Thanks for your help.
I added printf statements and it is throwing error at strcpy.
[root@localhost~]# more seg1.c
[root@localhost~]# gcc -g seg1.c -o seg1
I debug this using core file. It is also showing the error at strcpy
[root@localhost~]# gdb ./seg1 ./core.3121
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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");
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.