Segmentation fault with char* array

simopal6 · 06-15-2006, 04:36 AM

Hi to all! I'm new to the forum, and i need urgent help!
If i try to run this program:

#include <stdio.h>
int main()
{
char *str[3];
int i;
for(i=0; i<3; i++)
{
printf("i: %d\n", i);
sprintf(str[i], "string_%d", i);
}
}

i get segmentation fault when i=1 after sprintf.
What might cause this problem?
Thank you!

jschiwal · 06-15-2006, 05:29 AM

This part: char *str[3]; creates an array pointers to chars. You haven't created strings for them to point to or reserved memory for the strings. The pointers may be NUL also.

simopal6 · 06-15-2006, 06:15 AM

Mmm..How do you think i can solve this problem...
I need an array of strings to which assign values dynamically.
Thank you!

jtshaw · 06-15-2006, 07:28 AM

Moved: This thread is more suitable in Programming and has been moved accordingly to help your thread/question get the exposure it deserves.

jtshaw · 06-15-2006, 07:35 AM

Quote:

Originally Posted by simopal6

Mmm..How do you think i can solve this problem...
I need an array of strings to which assign values dynamically.
Thank you!

Your going to have to allocate space for the strings before you assign them.

I'd start by assigning all the pointers to NULL at the declaration.

Code:

char *str[3];
for (i = 0; i < 3; i++) {
     str[i] = NULL;
}

Then I would allocate memory for the str[i] in the loop before assigning it. What I've done below allows it to take in a string of 20 characters or less. There are probably some more elegant solutions, but I'll start with a simple example and we will see where it goes.

You'll also notice I used snprintf instead of sprintf. If you are dealing with cstrings (aka arrays of characters) it is always wise to use snprintf to ensure you don't try and copy more data then the string can hold into it. Not doing this can lead to wierd behavor as a result of buffer overflows.

Code:

if (str[i] == NULL) {
     str[i] = malloc (sizeof(char) * 20);
}
snprintf(str[i] ,20, "string_%d", i);

simopal6 · 06-15-2006, 09:20 AM

Yes, if i set the pointers to NULL and malloc enough memory for them, it works! I didn't use snprintf but i was careful to not put more data then the amount i malloc'd.
So, if i've understood the situation, the problem was that i put data in memory starting from pointers i had not allocated memory to, so the characters i wrote went out of the space allocated for the process, is this correct??
Thank you very much, also for answering so fast!! And please forgive my english, hope i didn't make too many mistakes! :-)

jtshaw · 06-15-2006, 09:34 AM

Quote:

Originally Posted by simopal6

So, if i've understood the situation, the problem was that i put data in memory starting from pointers i had not allocated memory to, so the characters i wrote went out of the space allocated for the process, is this correct??

Sort of. When you declare a variable in C it does not initialize it to any particular value. In the case of a pointer this means it could theoretically be pointed anywhere. When you write to it either you'll get a segmentation fault because you attempt to write to memory you don't have permissions to write too or if you get lucky (or unlucky as the case might be) it will appear to succeed and write to where ever it happens to be pointing too. In the second case you have potentially overwritten something important like other variables in your program.

simopal6 · 06-16-2006, 08:15 AM

Ok..Got it! Thanks very much for you explanation!

elyk1212 · 06-16-2006, 01:05 PM

Yeah, always initialize pointers to null if you are not going to malloc them on the next line, or you will run the risk of strange behavior when you forget. It can be a debugging nightmare that can be avoided with proper planning.

The inverse of this is, always check that the pointer is not null before writing to it. Then, don't forget to free the memory when you are done with the heap pointer.