我应该使用编译器指令gcc -pedantic -Wall -ansi -O4。但是-O4标志给了我一些问题。它没有O4标志，但有了它，我得到了意想不到的结果：
$ gcc -pedantic -Wall -ansi openshell.c
$ ./a.out
'PATH' is set to /home/dac/proj/google-cloud-sdk/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games.
miniShell>> ls | wc -l
82
6318: child 6319 status 0x0000
Execution time 1.197 ms
miniShell>> exit
$ gcc -pedantic -Wall -ansi -O4 openshell.c
$ ./a.out
'PATH' is set to /home/dac/proj/google-cloud-sdk/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games.
miniShell>> ls | wc -l
./a.out:6332: Syntax error: pipe with no command following (10: No child processes)
我该怎么办？相关代码是：
static void exec_arguments(int argc, char **argv)
{
/* Split the command line into sequences of arguments */
/* Break at pipe symbols as arguments on their own */
int i = 1;
char ***cmdv = malloc(sizeof *cmdv * (argc/2));
char **args = malloc(sizeof *args * (argc+1));
int cmdn = 0;
int argn = 0;
cmdv[cmdn++] = &args[argn];
for (i=1; i < argc; i++)
{
char *arg = argv[i];
if (strcmp(arg, "|") == 0)
{
if (i == 1)
err_sysexit("Syntax error: pipe before any command");
if (args[argn-1] == 0)
err_sysexit("Syntax error: two pipes with no command between");
arg = 0;
}
args[argn++] = arg;
if (arg == 0)
cmdv[cmdn++] = &args[argn];
}
if (args[argn-1] == 0)
err_sysexit("Syntax error: pipe with no command following");
args[argn] = 0;
exec_pipeline(cmdn, cmdv);
free(cmdv);
free(args);
}
如果我使用O3和O4进行编译但不使用O2进行编译，则会发生这种情况。对上述函数的调用是这段代码：
argv2[1] = line;
i = 1;
p = strtok (line, " ");
while (p != NULL)
{
array[i++] = p;
p = strtok (NULL, " ");
}
exec_arguments(i, array);
corpse_collector();
这是valgrind的分析：
$ valgrind --leak-check=full -v ./a.out
==9145== Memcheck, a memory error detector
==9145== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==9145== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
==9145== Command: ./a.out
==9145==
--9145-- Valgrind options:
--9145-- --leak-check=full
--9145-- -v
--9145-- Contents of /proc/version:
--9145-- Linux version 3.16.0-48-generic (buildd@lcy01-10) (gcc version 4.8.2 (Ubuntu 4.8.2-19ubuntu1) ) #64~14.04.1-Ubuntu SMP Thu Aug 20 23:03:57 UTC 2015
--9145--
--9145-- Arch and hwcaps: AMD64, LittleEndian, amd64-cx16-lzcnt-rdtscp-sse3-avx-avx2-bmi
--9145-- Page sizes: currently 4096, max supported 4096
--9145-- Valgrind library directory: /usr/lib/valgrind
--9145-- Reading syms from /home/dac/ClionProjects/openshell/a.out
--9145-- Reading syms from /lib/x86_64-linux-gnu/ld-2.21.so
--9145-- Considering /lib/x86_64-linux-gnu/ld-2.21.so ..
--9145-- .. CRC mismatch (computed c1319e6e wanted c87fa39c)
--9145-- Considering /usr/lib/debug/lib/x86_64-linux-gnu/ld-2.21.so ..
--9145-- .. CRC is valid
--9145-- Reading syms from /usr/lib/valgrind/memcheck-amd64-linux
--9145-- Considering /usr/lib/valgrind/memcheck-amd64-linux ..
--9145-- .. CRC mismatch (computed cd34a87b wanted 936d30dc)
--9145-- object doesn't have a symbol table
--9145-- object doesn't have a dynamic symbol table
--9145-- Scheduler: using generic scheduler lock implementation.
--9145-- Reading suppressions file: /usr/lib/valgrind/default.supp
==9145== embedded gdbserver: reading from /tmp/vgdb-pipe-from-vgdb-to-9145-by-dac-on-???
==9145== embedded gdbserver: writing to /tmp/vgdb-pipe-to-vgdb-from-9145-by-dac-on-???
==9145== embedded gdbserver: shared mem /tmp/vgdb-pipe-shared-mem-vgdb-9145-by-dac-on-???
