usage of strings in C

[krishna chaithanya]

Hi,

Just wanted to know the consequences if I write this piece of a code in C.

int main()
{
char* s1 = "abcdefghijklmnopqrstuvwxyz";
char* s2 = (char*) malloc (100);
strcpy(s2,"Alphabets:");
strcat(s2,s1);
strcat(s2," ");
strcat(s2,s1);
printf("s1 is %s and s2 is %s",s1,s2);
free(s2);
return 0;
}

Where does s1 point to?

Assume that my environment is having less memory.

Thanks and Regards,
krishna

[gerard4143]

String s1 - char* s1 = "abcdefghijklmnopqrstuvwxyz";
points to the const string "abcdefghijklmnopqrstuvwxyz".

[krishna chaithanya]

so, where will be the memory allocated? In the code segment?

[gerard4143]

Maybe this example will clarify some of your questions:

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

int main()
{
	char* s1 = "abcdefghijklmnopqrstuvwxyz";
	char* s2 = (char*) malloc (100);

	fprintf(stdout, "s1->%p\n", (void*)&s1[0]);
	fprintf(stdout, "s2->%p\n", (void*)&s2[0]);

	strcpy(s2,"Alphabets:");
	strcat(s2,s1);
	strcat(s2," ");
	strcat(s2,s1);

	fprintf(stdout, "s1->%p\n", (void*)&s1[0]);
	fprintf(stdout, "s2->%p\n", (void*)&s2[0]);

	printf("s1 is %s and s2 is %s",s1,s2);

	free(s2);

	return 0;
}

[GNU-Fan]

so, where will be the memory allocated? In the code segment?

Only the second one.
The first one is reserved by the compiler in so called static-storage (read-only).
This is why you had better write

const char* s1 = "abcdefghijklmnopqrstuvwxyz";

[Cabhan]

On the standard Linux architecture, your s1 string gets allocated at compile time (it's a constant!), so it is located in the read-only data segment of memory. This is why in the following code:

char *s1 = "hello";
char *s2 = "hello";

s1 and s2 are actually the same pointer.

bricka@joust ~/test/c $ cat pointers_equal.c 
#include <stdio.h>

int main()
{
    char *s1 = "hello";
    char *s2 = "hello";

    printf("s1 = %p, s2 = %p\n", s1, s2);

    return 0;
}
bricka@joust ~/test/c $ ./a.out 
s1 = 0x80484f0, s2 = 0x80484f0

On the other hand, when you invoke malloc, memory is allocated on the heap, which is a read-writable section of memory.