C String Pointers

Question

why is it not possible for sscanf() to write the string into char* s? I initialised it to NULL as I do not want uninitialised variables.

#include <stdio.h>
#include <string.h>

int main()
{

    char* t = "I am a monkey";
    char *s = NULL;
    sscanf(t, "%s",s);
    printf("%s\n",s);


}

There is a need to secure an area to be written. char *s = malloc(16); instead of char *s = NULL;. (malloc #include <stdlib.h> then printf("%s\n",s);free(s);） — BLUEPIXY
– BLUEPIXY, Commented Dec 20, 2015 at 5:25
@kuan It can't because it is specified to do something else. Read the docs. — juanchopanza
– juanchopanza, Commented Dec 20, 2015 at 5:26
@kuan re: your comment. C functions are pass by value, so sscanf is passed a copy of s (which contains NULL). It cannot change the value of s. Look up pass by value if you are still confused. — Dylan Kirkby
– Dylan Kirkby, Commented Dec 20, 2015 at 5:33

Dylan Kirkby · Accepted Answer · 2015-12-20 05:29:33Z

8

The line char *s = NULL creates a variable that holds the memory address of a character. Then it sets that memory address to zero (NULL is address zero).

Then the line sscanf(t, "%s",s); tries to write the contents of t to the string at the location s. This will segfault because your process cannot access address zero.

Your instincts were good to avoid uninitialized variables, but you traded this for unallocated pointers!

Fix this by allocating some space on the stack (or heap) for s by declaring:

char s[STRING_LENGTH];

Where STRING_LENGTH is #defined to be however many characters you want to allocate. This allocates a chunk of memory to hold the null-terminated character array and sets s to the address of the first character

edited Dec 20, 2015 at 5:29

answered Dec 20, 2015 at 5:24

Dylan Kirkby

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Jeegar Patel · Accepted Answer · 2015-12-20 05:24:05Z

1

Here sscanf defination says that,,,

int sscanf ( const char * s, const char * format, ...);

sscanf(t, "%s",s);

So here it will take text from source as t but on destination side s is just pointer to Null there is no memory.

So allocate some memory to s using malloc or assign s to some valid memory location.

answered Dec 20, 2015 at 5:24

Jeegar Patel

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David Turnbull · Accepted Answer · 2015-12-20 07:48:20Z

0

why is it not possible for sscanf() to write the string into char* s? I initialised it to NULL

The string functions in the C standard library do not allocate storage. All they do is copy things around. You have to manage every byte of string storage yourself. This will seem strange to someone coming from pretty much every other language.

In other words, you have to point *s to some memory for sscanf to write into. If you use NULL then sscanf writes to memory that starts at address NULL (usually 0).

answered Dec 20, 2015 at 7:48

David Turnbull

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