栈向上跳转到仍在执行的函数。一旦对函数的调用停止执行（如 returns 时），您将无法跳回该调用。您的代码将上下文保存在 routineB 中，然后跳转到 routineA（结束 routineB 的执行），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：
… if the function containing the invocation of the setjmp macro has terminated execution in the interim,… the behavior is undefined.
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，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" );
}
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
第 12 行之前的输出都是正常的：This is not the first iteration - longjmp to routineB，但是，在此之后它应该转到 routineB，并打印 Got to this line via longjmp (in this program it must be from routineA) - continue 但它停留在 routineA，并显示 Got to this line via longjmp (in this program it must be from routineB) - continue。
我的代码有错误还是我对 setjmp / longjmp 有什么误解？
更新
根据 Eric Postpischil 的回答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）。
好的，我把sjr = setjmp( *s_pEnvA );（和routineB的对应部分）换成了
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;
}
我认为这完全符合要点“选择或迭代语句的整个控制表达式”。
结果还是一样
最后更新
Eric 和 Nate 是对的，这是未定义的行为。但由于我想知道为什么 this answer 中的代码有效，我逐渐将我的代码转换为那个代码，并观察它在哪一点中断。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" );
}
如果你用 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，但是如果你用 gcc -DDOESNOTWORK -o ./jump_doesnotwork ./jump.c 编译它，那么它不会（编译器: GNU C11 (Debian 6.3.0-18+deb9u1) 版本 6.3.0 20170516 (x86_64-linux-gnu)).
我认为这么小的变化（不同的字符串格式来打印）破坏了它，清楚地表明 setjmp / longjmp 不能用于此目的。
首先，您调用 setjmp 的行为未由 C 标准定义，因为它们违反了 C 2018 7.13.1.1 4 和 5 中的约束：
An invocation of the
setjmpmacro shall appear only in one of the following contexts:— the entire controlling expression of a selection or iteration statement;
— one operand of a relational or equality operator with the other operand an integer constant expression, with the resulting expression being the entire controlling expression of a selection or iteration statement;
— the operand of a unary
!operator with the resulting expression being the entire controlling expression of a selection or iteration statement; or— the entire expression of an expression statement (possibly cast to
void).If the invocation appears in any other context, the behavior is undefined.
例如，在if( ( sjr = setjmp( *s_pEnvA ) ) == 0 )中，setjmp调用不是整个控制表达式，它不是关系或相等运算符的操作数（<，<=, >, >=, ==, 或 !=), 它不是 ! 的操作数, 也不是整个表达式表达式语句。
其次，longjmp只能将调用堆栈向上跳转到仍在执行的函数。一旦对函数的调用停止执行（如 returns 时），您将无法跳回该调用。您的代码将上下文保存在 routineB 中，然后跳转到 routineA（结束 routineB 的执行），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：
… if the function containing the invocation of the
setjmpmacro has terminated execution in the interim,… the behavior is undefined.