C: Concatenating Dynamically Allocated Strings

Question

I have searched high and low for an applicable answer, but I couldn't find one. I am well aware my inability to solve this issue is due to a unfamiliarity of C.

I am working on a function to concatenate one dynamically allocated string onto another, and it seems to work outside the realm of memory leaks and valgrind; however, when I run it with valgrind it is evident I have some glaring memory leaks I just cannot find. It seems to be a issue with how I am using realloc.

here is the code:

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

void dynamCat(char *dest, const char *src) {

    size_t len = strlen(dest)  + strlen(src) + 1;

    printf("strLen: %ld\n", len);

    char *tmp = realloc(dest, len);

    if(tmp == NULL){
        fprintf(stderr, "strAppend: realloc messed up\n");
        free(tmp);
        return;
    } else {
       dest = tmp;
    }

    strcat(dest, src);
}

int main (int argc, char *argv[]) {

        char *one = malloc(4);
        strcpy(one, "tee");

        char *two = malloc(4);
        strcpy(two, "hee");

        printf("one: %s\n", one);
        printf("two: %s\n", two);

        dynamCat(one, two);

        printf("one: %s\n", one);
        printf("two: %s\n", two);

        //freeee
        free(one);
        free(two);


        return 0;

}

the output on the command line:

$ ./dynamCat
one: tee
two: hee
strLen: 7
one: teehee
two: hee

here's where the bad stuff comes.

output with valgrind on the commandline:

==28804== Memcheck, a memory error detector
==28804== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==28804== Using Valgrind-3.16.1 and LibVEX; rerun with -h for copyright info
==28804== Command: ./dynamCat
==28804==
==28804== Invalid read of size 1
==28804==    at 0x4841C72: strlen (vg_replace_strmem.c:459)
==28804==    by 0x48D9407: __vfprintf_internal (in /usr/lib/libc-2.33.so)
==28804==    by 0x48C463E: printf (in /usr/lib/libc-2.33.so)
==28804==    by 0x1092EE: main (dynamCat.c:37)
==28804==  Address 0x4a3c040 is 0 bytes inside a block of size 4 free'd
==28804==    at 0x4840D7B: realloc (vg_replace_malloc.c:834)
==28804==    by 0x1091FA: dynamCat (dynamCat.c:11)
==28804==    by 0x1092D6: main (dynamCat.c:35)
==28804==  Block was alloc'd at
==28804==    at 0x483E77F: malloc (vg_replace_malloc.c:307)
==28804==    by 0x10926D: main (dynamCat.c:26)
==28804==
==28804== Invalid read of size 1
==28804==    at 0x4841C84: strlen (vg_replace_strmem.c:459)
==28804==    by 0x48D9407: __vfprintf_internal (in /usr/lib/libc-2.33.so)
==28804==    by 0x48C463E: printf (in /usr/lib/libc-2.33.so)
==28804==    by 0x1092EE: main (dynamCat.c:37)
==28804==  Address 0x4a3c041 is 1 bytes inside a block of size 4 free'd
==28804==    at 0x4840D7B: realloc (vg_replace_malloc.c:834)
==28804==    by 0x1091FA: dynamCat (dynamCat.c:11)
==28804==    by 0x1092D6: main (dynamCat.c:35)
==28804==  Block was alloc'd at
==28804==    at 0x483E77F: malloc (vg_replace_malloc.c:307)
==28804==    by 0x10926D: main (dynamCat.c:26)
==28804==
==28804== Invalid read of size 1
==28804==    at 0x48460D0: mempcpy (vg_replace_strmem.c:1536)
==28804==    by 0x48ED211: _IO_file_xsputn@@GLIBC_2.2.5 (in /usr/lib/libc-2.33.so)
==28804==    by 0x48D915A: __vfprintf_internal (in /usr/lib/libc-2.33.so)
==28804==    by 0x48C463E: printf (in /usr/lib/libc-2.33.so)
==28804==    by 0x1092EE: main (dynamCat.c:37)
==28804==  Address 0x4a3c042 is 2 bytes inside a block of size 4 free'd
==28804==    at 0x4840D7B: realloc (vg_replace_malloc.c:834)
==28804==    by 0x1091FA: dynamCat (dynamCat.c:11)
==28804==    by 0x1092D6: main (dynamCat.c:35)
==28804==  Block was alloc'd at
==28804==    at 0x483E77F: malloc (vg_replace_malloc.c:307)
==28804==    by 0x10926D: main (dynamCat.c:26)
==28804==
==28804== Invalid read of size 1
==28804==    at 0x48460DE: mempcpy (vg_replace_strmem.c:1536)
==28804==    by 0x48ED211: _IO_file_xsputn@@GLIBC_2.2.5 (in /usr/lib/libc-2.33.so)
==28804==    by 0x48D915A: __vfprintf_internal (in /usr/lib/libc-2.33.so)
==28804==    by 0x48C463E: printf (in /usr/lib/libc-2.33.so)
==28804==    by 0x1092EE: main (dynamCat.c:37)
==28804==  Address 0x4a3c040 is 0 bytes inside a block of size 4 free'd
==28804==    at 0x4840D7B: realloc (vg_replace_malloc.c:834)
==28804==    by 0x1091FA: dynamCat (dynamCat.c:11)
==28804==    by 0x1092D6: main (dynamCat.c:35)
==28804==  Block was alloc'd at
==28804==    at 0x483E77F: malloc (vg_replace_malloc.c:307)
==28804==    by 0x10926D: main (dynamCat.c:26)
==28804==
==28804== Invalid free() / delete / delete[] / realloc()
==28804==    at 0x483F9AB: free (vg_replace_malloc.c:538)
==28804==    by 0x109312: main (dynamCat.c:41)
==28804==  Address 0x4a3c040 is 0 bytes inside a block of size 4 free'd
==28804==    at 0x4840D7B: realloc (vg_replace_malloc.c:834)
==28804==    by 0x1091FA: dynamCat (dynamCat.c:11)
==28804==    by 0x1092D6: main (dynamCat.c:35)
==28804==  Block was alloc'd at
==28804==    at 0x483E77F: malloc (vg_replace_malloc.c:307)
==28804==    by 0x10926D: main (dynamCat.c:26)
==28804==
==28804==
==28804== HEAP SUMMARY:
==28804==     in use at exit: 7 bytes in 1 blocks
==28804==   total heap usage: 4 allocs, 4 frees, 4,111 bytes allocated
==28804==
==28804== 7 bytes in 1 blocks are definitely lost in loss record 1 of 1
==28804==    at 0x4840D7B: realloc (vg_replace_malloc.c:834)
==28804==    by 0x1091FA: dynamCat (dynamCat.c:11)
==28804==    by 0x1092D6: main (dynamCat.c:35)
==28804==
==28804== LEAK SUMMARY:
==28804==    definitely lost: 7 bytes in 1 blocks
==28804==    indirectly lost: 0 bytes in 0 blocks
==28804==      possibly lost: 0 bytes in 0 blocks
==28804==    still reachable: 0 bytes in 0 blocks
==28804==         suppressed: 0 bytes in 0 blocks
==28804==
==28804== For lists of detected and suppressed errors, rerun with: -s
==28804== ERROR SUMMARY: 9 errors from 6 contexts (suppressed: 0 from 0)

