You can use valgrind
to debug it.
#include<stdio.h>
#include<stdlib.h>
int main()
{
char *x = malloc(100);
free(x);
free(x);
return 0;
}
[sand@PS-CNTOS-64-S11 testbox]$ vim t1.c
[sand@PS-CNTOS-64-S11 testbox]$ cc -g t1.c -o t1
[sand@PS-CNTOS-64-S11 testbox]$ ./t1
*** glibc detected *** ./t1: double free or corruption (top): 0x00000000058f7010 ***
======= Backtrace: =========
/lib64/libc.so.6[0x3a3127245f]
/lib64/libc.so.6(cfree+0x4b)[0x3a312728bb]
./t1[0x400500]
/lib64/libc.so.6(__libc_start_main+0xf4)[0x3a3121d994]
./t1[0x400429]
======= Memory map: ========
00400000-00401000 r-xp 00000000 68:02 30246184 /home/sand/testbox/t1
00600000-00601000 rw-p 00000000 68:02 30246184 /home/sand/testbox/t1
058f7000-05918000 rw-p 058f7000 00:00 0 [heap]
3a30e00000-3a30e1c000 r-xp 00000000 68:03 5308733 /lib64/ld-2.5.so
3a3101b000-3a3101c000 r--p 0001b000 68:03 5308733 /lib64/ld-2.5.so
3a3101c000-3a3101d000 rw-p 0001c000 68:03 5308733 /lib64/ld-2.5.so
3a31200000-3a3134e000 r-xp 00000000 68:03 5310248 /lib64/libc-2.5.so
3a3134e000-3a3154e000 ---p 0014e000 68:03 5310248 /lib64/libc-2.5.so
3a3154e000-3a31552000 r--p 0014e000 68:03 5310248 /lib64/libc-2.5.so
3a31552000-3a31553000 rw-p 00152000 68:03 5310248 /lib64/libc-2.5.so
3a31553000-3a31558000 rw-p 3a31553000 00:00 0
3a41c00000-3a41c0d000 r-xp 00000000 68:03 5310264 /lib64/libgcc_s-4.1.2-20080825.so.1
3a41c0d000-3a41e0d000 ---p 0000d000 68:03 5310264 /lib64/libgcc_s-4.1.2-20080825.so.1
3a41e0d000-3a41e0e000 rw-p 0000d000 68:03 5310264 /lib64/libgcc_s-4.1.2-20080825.so.1
2b1912300000-2b1912302000 rw-p 2b1912300000 00:00 0
2b191231c000-2b191231d000 rw-p 2b191231c000 00:00 0
7ffffe214000-7ffffe229000 rw-p 7ffffffe9000 00:00 0 [stack]
7ffffe2b0000-7ffffe2b4000 r-xp 7ffffe2b0000 00:00 0 [vdso]
ffffffffff600000-ffffffffffe00000 ---p 00000000 00:00 0 [vsyscall]
Aborted
[sand@PS-CNTOS-64-S11 testbox]$
[sand@PS-CNTOS-64-S11 testbox]$ vim t1.c
[sand@PS-CNTOS-64-S11 testbox]$ cc -g t1.c -o t1
[sand@PS-CNTOS-64-S11 testbox]$ valgrind --tool=memcheck ./t1
==20859== Memcheck, a memory error detector
==20859== Copyright (C) 2002-2009, and GNU GPL'd, by Julian Seward et al.
==20859== Using Valgrind-3.5.0 and LibVEX; rerun with -h for copyright info
==20859== Command: ./t1
==20859==
==20859== Invalid free() / delete / delete[]
==20859== at 0x4A05A31: free (vg_replace_malloc.c:325)
==20859== by 0x4004FF: main (t1.c:8)
==20859== Address 0x4c26040 is 0 bytes inside a block of size 100 free'd
==20859== at 0x4A05A31: free (vg_replace_malloc.c:325)
==20859== by 0x4004F6: main (t1.c:7)
==20859==
==20859==
==20859== HEAP SUMMARY:
==20859== in use at exit: 0 bytes in 0 blocks
==20859== total heap usage: 1 allocs, 2 frees, 100 bytes allocated
==20859==
==20859== All heap blocks were freed -- no leaks are possible
==20859==
==20859== For counts of detected and suppressed errors, rerun with: -v
==20859== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 4 from 4)
[sand@PS-CNTOS-64-S11 testbox]$
[sand@PS-CNTOS-64-S11 testbox]$ valgrind --tool=memcheck --leak-check=full ./t1
==20899== Memcheck, a memory error detector
==20899== Copyright (C) 2002-2009, and GNU GPL'd, by Julian Seward et al.
==20899== Using Valgrind-3.5.0 and LibVEX; rerun with -h for copyright info
==20899== Command: ./t1
==20899==
==20899== Invalid free() / delete / delete[]
==20899== at 0x4A05A31: free (vg_replace_malloc.c:325)
==20899== by 0x4004FF: main (t1.c:8)
==20899== Address 0x4c26040 is 0 bytes inside a block of size 100 free'd
==20899== at 0x4A05A31: free (vg_replace_malloc.c:325)
==20899== by 0x4004F6: main (t1.c:7)
==20899==
==20899==
==20899== HEAP SUMMARY:
==20899== in use at exit: 0 bytes in 0 blocks
==20899== total heap usage: 1 allocs, 2 frees, 100 bytes allocated
==20899==
==20899== All heap blocks were freed -- no leaks are possible
==20899==
==20899== For counts of detected and suppressed errors, rerun with: -v
==20899== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 4 from 4)
[sand@PS-CNTOS-64-S11 testbox]$
One possible fix:
#include<stdio.h>
#include<stdlib.h>
int main()
{
char *x = malloc(100);
free(x);
x=NULL;
free(x);
return 0;
}
[sand@PS-CNTOS-64-S11 testbox]$ vim t1.c
[sand@PS-CNTOS-64-S11 testbox]$ cc -g t1.c -o t1
[sand@PS-CNTOS-64-S11 testbox]$ ./t1
[sand@PS-CNTOS-64-S11 testbox]$
[sand@PS-CNTOS-64-S11 testbox]$ valgrind --tool=memcheck --leak-check=full ./t1
==20958== Memcheck, a memory error detector
==20958== Copyright (C) 2002-2009, and GNU GPL'd, by Julian Seward et al.
==20958== Using Valgrind-3.5.0 and LibVEX; rerun with -h for copyright info
==20958== Command: ./t1
==20958==
==20958==
==20958== HEAP SUMMARY:
==20958== in use at exit: 0 bytes in 0 blocks
==20958== total heap usage: 1 allocs, 1 frees, 100 bytes allocated
==20958==
==20958== All heap blocks were freed -- no leaks are possible
==20958==
==20958== For counts of detected and suppressed errors, rerun with: -v
==20958== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 4 from 4)
[sand@PS-CNTOS-64-S11 testbox]$
Check out the blog on using Valgrind Link