two C functions trying to return the first word in a string

Question

I have created the two following functions. The first, eatWrd, returns the first word in a string without any white spaces, and removes the first word from the input string:

MAX is a number representing the max length of a string

char* eatWrd(char * cmd)
{
  int i = 0;        //i will hold my place in cmd
  int count = 0;    //count will hold the position of the second word
  int fw = 0;       //fw will hold the position of the first word
  char rest[MAX]; // rest will hold cmd without the first word
  char word[MAX]; // word will hold the first word

  // start by removing initial white spaces
  while(cmd[i] == ' ' || cmd[i] == '\t'){
    i++;
    count++;
    fw++;
  }

  // now start reading the first word until white spaces or terminating characters
  while(cmd[i] != ' ' && cmd[i] != '\t' && cmd[i] != '\n' && cmd[i] != '\0'){
    word[i-fw] = cmd[i];
    i++;
    count++;
  }
  word[i-fw] = '\0';

  // now continue past white spaces after the first word
  while(cmd[i] == ' ' || cmd[i] == '\t'){
    i++;
    count++;
  }

  // finally save the rest of cmd
  while(cmd[i] != '\n' && cmd[i] != '\0'){
    rest[i-count] = cmd[i];
    i++;
  }
  rest[i-count] = '\0';

  // reset cmd, and copy rest back into it
  memset(cmd, 0, MAX);
  strcpy(cmd, rest);

  // return word as a char *
  char *ret = word;
  return ret;
}

The second, frstWrd, just returns the first word without modifying the input string:

// this function is very similar to the first without modifying cmd
char* frstWrd(char * cmd)
{
  int i = 0;
  int fw = 0;
  char word[MAX];

  while(cmd[i] == ' ' || cmd[i] == '\t'){
    i++;
    fw++;
  }

  while(cmd[i] != ' ' && cmd[i] != '\t' && cmd[i] != '\n' && cmd[i] != '\0'){
    word[i-fw] = cmd[i];
    i++;
  }
  word[i-fw] = '\0';

  char *ret = word;
  return ret;
}

To test the function, I used fgets to read a string from the User(me), and then I printed three strings (frstWrd(input), eatWrd(input), eatWrd(input)). I would have expected that given a string, "my name is tim" for example, the program would print "my my name", but instead it prints the third word three times over, "is is is":

// now simply test the functions
main()
{
  char input[MAX];
  fgets(input, MAX - 1, stdin);
  printf("%s %s %s", frstWrd(input), eatWrd(input), eatWrd(input));
}

I have looked over my functions over and over and cannot see the mistake. I believe there is simply something I don't know about printf, or about using multiple string modification functions as arguments in another function. Any insight would be helpful thanks.

Jut one long paragraph describing your problem is hard to read. Try to split up the information somehow. — bolov, Feb 01 '15 at 20:55
Related: You should wonder whether there are any guarantees as to the order of execution of those three function calls in your `printf()` arguments. (spoiler: there is no such guarantee). And there is clearly *undefined behavior* in your return values (see Volands answer below). — WhozCraig, Feb 01 '15 at 20:58
duplicate, http://stackoverflow.com/questions/25798977/returning-string-from-c-function (and a dozen other threads) — M.M, Feb 01 '15 at 22:51

VolAnd · Accepted Answer · 2015-02-01T21:16:09.700

1

As I see rest and word are local variables in the function eatWrd. So it is bad practice to return pointer to such memory outside functions.

EDIT 1:

Also you should understand, that in line

printf("%s %s %s", frstWrd(input), eatWrd(input), eatWrd(input));

function eatWrd(input) could be called the first (before frstWrd(input)).

EDIT 2:

This can be usefull in finction eatWrd

  //char rest[MAX]; // rest will hold cmd without the first word
  char * rest = (char*) malloc(MAX);

And new main let be as:

int main()
{
  char input[MAX];
  fgets(input, MAX - 1, stdin);
  printf("%s ", frstWrd(input));
  printf("%s ", eatWrd(input));
  printf("%s\n", eatWrd(input));
}

And in the end my solution for frstWrd (just to show how standard functions can be useful):

char* frstWrd(char * cmd)
{
  char * word = (char *) malloc(MAX);
  sscanf(cmd, "%s", word);
  return word;
}

edited Feb 01 '15 at 21:16

answered Feb 01 '15 at 20:59

VolAnd

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Ok thanks, so is there any good way to return a string from a c function? – quigs Feb 01 '15 at 21:15
@quigs You can also just `malloc()` the memory you need for the string and return that. The big advantage of this approach is that you don't need to use a preallocated buffer with a given size, instead you call allocate your memory to fit the string. This eliminates a huge error source. This is even more convenient, as there are already some standard functions (POSIX-2008 standard) that return malloc'ed strings (`asprintf()`, `strdup()`, etc.). These can usually be used to do the heavy lifting. The only thing you need to remember is the `free()` call. – cmaster - reinstate monica Feb 01 '15 at 21:23
@quigs: Functions of type char* return pointers, and very importnt to use memory that is available after function is finished. So there are three options: a) use memory from heap (allocate with malloc or similar functions); b) use static local variables (memory will be reserved and available between function is working); c) return pointers to memory that occupied by global variables or memory allocated outside (e.g. memory pointed by cmd in your example). – VolAnd Feb 01 '15 at 21:25

two C functions trying to return the first word in a string

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