[英]C - Why does getopt return 255 on linux?
I've been fooling around with getopt
(from unistd.h) recently. 我最近一直在使用
getopt
(来自unistd.h)。 I wrote some code that worked fine under Windows 7 compiled with gcc from MinGW, while not working under Raspbian Linux on my Raspberry Pi (I compiled them both with gcc, no options; gcc tc
). 我写了一些在使用MinGW的gcc编译的Windows 7下运行良好的代码,而不是在我的Raspberry Pi上使用Raspbian Linux工作(我用gcc编译它们,没有选项;
gcc tc
)。 For some reason getopt returns int 255 or char ÿ when faced with no switches, when really it should return -1. 出于某种原因,当面对没有开关时,getopt返回int 255或charÿ,实际上它应该返回-1。
#include <stdio.h>
#include <unistd.h>
int main(int argc, char *argv[])
{
char t;
opterr = 0;
while ((t = getopt(argc, argv, "a:")) != -1)
switch (t) {
case 'a':
printf("-a with argument %s\n", optarg);
return 0;
case '?':
printf("uknown option\n");
return 1;
default:
/* This is always 255 under linux, and is never reached under windows */
printf("getopt returned int %d (char %c)\n", t, t);
return 2;
}
return 0;
}
One tought I had was that, actually 255 is -1 in unsinged 8-bit arithmetic, so I tried to put an int cast in the while conditional, but that did nothing. 我得到的是,在未编程的8位算术中实际上255 是 -1,所以我试图在while条件中放置一个int转换,但是什么也没做。
It looks like your system/toolchain defaults to an unsigned char
type. 看起来您的系统/工具链默认为unsigned
char
类型。 That means when getopt()
returns -1, it gets converted to 255 and stored in t
. 这意味着当
getopt()
返回-1时,它将转换为255并存储在t
。 Then that 255 gets promoted to int
type (staying 255) and compared to -1
, which can't ever match. 然后将255提升为
int
类型(保持255)并与-1
进行比较,这是无法匹配的。
getopt()
returns int
, so you should really declare t
as int
to match, but if you're set on using char
, you're going to need to use signed char
. getopt()
返回int
,所以你应该把t
声明为int
来匹配,但如果你使用char
设置,你将需要使用signed char
。
Aside: Since you say you're compiling with gcc, you might also find the -fsigned-char
flag helpful if you want this and other char
variables in your program to be signed. 旁白:既然你说你正在用gcc进行编译,你可能会发现
-fsigned-char
标志有用,如果你想要在你的程序中对这个和其他char
变量进行签名。
Second Aside: You can duplicate the failure by passing the -funsigned-char
flag or by changing t
to be an unsigned char
in your Windows test, if that makes it easier to debug. 第二个:您可以通过传递
-funsigned-char
标志或在Windows测试中将t
更改为unsigned char
来复制失败,如果这样可以更容易调试。
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