C's strtok() and read only string literals

Question

char *strtok(char *s1, const char *s2)

repeated calls to this function break string s1 into \"tokens\"--that is the string is broken into substr

Answer 1

What did you initialize the char * to?

If something like

char *text = "foobar";

then you have a pointer to some read-only characters

For

char text[7] = "foobar";

then you have a seven element array of characters that you can do what you like with.

strtok writes into the string you give it - overwriting the separator character with null and keeping a pointer to the rest of the string.

Hence, if you pass it a read-only string, it will attempt to write to it, and you get a segfault.

Also, becasue strtok keeps a reference to the rest of the string, it's not reeentrant - you can use it only on one string at a time. It's best avoided, really - consider strsep(3) instead - see, for example, here: http://www.rt.com/man/strsep.3.html (although that still writes into the string so has the same read-only/segfault issue)