==9145==
==9145== TO CONTROL THIS PROCESS USING vgdb (which you probably
==9145== don't want to do, unless you know exactly what you're doing,
==9145== or are doing some strange experiment):
==9145== /usr/lib/valgrind/../../bin/vgdb --pid=9145 ...command...
==9145==
==9145== TO DEBUG THIS PROCESS USING GDB: start GDB like this
==9145== /path/to/gdb ./a.out
==9145== and then give GDB the following command
==9145== target remote | /usr/lib/valgrind/../../bin/vgdb --pid=9145
==9145== --pid is optional if only one valgrind process is running
==9145==
--9145-- REDIR: 0x401add0 (ld-linux-x86-64.so.2:strlen) redirected to 0x3809e1b1 (???)
--9145-- Reading syms from /usr/lib/valgrind/vgpreload_core-amd64-linux.so
--9145-- Considering /usr/lib/valgrind/vgpreload_core-amd64-linux.so ..
--9145-- .. CRC mismatch (computed 1c3ef3cc wanted d1ae2653)
--9145-- object doesn't have a symbol table
--9145-- Reading syms from /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so
--9145-- Considering /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so ..
--9145-- .. CRC mismatch (computed 6e6e6f70 wanted ea7b69f1)
--9145-- object doesn't have a symbol table
==9145== WARNING: new redirection conflicts with existing -- ignoring it
--9145-- old: 0x0401add0 (strlen ) R-> (0000.0) 0x3809e1b1 ???
--9145-- new: 0x0401add0 (strlen ) R-> (2007.0) 0x04c2f060 strlen
--9145-- REDIR: 0x401ab30 (ld-linux-x86-64.so.2:index) redirected to 0x4c2ec00 (index)
--9145-- REDIR: 0x401ad50 (ld-linux-x86-64.so.2:strcmp) redirected to 0x4c30110 (strcmp)
--9145-- REDIR: 0x401bac0 (ld-linux-x86-64.so.2:mempcpy) redirected to 0x4c33330 (mempcpy)
--9145-- Reading syms from /lib/x86_64-linux-gnu/libc-2.21.so
--9145-- Considering /lib/x86_64-linux-gnu/libc-2.21.so ..
--9145-- .. CRC mismatch (computed 871d6e12 wanted 63ccc9d2)
--9145-- Considering /usr/lib/debug/lib/x86_64-linux-gnu/libc-2.21.so ..
--9145-- .. CRC is valid
--9145-- REDIR: 0x4ec7950 (libc.so.6:strcasecmp) redirected to 0x4a26730 (_vgnU_ifunc_wrapper)
--9145-- REDIR: 0x4ec9c40 (libc.so.6:strncasecmp) redirected to 0x4a26730 (_vgnU_ifunc_wrapper)
--9145-- REDIR: 0x4ec70d0 (libc.so.6:memcpy@GLIBC_2.2.5) redirected to 0x4a26730 (_vgnU_ifunc_wrapper)
--9145-- REDIR: 0x4ec5370 (libc.so.6:rindex) redirected to 0x4c2e8e0 (rindex)
==9145== Syscall param rt_sigaction(act->sa_mask) points to uninitialised byte(s)
==9145== at 0x4E6D3F1: __libc_sigaction (sigaction.c:62)
==9145== by 0x401061: main (in /home/dac/ClionProjects/openshell/a.out)
==9145== Address 0xffeffefc8 is on thread 1's stack
==9145==
--9145-- REDIR: 0x4ec3670 (libc.so.6:strlen) redirected to 0x4c2efa0 (strlen)
--9145-- REDIR: 0x4ec3ae0 (libc.so.6:__GI_strncmp) redirected to 0x4c2f750 (__GI_strncmp)
--9145-- REDIR: 0x4ece6b0 (libc.so.6:strchrnul) redirected to 0x4c32e60 (strchrnul)
--9145-- REDIR: 0x4ec7300 (libc.so.6:__GI_mempcpy) redirected to 0x4c33060 (__GI_mempcpy)
'PATH' is set to /home/dac/proj/google-cloud-sdk/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games.
