Question

I have a fairly simple program, I write to a string with snprintf, if I need to, I extend the string length to accomodate for what I’m trying to write to it. Then, I try to add to that string. It does work, but when I free the malloc’d variable, it crashes.

The code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(int ac, char **av) {
    char buf[16];
    char *str = buf;
    char *extra = NULL;
    int len;

    if(!av[1]) {
        return 0;
    }

    if((len = snprintf(str, 16, "%s", av[1])) >= 16) {
        printf("more than 15 chars, %d\n",len);
        if (extra = malloc((len + 1) * sizeof(char))) {
            snprintf(str = extra, len + 1, "%s", av[1]);
        }
    } else {
        printf("less than 15 chars, %d\n",len);
    }

    printf("%s\n", str);


    if((len = snprintf(str+strlen(str), 16, "%s", av[1])) >= 16) {
        printf("more than 15 chars, %d, required: %d\n",len,(len + strlen(av[1]) + 1));
        if ((extra = malloc((len + strlen(av[1]) + 1) * sizeof(char)) ) && strcpy(extra,str)) {
            str = extra;
            snprintf(extra+strlen(av[1]), len + 1, "%s", av[1]);
        }
    } else {
        printf("less than 15 chars, %d\n",len);
    }
    printf("%s\n", str);

    if(extra) {
        free(extra);
    }

    return 1;
}

the output:

more than 15 chars, 16
1234567890123456
more than 15 chars, 16, required: 33
12345678901234561234567890123456
*** glibc detected *** ./testprog: free(): invalid next size (fast): 0x0000000000ce0030 ***
======= Backtrace: =========
/lib/libc.so.6(+0x78a56)[0x7fe46c0b9a56]
./testprog[0x4008c7]
/lib/libc.so.6(__libc_start_main+0xf5)[0x7fe46c062455]
./testprog[0x4005a9]
======= Memory map: ========
00400000-00401000 r-xp 00000000 08:03 26355255                           /home/joachim/kc/testprog
00600000-00601000 rw-p 00000000 08:03 26355255                           /home/joachim/kc/testprog
00ce0000-00d01000 rw-p 00000000 00:00 0                                  [heap]
7fe46be2c000-7fe46be41000 r-xp 00000000 08:03 15077980                   /usr/lib/libgcc_s.so.1
7fe46be41000-7fe46c040000 ---p 00015000 08:03 15077980                   /usr/lib/libgcc_s.so.1
7fe46c040000-7fe46c041000 rw-p 00014000 08:03 15077980                   /usr/lib/libgcc_s.so.1
7fe46c041000-7fe46c1d8000 r-xp 00000000 08:03 393241                     /lib/libc-2.15.so
7fe46c1d8000-7fe46c3d8000 ---p 00197000 08:03 393241                     /lib/libc-2.15.so
7fe46c3d8000-7fe46c3dc000 r--p 00197000 08:03 393241                     /lib/libc-2.15.so
7fe46c3dc000-7fe46c3de000 rw-p 0019b000 08:03 393241                     /lib/libc-2.15.so
7fe46c3de000-7fe46c3e2000 rw-p 00000000 00:00 0 
7fe46c3e2000-7fe46c403000 r-xp 00000000 08:03 393253                     /lib/ld-2.15.so
7fe46c5d5000-7fe46c5d8000 rw-p 00000000 00:00 0 
7fe46c600000-7fe46c603000 rw-p 00000000 00:00 0 
7fe46c603000-7fe46c604000 r--p 00021000 08:03 393253                     /lib/ld-2.15.so
7fe46c604000-7fe46c605000 rw-p 00022000 08:03 393253                     /lib/ld-2.15.so
7fe46c605000-7fe46c606000 rw-p 00000000 00:00 0 
7fff72b84000-7fff72ba5000 rw-p 00000000 00:00 0                          [stack]
7fff72bff000-7fff72c00000 r-xp 00000000 00:00 0                          [vdso]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]
Aborted

Now I’m not exactly what you would call a C-expert so the mistake is probably fairly simple, but I’ve not been able to fix this myself/find anything useful online.

Thanks for the help!

edit:

the fixed code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(int ac, char **av) {
    char buf[16];
    char *str = buf;
    char *extra = NULL;
    int len;
    int ssize = 16;
    if(ac != 2)
    {
        return 0;
    }


    if((len = snprintf(str, 16, "%s", av[1])) >= 16) {
        printf("more than 15 chars, %d\n",len);
        if (extra = malloc((len + 1) * sizeof(char))) {
        ssize = (len + 1);
          snprintf(str = extra, len + 1, "%s", av[1]);
        } 
    } else {
        printf("less than 15 chars, %d\n",len);
    }

    printf("%s\n", str);

    char *extra2 = NULL;
    printf("space: %d (%d-%d)\n",ssize-strlen(str),ssize,strlen(str));
    if((len = snprintf(str+strlen(str), ssize-strlen(str), "%s", av[1])) >= ssize-strlen(str)) {
        printf("more than 15 chars, %d, required: %d (strlen: %d)\n",len,(len + strlen(av[1]) + 1),strlen(str));


        if ((extra2 = malloc((len + strlen(av[1]) + 1) * sizeof(char)) ) && strcpy(extra2,str)) {
          str = extra2;
        ssize = (len + strlen(av[1]) + 1);
          snprintf(extra2+strlen(av[1]), len + 1, "%s", av[1]);
        } else printf("failed extraing");
    } else {
        printf("less than 15 chars, %d\n",len);
    }
    printf("%s\n", str);

    if(extra) {
        free(extra);
    }
    if(extra2) {
        free(extra2);
    }

    return 1;
}

Answer 1

In the second copy to the string you use this code, which tells snprintf() that there are 16 bytes in the buffer:

if((len = snprintf(str+strlen(str), 16, "%s", av[1])) >= 16) {

But you have passed str + strlen(str) as the address to write to. Obviously you don’t have 16 bytes available from this address, you only have 1 (assuming you initially entered a string longer than 16 and you then allocated len + 1 bytes). So when you tell snprintf() there are 16 it writes past the end of the memory you have allocated, which can sometimes only show up as a problem when you try to free it.