*** glibc detected *** ./a.out: corrupted double-linked list: 0x0000000000a5e240 ***

[deepikarc14]

Hi i am Deepika. am new to this forum. I am working on ubuntu 10.04 LTS and using C lang. to write prgrms.
I am trying to read strings from a file where i am using malloc concept to declare and initialize string arrays. After execution i get the above mentioned error. I don't know where i am going wrong and i don't know how to solve the problem. Help would be greatly appreciated. My program code(C language) is as follows :

#include<stdio.h>
#include<string.h>
#include<malloc.h>
int main()
{
char **a, **b, str1[10],str2[10],s[80];
int i=0,nl=0;
FILE *fp;
 
fp=fopen("abc.c","r"); 
 
while(fgets(s,79,fp)!=NULL)               //Counts the no. of lines in a file
       nl++;
printf("\n The no. of lines are : %d ",nl); 
 
rewind(fp);
a=(char **)malloc(nl);
if(a==NULL)
  {
    printf("\n Not enough memory ");
    return 0;
  }
b=(char **)malloc(nl);
if(b==NULL)
  {
    printf("\n Not enough memory ");
    return 0;
  }
 
for(i=0;i<nl;i++)
  {
    fscanf(fp,"%s%s",str1,str2);
 
    a[i]=(char *)malloc(strlen(str1)+1);
    b[i]=(char *)malloc(strlen(str2)+1);
    strcpy(a[i],str1);
    strcpy(b[i],str2);
 
  }
 
fclose(fp);
 
printf("\n");
for(i=0;i<nl;i++)
 {
   printf("\n%s\t\t%s",a[i],b[i]);
 }
 
for(i=0;i<nl;i++)
 {
    free(a[i]);
    free(b[i]);
 }
free(a);
free(b);         
 
printf("\n");
return 0;
}

After execution the output displays :

*** glibc detected *** ./a.out: corrupted double-linked list: 0x000000000187d240 ***
======= Backtrace: =========
/lib/libc.so.6(+0x775b6)[0x7ff28bc005b6]
/lib/libc.so.6(+0x7aa25)[0x7ff28bc03a25]
/lib/libc.so.6(cfree+0x73)[0x7ff28bc06e53]
/lib/libc.so.6(fclose+0x14d)[0x7ff28bbf0abd]
./a.out[0x400ad2]
/lib/libc.so.6(__libc_start_main+0xfd)[0x7ff28bba7c4d]
./a.out[0x4007f9]
======= Memory map: ========
00400000-00401000 r-xp 00000000 08:08 123 /home/deepika/prg/a.out
00600000-00601000 r--p 00000000 08:08 123 /home/deepika/prg/a.out
00601000-00602000 rw-p 00001000 08:08 123 /home/deepika/prg/a.out
0187d000-0189e000 rw-p 00000000 00:00 0 [heap]
7ff284000000-7ff284021000 rw-p 00000000 00:00 0
7ff284021000-7ff288000000 ---p 00000000 00:00 0
7ff28b972000-7ff28b988000 r-xp 00000000 08:07 21469 /lib/libgcc_s.so.1
7ff28b988000-7ff28bb87000 ---p 00016000 08:07 21469 /lib/libgcc_s.so.1
7ff28bb87000-7ff28bb88000 r--p 00015000 08:07 21469 /lib/libgcc_s.so.1
7ff28bb88000-7ff28bb89000 rw-p 00016000 08:07 21469 /lib/libgcc_s.so.1
7ff28bb89000-7ff28bd01000 r-xp 00000000 08:07 19153 /lib/libc-2.11.1.so
7ff28bd01000-7ff28bf01000 ---p 00178000 08:07 19153 /lib/libc-2.11.1.so
7ff28bf01000-7ff28bf05000 r--p 00178000 08:07 19153 /lib/libc-2.11.1.so
7ff28bf05000-7ff28bf06000 rw-p 0017c000 08:07 19153 /lib/libc-2.11.1.so
7ff28bf06000-7ff28bf0b000 rw-p 00000000 00:00 0
7ff28bf0b000-7ff28bf2b000 r-xp 00000000 08:07 6347 /lib/ld-2.11.1.so
7ff28c10b000-7ff28c10e000 rw-p 00000000 00:00 0
7ff28c127000-7ff28c128000 rw-p 00000000 00:00 0
7ff28c129000-7ff28c12b000 rw-p 00000000 00:00 0
7ff28c12b000-7ff28c12c000 r--p 00020000 08:07 6347 /lib/ld-2.11.1.so
7ff28c12c000-7ff28c12d000 rw-p 00021000 08:07 6347 /lib/ld-2.11.1.so
7ff28c12d000-7ff28c12e000 rw-p 00000000 00:00 0
7fffe73e5000-7fffe73fa000 rw-p 00000000 00:00 0 [stack]
7fffe73ff000-7fffe7400000 r-xp 00000000 00:00 0 [vdso]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0 [vsyscall]
The no. of lines are : 9 Aborted

[weaknessforcats]

I expect the heap is trashed and you crash making the calls to free() to clean up your mallocs.

I would use my debugger and carefully record the mallocs and verify all of the strcpy fits in those allocation.

Also be sure you don't free the same memory twice.

[rockcesar]

I post the answer, compiling in Debian GNU/Linux 8 testing, and also in Debian 7 stable, both:

When you do a malloc, you have to take in consideration all the array, in case of char**:

(char **)malloc(nl*sizeof(char *));

Because is an array of char*. nl is the size of the array, and sizeof(char *) means that each position have an char *.

In the case of an array of char, you have to malloc like this;

(char *)malloc((strlen(str1)+1)*sizeof(char));

(strlen(str1)+1) is the size of the array, and sizeof(char) means that each position have an char.

#include<stdio.h>
    #include<string.h>
    #include<malloc.h>
    int main()
    {
    char **a, **b, str1[10],str2[10],s[80];
    int i=0,nl=0;
    FILE *fp;
 
    fp=fopen("abc.c","r"); 
 
    while(fgets(s,79,fp)!=NULL)               //Counts the no. of lines in a file
           nl++;
    printf("\n The no. of lines are : %d ",nl); 
 
    rewind(fp);
    a=(char **)malloc(nl*sizeof(char *));//(char **)malloc(nl);
    if(a==NULL)
      {
        printf("\n Not enough memory ");
        return 0;
      }
    b=(char **)malloc(nl*sizeof(char *));
    if(b==NULL)
      {
        printf("\n Not enough memory ");
        return 0;
      }
 
    for(i=0;i<nl;i++)
      {
        fscanf(fp,"%s%s",str1,str2);
 
        a[i]=(char *)malloc((strlen(str1)+1)*sizeof(char));
        b[i]=(char *)malloc((strlen(str2)+1)*sizeof(char));
        strcpy(a[i],str1);
        strcpy(b[i],str2);
 
      }
 
    fclose(fp);
 
    printf("\n");
    for(i=0;i<nl;i++)
     {
       printf("\n%s\t\t%s",a[i],b[i]);
     }
 
    for(i=0;i<nl;i++)
     {
        free(a[i]);
        free(b[i]);
     }
    free(a);
    free(b);
 
    printf("\nSaliendo\n");
    return 0;
    }