Thread: error with realloc() (*** glibc detected *** ... invalid pointer)

Hello everybody!
I have a problem with realloc() which I cannot understand even after having searched for solutions in the forum and with google...
I have this simple program:

Code:

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

int main(void)
{
	printf("Insert a tel number (e.g: (555) 555-5555): ");
	char buffer[51];
	fgets(buffer,49,stdin);
	char *pref, *startnum, *endnum;
	pref=(char *)malloc(4*sizeof(char));
	startnum=(char *)malloc(4*sizeof(char));
	endnum=(char *)malloc(4*sizeof(char));

	if (pref==NULL||startnum==NULL||endnum==NULL)
	{
		printf("Not enough memory! Ending...\n");
		exit(1);
	}

	pref=strtok (buffer,"()");
	startnum=strtok (NULL,"- ");
	endnum=strtok (NULL,"\n");

	printf("%s %s %s\n",pref,startnum,endnum);
	
	char *tmp;
	tmp=realloc(startnum,8*sizeof(char));
	if (tmp==NULL)
	{
		printf("Not enough memory when reallocating memory! Ending...\n");
		exit(1);
	}
	startnum=tmp;
	strcat(startnum,endnum);
	printf("%s %s\n",pref,startnum);

	return 0;
}

and i get this output:

Code:

Insert a tel number (e.g: (555) 555-5555): (555) 555-5555
555 555 5555
*** glibc detected *** ./e8_14: realloc(): invalid pointer: 0xbfc719e3 ***
======= Backtrace: =========
/lib/tls/i686/cmov/libc.so.6(realloc+0x39c)[0xb7ec49ec]
/lib/tls/i686/cmov/libc.so.6(realloc+0x3c)[0xb7ec468c]
./e8_14[0x8048677]
/lib/tls/i686/cmov/libc.so.6(__libc_start_main+0xe0)[0xb7e6b450]
./e8_14[0x8048501]
======= Memory map: ========
08048000-08049000 r-xp 00000000 08:02 3408516    /home/federico/università/programmazione C/esercizi/e8_14
08049000-0804a000 rw-p 00000000 08:02 3408516    /home/federico/università/programmazione C/esercizi/e8_14
0804a000-0806b000 rw-p 0804a000 00:00 0          [heap]
b7e54000-b7e55000 rw-p b7e54000 00:00 0
b7e55000-b7f9e000 r-xp 00000000 08:02 1048704    /lib/tls/i686/cmov/libc-2.7.so
b7f9e000-b7f9f000 r--p 00149000 08:02 1048704    /lib/tls/i686/cmov/libc-2.7.so
b7f9f000-b7fa1000 rw-p 0014a000 08:02 1048704    /lib/tls/i686/cmov/libc-2.7.so
b7fa1000-b7fa4000 rw-p b7fa1000 00:00 0
b7fa6000-b7fb0000 r-xp 00000000 08:02 1015876    /lib/libgcc_s.so.1
b7fb0000-b7fb1000 rw-p 0000a000 08:02 1015876    /lib/libgcc_s.so.1
b7fb1000-b7fb5000 rw-p b7fb1000 00:00 0
b7fb5000-b7fb6000 r-xp b7fb5000 00:00 0          [vdso]
b7fb6000-b7fd0000 r-xp 00000000 08:02 1016004    /lib/ld-2.7.so
b7fd0000-b7fd2000 rw-p 00019000 08:02 1016004    /lib/ld-2.7.so
bfc5e000-bfc73000 rw-p bffeb000 00:00 0          [stack]
Aborted

can you please explain me the origin of the issue? Thanks a lot for your help! Bye!

Code:

	startnum=strtok (NULL,"- ");
	endnum=strtok (NULL,"\n");

Right there is the problem. You are giving realloc() pointers to the local stack instead of the heap pointers that were possibly returned by malloc().

Originally Posted by master5001

Code:

	startnum=strtok (NULL,"- ");
	endnum=strtok (NULL,"\n");

Right there is the problem. You are giving realloc() pointers to the local stack instead of the heap pointers that were possibly returned by malloc().

ops.. it's true i forgot that strtok returns a pointer to an already initialized portion of the stack so my use of malloc was completely useless in this case, isn't it?
Thanks again!

No problem. Its a very common mistake that can even come up unintentionally. realloc() is not generally written to identify the nature of the pointer passed into it, thus it typically does little to avoid these sorts of problems from happening.