thank you so much for your help. I am still very new to C and really appreciate your help.

Answer 1

If realloc returns a null pointer, do not free its return value; there is no point in passing a null pointer to free :

    char *tmp = realloc(dest, len);

    if(tmp == NULL){
        fprintf(stderr, "strAppend: realloc messed up\n");
        free(tmp); // This does nothing.

dest is a parameter, so assigning to it changes only the parameter. It does not change the original argument:

       dest = tmp; // This changes only the dest inside dynamCat.

To give the caller a new address, you must either return it as a pointer:

char *dynamCat(char *dest, const char *src)
{
    …
    dest = something;
    …
    return dest;
}

or you must put it into a pointer passed by the caller:

void dynamCat(char **dest, const char *src)
{
    …
    char *tmp = realloc(*dest, len);
    …
        *dest = tmp;
    }

    strcat(*dest, src);
}

In the latter case, when calling dynamCat , you must pass it the address of a pointer:

        dynamCat(&one, two);

In the former case, you must take the new pointer from the return value of dynamCat :

    one = dynamCat(one, two);

Answer 2

The realloc function can change the address of the object. When he object is being made bigger, the need for this is obvious: there may not be space at the original location. It's potentially true when shrinking the object also.

In abstract terms, when we call realloc(x, newsize) , the pointer x becomes indeterminate. We must capture the returned pointer y = realloc(x, newsize) and then use that in all places that previously referred to x .

In your program the catenating function returns void , and main keeps referring to the old string.

Your dynamCat should be like this:

/* dest is destroyed: use the return value
 * in place of dest!
 */
char *dynamCat(char *dest, const char *src) {
    size_t len = strlen(dest)  + strlen(src) + 1;

    printf("strLen: %ld\n", len);

    char *tmp = realloc(dest, len);

    if(tmp == NULL){
        fprintf(stderr, "strAppend: realloc messed up\n");
        /* better handling needed here, too */
    }

    strcat(tmp, src);
    return tmp;
}