--9145-- REDIR: 0x4ebc440 (libc.so.6:malloc) redirected to 0x4c2bb60 (malloc)
--9145-- REDIR: 0x4ecc390 (libc.so.6:__GI_memcpy) redirected to 0x4c30b60 (__GI_memcpy)
--9145-- REDIR: 0x4ec30a0 (libc.so.6:strcpy) redirected to 0x4a26730 (_vgnU_ifunc_wrapper)
--9145-- REDIR: 0x4edcab0 (libc.so.6:__strcpy_sse2_unaligned) redirected to 0x4c2f080 (strcpy)
--9145-- REDIR: 0x4ebc7f0 (libc.so.6:free) redirected to 0x4c2cdc0 (free)
miniShell>> ls | wc -l
--9145-- REDIR: 0x4ec67c0 (libc.so.6:memchr) redirected to 0x4c301b0 (memchr)
--9145-- REDIR: 0x4ec5330 (libc.so.6:strncpy) redirected to 0x4a26730 (_vgnU_ifunc_wrapper)
--9145-- REDIR: 0x4edd0e0 (libc.so.6:__strncpy_sse2_unaligned) redirected to 0x4c2f5b0 (__strncpy_sse2_unaligned)
--9145-- REDIR: 0xffffffffff600000 (???:???) redirected to 0x3809e193 (???)
./a.out:9145: Syntax error: pipe with no command following--9145-- REDIR: 0x4ec77e0 (libc.so.6:__GI_stpcpy) redirected to 0x4c31fe0 (__GI_stpcpy)
(10: No child processes)
==9145==
==9145== HEAP SUMMARY:
==9145== in use at exit: 64 bytes in 2 blocks
==9145== total heap usage: 8 allocs, 6 frees, 300 bytes allocated
==9145==
==9145== Searching for pointers to 2 not-freed blocks
==9145== Checked 73,312 bytes
==9145==
==9145== LEAK SUMMARY:
==9145== definitely lost: 0 bytes in 0 blocks
==9145== indirectly lost: 0 bytes in 0 blocks
==9145== possibly lost: 0 bytes in 0 blocks
==9145== still reachable: 64 bytes in 2 blocks
==9145== suppressed: 0 bytes in 0 blocks
==9145== Reachable blocks (those to which a pointer was found) are not shown.
==9145== To see them, rerun with: --leak-check=full --show-leak-kinds=all
==9145==
==9145== Use --track-origins=yes to see where uninitialised values come from
==9145== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)
==9145==
==9145== 1 errors in context 1 of 1:
==9145== Syscall param rt_sigaction(act->sa_mask) points to uninitialised byte(s)
==9145== at 0x4E6D3F1: __libc_sigaction (sigaction.c:62)
==9145== by 0x401061: main (in /home/dac/ClionProjects/openshell/a.out)
==9145== Address 0xffeffefc8 is on thread 1's stack
==9145==
==9145== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)
$
使用clang编译器编译并运行干净。我想如果我想继续解决这个问题，我将留下一个最小的例子并比较生成的程序集。整个代码可用here。
奇怪的是，如果我们在循环中插入一个print语句，那么代码就可以了：
miniShell>> ls | wc -l
Processing 1 [ls]
Processing 2 [|]
Processing 3 [wc]
Processing 4 [-l]
84
27892: child 27900 status 0x0000
Execution time 1.358 ms
static void exec_arguments(int argc, char **argv)
{
/* Split the command line into sequences of arguments */
/* Break at pipe symbols as arguments on their own */
int i = 1;
char ***cmdv = malloc(sizeof *cmdv * (argc/2));
char **args = malloc(sizeof *args * (argc+1));
int cmdn = 0;
int argn = 0;
char *arg;
cmdv[cmdn++] = &args[argn];
for (i=1; i < argc; i++)
{
arg = argv[i];
printf("Processing %d [%s]\n", i, arg);
if (strcmp(arg, "|") == 0)
{
if (i == 1)
err_sysexit("Syntax error: pipe before any command");
if (args[argn-1] == 0)
err_sysexit("Syntax error: two pipes with no command between");
arg = 0;
}
args[argn] = arg;
argn++;
/*printf("argn %d", argn);*/
if (arg == 0)
cmdv[cmdn++] = &args[argn];
}
if (args[argn-1] == 0)
err_sysexit("Syntax error: pipe with no command following");
args[argn] = 0;
