Segmentation fault

ravi.chakram

Hi,

I am learning C. I got segmentation fault when I ran the below program.

[root@localhost ~]# more segmentationfault.c

#include

#include

int main()

{

char *mystr="Hi";

strcpy(mystr,"new value") ;

puts(mystr);

return 0;

}

[root@localhost ~]# ./a.out

Segmentation fault

[root@localhost ~]#

Could you please tell me the ways to debug the segmentation fault errors.

TsaniApani

The notorious "Segmentation fault" is basically the default sink-state error message when no others can be found. Until some point you need to climb the massive learning curve of the GDB (GNU Debugger) you will have to debug the old-fashioned way:

printf( "1" );
char *mystr="Hi";
printf( "2" );
strcpy(mystr,"new value" ;
printf( "3" );
puts(mystr);
printf( "4" );
return 0;

henok

you can use gdb to debug programs. segmentation faults are almost always memory issues. in your case, you cannot ust strcpy until you allocate memory for the variable mystr. you need to say have something like mystr = malloc(50). Don't forget to include stdlib.h for malloc.

ravi.chakram

Thanks for your help.

I added printf statements and it is throwing error at strcpy.

[root@localhost~]# more seg1.c
#include<stdio.h>
#include<string.h>
int main()
{
char *mystr="hi";
printf("1\n");
printf("2\n");
printf("3\n");
strcpy(mystr,"new value");
printf("4\n");
puts(mystr);
return 0;
}
[root@localhost~]# gcc -g seg1.c -o seg1
[root@localhost~]# ./seg1
1
2
3
Segmentation fault
[root@localhost~]#




I debug this using core file. It is also showing the error at strcpy

[root@localhost~]# gdb ./seg1 ./core.3121
GNU gdb Red Hat Linux (6.5-16.el5rh)
Copyright (C) 2006 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB. Type "show warranty" for details.
This GDB was configured as "i386-redhat-linux-gnu"...Using host libthread_db library "/lib/libthread_db.so.1".


warning: Can't read pathname for load map: Input/output error.
Reading symbols from /lib/libc.so.6...done.
Loaded symbols for /lib/libc.so.6
Reading symbols from /lib/ld-linux.so.2...done.
Loaded symbols for /lib/ld-linux.so.2
Core was generated by `./seg1'.
Program terminated with signal 11, Segmentation fault.
#0 0x08048393 in main () at seg1.c:9
9 strcpy(mystr,"new value");
(gdb)




Still I have one dought here. I assigned the value to mystr at the time of initialization so memory will be allocated for the mystr. Then why once again i need to assign space for mystr.

woboyle

Class exercise? Not sure we're supposed to help with class assignments - it's forbidden on the LinuxForums.org site. Anyway, the size and location of the variable 'mystr' is from the literal "Hi". Two points. First, a string literal might not be writable. Second, the string you are copying into that memory, "new value", is longer than the space reserved for "Hi", so even if "Hi" is writable, you will have corrupted memory, and the system slapped you down for that. There are several ways to deal with this, depending upon what your intention happens to be.

ravi.chakram

Thanks for you help. Actually I am learning C.

woboyle

Well, I started out that way, about 30 years ago! :-) Fortunately, I had a real C/Unix expert to mentor me, and point out errors like this. I wish you all the best in your learning endeavors. What is your solution, after viewing the answers given?