trouble with strcmp() in C

Question

char* mystr = calloc(25, sizeof(char));
fgets(mystr, 25, stdin); // I enter "6 7 *" in here, without the quotes

char* tok;
tok = strtok(mystr, " ");
while (tok != NULL) {
    if(strcmp(tok, "*") == 0)
        //It never meets this condition, but I don't understand why
    else
        //do something else here
    tok = strtok(NULL, " ");
}

The problem is that the strcmp(tok, "*") never returns as being equal, even though tok reads in the asterisk from the original string. I don't understand why it never meets this condition.

Learn how to use a debugger (like `gdb`). Maybe try with `strncmp(tok, "*", 1)` — Basile Starynkevitch, Oct 17 '13 at 06:07
Step through the code in a debugger, line by line, and see what `strtok` returns. — Some programmer dude, Oct 17 '13 at 06:07
Not only is `sizeof(char)` 1, it's better not to repeat the `char` unnecessarily: `TYPE *ptr = calloc(N, sizeof *ptr)`. — Kaz, Oct 17 '13 at 06:22

score 5 · Accepted Answer · answered Oct 17 '13 at 06:11

5

Your * token is likely also containing the \n character you typed to complete your input. Either compare a single character with one of:

  if(tok[0] == '*')

  if(strncmp(tok, "*", 1) == 0)

or add \n to your separator list:

  tok = strtok(NULL, " \n");

answered Oct 17 '13 at 06:11

Carl Norum

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I just used your exact program and both of these changes fixed it fine. You must not be showing us something important. – Carl Norum Oct 17 '13 at 06:20

score 0 · Answer 2 · answered Oct 17 '13 at 06:17

0

Because fgets is embedding the newline character into the variable mystr. This is throwing off the comparison. Try to remove "\n" from variable.

answered Oct 17 '13 at 06:17

user2889221

1

score 0 · Answer 3 · answered Oct 17 '13 at 06:18

0

The strcmp() compare two strings. You have to provide * as a string with the terminating character "*\n".

if(strcmp(tok, "*\n") == 0)

answered Oct 17 '13 at 06:18

Thanushan

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score 0 · Answer 4 · answered Oct 17 '13 at 06:32

0

Another input for you. strtok() is not thread safe. Be careful while using. If you are having multiple threads(both calling strtok() for some purpose), use strtok_r().

answered Oct 17 '13 at 06:32

Pallavi Gk

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trouble with strcmp() in C

4 Answers4