exec_pipeline(cmdn, cmdv);
free(cmdv);
free(args);
}
知道了上面的内容，我可以放入一个不写任何输出的调试语句，并使循环工作（但我们不知道为什么），这将允许我的代码以最高的优化级别进行编译。我们仍然怀疑一些指针处理，我们可能会通过制作独立的bug的最小例子来调查。现在，这将使我继续：
static void exec_arguments(int argc, char **argv)
{
/* Split the command line into sequences of arguments */
/* Break at pipe symbols as arguments on their own */
int i = 1;
char ***cmdv = malloc(sizeof *cmdv * (argc/2));
char **args = malloc(sizeof *args * (argc+1));
int cmdn = 0;
int argn = 0;
char *arg;
FILE* debug;
cmdv[cmdn++] = &args[argn];
for (i=1; i < argc; i++)
{
arg = argv[i];
debug = fopen("/dev/null", "w");
fprintf(debug, "Debug Information");
if (strcmp(arg, "|") == 0)
{
if (i == 1)
err_sysexit("Syntax error: pipe before any command");
if (args[argn-1] == 0)
err_sysexit("Syntax error: two pipes with no command between");
arg = 0;
}
args[argn] = arg;
argn++;
/*printf("argn %d", argn);*/
if (arg == 0)
cmdv[cmdn++] = &args[argn];
}
if (args[argn-1] == 0)
err_sysexit("Syntax error: pipe with no command following");
args[argn] = 0;
exec_pipeline(cmdn, cmdv);
free(cmdv);
free(args);
}
答案 0 :(得分：4)
在修剪代码之后，我发现了麻烦的原因。
正如我第一次suggested，问题是“最有可能的是，代码中存在一些未定义的行为，使用优化会揭示它”。
问题是main()中的数组定义为char *array[4];。代码溢出该数组，导致未定义的行为。将大小更改为40意味着程序可靠地运行。
虽然我已经删除了大量代码，但文件中仍然有超过400行代码。我确信可以做一个更彻底的减少工作，但我相信这是造成麻烦的主要原因。
以下是简化的main()函数，其char *array[4]l设置太小，以及两个新的辅助函数dump_argv()和AllWhiteSpace()。许多其他功能已被删除。前景/背景的东西已经消失了;内置命令（例如cd和exit以及unixinfo已被删除。corpse_collector()已收集的尸体; Janitor()函数也试图收集它们，很少（好吧，我的意思是永远！）发现了什么，因为尸体收藏家已经做了肮脏的工作。我删除了信号处理，因为没有立即相关。
#include <ctype.h>
#include <stdbool.h>
static void dump_argv(const char *tag, int argc, char **argv)
{
assert(argv != 0 && tag != 0);
printf("%s: (%d)", tag, argc);
for (int i = 0; i < argc; i++)
printf(" {%s}", (argv[i] == NULL) ? "(null)" : argv[i]);
putchar('\n');
fflush(stdout);
assert(argv[argc] == 0);
}
static bool AllWhiteSpace(char *source)
{
unsigned char c;
while (isspace(c = *source))
source++;
return(c == '\0');
}
int main(int argc, char *argv[])
{
err_setarg0(argv[argc-argc]);
while (1)
{
char *p;
char *array[40];
char line[BUFFER_LEN];
size_t length;
long time;
struct timeval time_start;
struct timeval time_end;
printf("miniShell>> ");
fflush(stdout);
if (!fgets(line, BUFFER_LEN, stdin))
{
putchar('\n');
break;
}
if (AllWhiteSpace(line))
continue;
length = strlen(line);
if (line[length - 1] == '\n')
line[length - 1] = '\0';
gettimeofday(&time_start, NULL);
printf("Command line: %s\n", line);
int i = 1;
p = strtok(line, " ");
array[0] = NULL;
while (p != NULL)
{
array[i++] = p;
p = strtok(NULL, " ");
}
array[i] = NULL;
dump_argv("Before exec_arguments", i, array);
exec_arguments(i, array);
corpse_collector();
gettimeofday(&time_end, NULL);
time = (time_end.tv_sec - time_start.tv_sec) * 1000000 +
time_end.tv_usec - time_start.tv_usec;
printf("Execution time %ld.%03ld ms\n", time / 1000, time % 1000);
}
return(0);
}
现在麻烦的症状有些不同 - 要执行的字符串没有意义，因此事情无法执行，但是空间增加得足够大（40对于使用中的命令行来说是过度的; 6就足够了）它工作正常。
答案 1 :(得分：3)
您的计划中存在错误。 -O4选项与-O3不同。快速查看man gcc将列出执行任何操作的选项列表以及完整性检查：
diff <(gcc -Q --help=optimizers -O4) <(gcc -Q --help=optimizers -O3)
没有区别。
通常，更改优化级别是查找程序中的错误的好方法，因为它可能在一个优化级别中表现完美，或在另一个优化级别中断。这表示未定义的行为，等等。我不会浪费时间为您调试程序，但至少您知道问题不是由-O4直接引起的。
旁注：我假设-ansi是您无法更改的要求，但是以C90模式编译，即古代C模式。我建议C99或C11以避免意外。