Null-terminating a string in C

Question

I have a string with some spaces on the end. I would like to terminate this string at the position when the first space occurs, so that when I later do strncpy() on it, it would copy only the part of the string which doesn't contain spaces.

This is some try which gives me a segault obviously. How can I do what I intend to do?

int main() { char* s1 = "SomeString "; *(s1 + 10)='\0'; printf("%s\n",s1); return 0; }

The segfault is due to the fact that s1 points to a string which is allocated in read-only memory segment. Attempting to write into this segment causes a memory-access violation. That being said, you can use strtok for your purpose (again, as long as your string is allocated in a writable memory segment). — barak manos
– barak manos, Commented Apr 26, 2015 at 11:18
@naltipar: I'm not the down-voter, but I'd get rid of that 50 there. In addition, one good reason for the down-vote might be the fact that you've shown a very "hard-coded" example of how you truncate that string at index 10. — barak manos
– barak manos, Commented Apr 26, 2015 at 11:27
@naltipar: Most importantly, the segfault itself is not because OP has "not allocated space for your array". An array (or any piece of data for that matter) always has space allocated to it (how else would it exist?). The segfault here is due to the location of the array within the executable image (namely, the RO-data section instead of the stack). In fact, that is probably the main reason for the down-vote. — barak manos
– barak manos, Commented Apr 26, 2015 at 11:27
BTW: *(s1 + 10) is by definition equivalent to the more common s1[10]. It is also equivalent to *(10 + s1) and 10[s1] interestingly. — Ulrich Eckhardt
– Ulrich Eckhardt, Commented Apr 26, 2015 at 12:30

Oswald · Accepted Answer · 2015-04-26 11:12:19Z

5

Modifying the content of a literal string like you do in *(s1 + 10)='\0'; is undefined behavior.

answered Apr 26, 2015 at 11:12

Oswald

31.8k3 gold badges45 silver badges72 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Moldova · Accepted Answer · 2015-04-26 11:17:14Z

You're attempting to modify a string that is read only because of the way you declared it pointing to a constant value. You'll have to make a copy first:

#include <stdio.h> #include <string.h> int main() { char* s1 = "SomeString "; char *s2 = strdup(s1); char *s3 = strchr(s2, ' '); *s3 = '\0'; printf("%s\n",s2); /* if not needed any more, because strdup allocates heap memory */ free(s2); return 0; }

Jahid · Accepted Answer · 2015-04-26 11:55:35Z

1

You can just use

char s1[] = "SomeString ";

instead of

char* s1 = "SomeString ";

answered Apr 26, 2015 at 11:55

Jahid

22.6k10 gold badges97 silver badges114 bronze badges

Comments

John Hascall · Accepted Answer · 2015-04-26 13:20:35Z

A more generalized example where you actually find that first space character (if any). Also note the change in how s1 is defined to avoid the issue with literal strings (i.e. "this") possibly being stored in readonly memory. In your example s1 points directly at the literal string. Below the literal string is used to initialize the array s1.

 int main() { char s1[] = "SomeString "; char * spc = strchr(s1, ' '); if (spc != NULL) *spc = '\0'; printf("%s\n",s1); return 0; }

Mouze · Accepted Answer · 2015-04-26 11:16:33Z

Maybe strstr could be useful in your case (http://www.cplusplus.com/reference/cstring/strstr/)

char * pFirstSpace = strstr (s1," "); if (pFirstSpace) *pFirstSpace=0;

UPDATE

As many others have noticed, this turn out in Access Violation. This runs only on dynamic allocated chars. So you need to copy your string in a dynamic buffer before changes its content

Collectives™ on Stack Overflow

Null-terminating a string in C

5 Answers 5

Comments

Comments

Comments

Comments

Comments

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

Comments

Comments

Comments

Comments

Comments

Related