Processing user input in C

Question

I'm trying to read user input on an infinite loop in a C program. The function that does this is as below.

void processUserInput(unsigned int io, char* cmd, char* buffer){

   char* usrInput;

   int rbytes;
   if ((rbytes = read(io, buffer, 256)) < 0) {
      perror("Read error: ");
      exit(-1);
   }
   buffer[rbytes-1] = 0;

   usrInput = (char*) calloc(256, sizeof(char));
   strcpy(usrInput, buffer);

   cmd = strtok(usrInput, " ");
}

In my main, this is what I do.

char buffer[256];
char* cmd = NULL;
processUserInput(STDIN_FILENO, cmd, buffer);

if(strcasecmp(cmd, "quit") == 0){

   breakTrigger = 1;

}
memset(buffer, 0, 256);

The code gives a segmentation fault the moment I enter an input on STDIN. Any help appreciated.

Why are you using `read` to get user input? You should use `getline` or `fgets(..., ..., stdin)`. — lurker, Nov 17 '15 at 14:09
Search for and read about *pass by value* and *pass by reference* and how to use *pass by reference* in C. — Some programmer dude, Nov 17 '15 at 14:09
why do you need the global buffer variable? since you are allocating memory to userinput to hold the value for the input it seems unnecessary — Cyclonecode, Nov 17 '15 at 14:29

score 2 · Answer 1 · edited May 23 '17 at 11:59

The issue you're having is to write

cmd = strtok(usrInput, " ");

This is not storing the content of the returned pointer into cmd. Therefore, in main(), your cmd remains NULL. Later, the call to strcasecmp() creates UB because of NULL pointer.

You need to

Allocate memory to cmd in main(), before passing that to processUserInput().
Check for non-null return of strtok()
strcpy() the returned token to cmd.

Also, as a side-note, please see this discussion on why not to cast the return value of malloc() and family in C..

score 2 · Accepted Answer · answered Nov 17 '15 at 14:10

because of the parameter type of cmd. In processUserInput, cmd is a pointer to a char (string), so, you can change what cmd points to, but cmd will not be changed when method returns. You should either call processUserInput with &cmd, and within processUserInput do *cmd = strtok(...), or better yet, just have processUserInput return cmd.

char* processUserInput(unsigned int io, char* buffer){

   char* usrInput;

   int rbytes;
   if ((rbytes = read(io, buffer, 256)) < 0) {
      perror("Read error: ");
      exit(-1);
   }
   buffer[rbytes-1] = 0;

   usrInput = (char*) calloc(256, sizeof(char));
   strcpy(usrInput, buffer);

   return strtok(usrInput, " ");
}

char buffer[256];
char* cmd = processUserInput(STDIN_FILENO, buffer);

Processing user input in C

2 Answers2