Question

I’m trying an example

int main(){
    int count = 5;
    char *a = "JOSUA";
    char *c = "JANETALAE";
    char *b = NULL;
    char *d = NULL;
    b = (char *)malloc(sizeof(char)*5);
    b = a;
    if(b == NULL){
        printf("\n malloc failed");
    }
    printf("\nChar is:%s\n",b);
    d = (char *)realloc(b,sizeof(char) * 9);
    if(d == NULL){
        printf("\n realloc failed");
    }
    else{
        b = d;
        b = c;                                                                                                                                                                             
    }
    printf("\nChar is:%s\n",b);
    return 0;
}

I’m trying a sample program to allocate and reallocate memory. But i couldn’t reallocate the memory and getting a compilation break as below.

Char is:JOSUA
*** glibc detected *** ./realloc: realloc(): invalid pointer: 0x080485b4 ***
======= Backtrace: ========= /lib/libc.so.6[0x88ffd1] /lib/libc.so.6(realloc+0x2cf)[0x89613f] /lib/libc.so.6(realloc+0x2e7)[0x896157] ./realloc[0x80484aa] /lib/libc.so.6(__libc_start_main+0xe6)[0x836e16] ./realloc[0x8048391]
======= Memory map: ======== 007fc000-0081c000 r-xp 00000000 fd:00 659977     /lib/ld-2.12.90.so 0081c000-0081d000 r--p 0001f000 fd:00 659977     /lib/ld-2.12.90.so 0081d000-0081e000 rw-p 00020000 fd:00 659977     /lib/ld-2.12.90.so 00820000-009ad000 r-xp 00000000 fd:00 660018     /lib/libc-2.12.90.so 009ad000-009af000 r--p 0018c000 fd:00 660018     /lib/libc-2.12.90.so 009af000-009b0000 rw-p 0018e000 fd:00 660018     /lib/libc-2.12.90.so 009b0000-009b3000 rw-p 00000000 00:00 0  00a29000-00a45000 r-xp 00000000 fd:00 660039     /lib/libgcc_s-4.5.1-20100924.so.1 00a45000-00a46000 rw-p 0001b000 fd:00 660039     /lib/libgcc_s-4.5.1-20100924.so.1 00a84000-00a85000 r-xp 00000000 00:00 0          [vdso] 08048000-08049000 r-xp 00000000 fd:02 9440701    /home/user/beata/c_samples/realloc 08049000-0804a000 rw-p 00000000 fd:02 9440701    /home/user/beata/c_samples/realloc 092ac000-092cd000 rw-p 00000000 00:00 0          [heap] b78bc000-b78bd000 rw-p 00000000 00:00 0  b78d3000-b78d5000 rw-p 00000000 00:00 0  bfbd8000-bfbf9000 rw-p 00000000 00:00 0          [stack] Aborted (core dumped)

Could not understand why the compilation break happened.

Answer 1

You are passing an invalid pointer to realloc(), just as the error says.

It’s from this line:

b = a;

where you overwrite the return value of malloc() with the address of a string literal. You cannot pass that to realloc().

Your overall logic in this program is very broken; if malloc() fails, there’s little reason to believe realloc() will succeed (assuming the above line is an editing error and shouldn’t be there).

Also, this is a run-time error, not a “compilation error” as you say.

Further, the typical errors apply:

1. In C, don’t cast the return value of malloc(). This applies just as well to realloc(), too.
2. Don’t use sizeof (char), it’s just a very cumbersome way to write 1.