如何在C中创建命令行？

Question

I'm trying to create a command line for an app in C.

int main(int argc, char const *argv[])
{
  char cmd[40];

  do {
    scanf("%[^\n]", cmd);

    if(strcmp(cmd, "start"))
      new_game();
  } while(strcmp(cmd, "exit"));
  return 0;
}

I can't use the "%s" because the command might have more than one argument so I need to use the exception and ignore the new line char. The problem is that the program won't call the scanf anymore after one insertion. Why is this happening?

Thank You

Answer 1

Use fgets instead to read the whole input as shown below:

char cmd[255];
fgets(cmd, 255, stdin);

The sample can be found here .

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Also, the strcmp function is not used correctly in your example. Strcmp returns:

• <0 when the first character that does not match has a lower value in ptr1 than in ptr2,
• 0 when they are equal
• '>0 when the first character that does not match has a greater value in ptr1 than in ptr2

So in your example, you should use strcmp(str1, str2) == 0 or !strcmp(str1, str2) for short.

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Note: Care should be taken when using fgets . A newline character is considered valid by fgets and it is included in the string. On the other side with scanf(" %s") a newline is not included in the resulting string.

Answer 2

strcmp(cmd, "start") means "when cmd does not equal start". You need !strcmp(cmd, "start") .

Answer 3

The program jams because you read everything up to the first newline, but then you go back again and the scanf() finds a newline, which isn't allowed by the scan set, so it reads no characters and leaves the newline in place for the next time round the loop.

You should be checking the return value from scanf() ; if it is zero, then the next character isn't valid for the format string.

You have to read the newline.

One simple way is to include a space before the %[…] scan set in the format string. That skips leading white space, including newlines, before reading the rest of a line of information (and the 39 prevents buffer overflows):

if (scanf(" %39[^\n]", cmd) == 1)
    …process the string in cmd…
else
    …report error…

Another simple way is to use a loop after the scanf() :

int c;
while ((c = getchar()) != EOF && c != '\n')
    ;

Alternatively, as others have suggested, read a line of input and then process that, perhaps with sscanf() to read the information into your command name variable.