Usage of fgets function in C

Question

One of my assignments in to write my own UNIX Shell. To receive input from the user, I am using fgets to capture the input as a string but I\'m not really sure how it works.

Answer 1

Probably the easiest way to handle this is to switch to using scanf to read the input:

char command[51];

scanf("%50[^\n]", command);

if (0 == strcmp(command, "exit"))
    do_something();

Answer 2

As noted, fgets(3) gives you the trailing '\n'. If you use gets(3), you don't gets the trailing newline. Nothing like consistency, sez I.

Perl has a hand chomp() function that trims the trailing newline if its present — you could do worse than to roll your own:

#define NUL ((char)0)
void chomp( char *s )
{
  if ( s != null )
  {
    int len = strlen(s) ;
    if ( len >= 1 && s[len-1] == "\n" )
    {
      s[len-1] = NUL ;
    }
  }
  return ;
}

Answer 3

fgets considers the line terminator as a valid character. That's the extra character you are receiving.

Just do something like command[strlen(command) - 1] = '\0'; to remove the line terminator. Then you are free to do all your strcmp's.

Answer 4

From the fgets manual page:

fgets() reads in at most one less than size characters from stream and stores them into the buffer pointed to by s. Reading stops after an EOF or a newline. If a newline is read, it is stored into the buffer. A '\0' is stored after the last character in the buffer.

Bottom-line: you have an extra newline at the end of your string when comparing.

Answer 5

fgets is capturing the line break, too.

Note that you can overcome this in a few ways, one might be using strncmp:

if((strncmp(command, "exit", 4)) == 0)

which checks if only the first 4 characters of command match (though this might not be the right option for you here).

Another tactic is to check with the line break in place:

if((strcmp(command, "exit\n")) == 0)

Answer 6

fgets will always include the line termination character in the input string. You can remove any space, including the newline characters, from the end of your "command" by doing:

char command[50];
fgets(command, sizeof(command), stdin);

size_t length = strlen(command);
// Trim off trailing "spaces" including newline characters
while ((length > 0) && isspace(command[length-1]))
      command[--length] = '\0';

printf("Your Command: %s\n", &command); // Include newline now...
// This is computed above...
// int length = strlen(command);

// Continue as before