警告：与字符串比较结果不确定的行为 [英] Warning: comparison with string literals results in unspecified behaviour
问题描述
我开始写在linux C.我一点都不精通C和使用Linux这正是我决定这将是一个好主意的原因简化外壳的一个项目。
与分析器开始，我已经遇到了一些问题。
在code应该是简单的，这就是为什么我不包括任何评论。
我正在与海湾合作委员会的警告：在用警告HERE评论行与不确定的行为字符串结果的比较（见下文code）。
我不知道为什么这会导致警告，但真正的问题是，即使我是一个比较＆LT;到一个＆下;是里面的，如果...
没有得到我要寻找的说明了问题的答案，但如果有你在code应改进请说出来看到的东西。只取一点，我并不精通，并且这仍是一项正在进行的工作（或更好，但在开始工作）。
先谢谢了。
的#include＆LT;＆stdio.h中GT;
＃包括LT＆;＆unistd.h中GT;
＃包括LT＆;＆string.h中GT;的typedef枚举{假的，真正的布尔};typedef结构{
焦炭** ARG;
字符* INFILE;
字符* OUTFILE;
INT背景;
} Command_Info;INT parse_cmd（字符* cmd_line，Command_Info * cmd_info）
{
字符* ARG;
字符* ARGS [100]; INT I = 0;
ARG = strtok的（cmd_line，\\ n）;
而（ARG！= NULL）{
ARGS [I] = ARG;
ARG =的strtok（NULL，\\ n）;
我++;
} 诠释num_elems = I; cmd_info-＆GT; INFILE = NULL;
cmd_info-＆GT; OUTFILE = NULL;
cmd_info-＆GT;背景= 0; INT iarg = 0;
对于（i = 0; I＆LT; num_elems;我++）
{
如果（参数[I] ==＆安培;）//警告HERE
返回-1;
否则，如果（参数[I] ==＆LT;）//警告HERE
如果（参数[I + 1]！= NULL）
cmd_info-＆GT; INFILE = ARGS [I + 1];
其他
返回-1; 否则，如果（参数[I] ==＆gt;中）//警告HERE
如果（参数[I + 1]！= NULL）
cmd_info-＆GT; OUTFILE = ARGS [I + 1];
其他
返回-1; 其他
cmd_info-＆GT; ARG [iarg ++] = ARGS [I]
} cmd_info-＆GT;精氨酸[iarg] = NULL; 返回0;
}无效print_cmd（Command_Info * cmd_info）
{
INT I;
对于（i = 0; cmd_info-＆GT; ARG [我] = NULL;！我++）
的printf（ARG内容[％d] = \\％s \\的\\ n，我，cmd_info-＆GT; ARG [I]）;
的printf（ARG内容[％d] = \\％s \\的\\ n，我，cmd_info-＆GT; ARG [I]）;
的printf（INFILE = \\％s \\的\\ n，cmd_info-＆GT; INFILE）;
的printf（OUTFILE = \\％s \\的\\ n，cmd_info-＆GT; OUTFILE）;
的printf（后台= \\％d个\\\\ n，cmd_info-＆GT;背景）;
}INT主（INT ARGC，CHAR *的argv []）
{
炭cmd_line [100];
Command_Info cmd_info; 的printf（＆GT;＆GT;＆gt;中）; 与fgets（cmd_line，100，标准输入）; parse_cmd（cmd_line，＆安培; cmd_info）; print_cmd（安培; cmd_info）; 返回0;
}
您想要使用的strcmp（）== 0 来比较字符串，而不是一个简单的 == ，这将只是比较，如果指针是相同的（他们不会在这种情况下）。
ARGS [I] 是一个指向字符串的指针（一个指向字符数组空值终止的），如＆放大器; 或＆LT;
这位前pression ARGC [I] ==＆放大器;检查，如果这两个指针是相同的（指向同一个内存位置）
这位前pression STRCMP（ARGC [I]，与＆amp;）== 0 将检查两个字符串的内容是相同的。
I am starting a project of writing a simplified shell for linux in C. I am not at all proficient with C nor with Linux that's exactly the reason I decided it would be a good idea.
Starting with the parser, I have already encountered some problems.
The code should be straightforward that's why I didn't include any comments.
I am getting a warning with gcc: "comparison with string literals results in unspecified behaviour" at the lines commented with "WARNING HERE" (see code below).
I have no idea why this causes an warning, but the real problem is that even though I am comparing an "<" to an "<" is doesn't get inside the if...
I am looking for an answer for the problem explained, however if there's something that you see in the code that should be improved please say so. Just take in mind I am not that proficient and that this is still a work in progress (or better yet, a work in start).
Thanks in advance.
#include <stdio.h>
#include <unistd.h>
#include <string.h>
typedef enum {false, true} bool;
typedef struct {
char **arg;
char *infile;
char *outfile;
int background;
} Command_Info;
int parse_cmd(char *cmd_line, Command_Info *cmd_info)
{
char *arg;
char *args[100];
int i = 0;
arg = strtok(cmd_line, " \n");
while (arg != NULL) {
args[i] = arg;
arg = strtok(NULL, " \n");
i++;
}
int num_elems = i;
cmd_info->infile = NULL;
cmd_info->outfile = NULL;
cmd_info->background = 0;
int iarg = 0;
for (i = 0; i < num_elems; i++)
{
if (args[i] == "&") //WARNING HERE
return -1;
else if (args[i] == "<") //WARNING HERE
if (args[i+1] != NULL)
cmd_info->infile = args[i+1];
else
return -1;
else if (args[i] == ">") //WARNING HERE
if (args[i+1] != NULL)
cmd_info->outfile = args[i+1];
else
return -1;
else
cmd_info->arg[iarg++] = args[i];
}
cmd_info->arg[iarg] = NULL;
return 0;
}
void print_cmd(Command_Info *cmd_info)
{
int i;
for (i = 0; cmd_info->arg[i] != NULL; i++)
printf("arg[%d]=\"%s\"\n", i, cmd_info->arg[i]);
printf("arg[%d]=\"%s\"\n", i, cmd_info->arg[i]);
printf("infile=\"%s\"\n", cmd_info->infile);
printf("outfile=\"%s\"\n", cmd_info->outfile);
printf("background=\"%d\"\n", cmd_info->background);
}
int main(int argc, char* argv[])
{
char cmd_line[100];
Command_Info cmd_info;
printf(">>> ");
fgets(cmd_line, 100, stdin);
parse_cmd(cmd_line, &cmd_info);
print_cmd(&cmd_info);
return 0;
}
You want to use strcmp() == 0 to compare strings instead of a simple ==, which will just compare if the pointers are the same (which they won't be in this case).
args[i] is a pointer to a string (a pointer to an array of chars null terminated), as is "&" or "<".
The expression argc[i] == "&" checks if the two pointers are the same (point to the same memory location).
The expression strcmp( argc[i], "&") == 0 will check if the contents of the two strings are the same.
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