problem : the program crashes when running code

*** Error in `./parsing': double free or corruption (fasttop): 0x00000000023d2350 ***
======= Backtrace: =========
/lib64/libc.so.6(+0x81679)[0x7f349ead0679]
./parsing[0x4011fe]
./parsing[0x401b07]
/lib64/libc.so.6(__libc_start_main+0xf5)[0x7f349ea71505]
./parsing[0x400f59]
======= Memory map: ========
00400000-00413000 r-xp 00000000 08:01 34603142                           /root/Lispy/parsing
00612000-00613000 r--p 00012000 08:01 34603142                           /root/Lispy/parsing
00613000-00614000 rw-p 00013000 08:01 34603142                           /root/Lispy/parsing
023a4000-023df000 rw-p 00000000 00:00 0                                  [heap]
7f3494000000-7f3494021000 rw-p 00000000 00:00 0
7f3494021000-7f3498000000 ---p 00000000 00:00 0
7f34980e5000-7f34980fa000 r-xp 00000000 08:01 2709898                    /usr/lib64/libgcc_s-4.8.5-20150702.so.1
7f34980fa000-7f34982f9000 ---p 00015000 08:01 2709898                    /usr/lib64/libgcc_s-4.8.5-20150702.so.1
7f34982f9000-7f34982fa000 r--p 00014000 08:01 2709898                    /usr/lib64/libgcc_s-4.8.5-20150702.so.1
7f34982fa000-7f34982fb000 rw-p 00015000 08:01 2709898                    /usr/lib64/libgcc_s-4.8.5-20150702.so.1
7f34982fb000-7f349e825000 r--p 00000000 08:01 2129632                    /usr/lib/locale/locale-archive
7f349e825000-7f349e84a000 r-xp 00000000 08:01 2137260                    /usr/lib64/libtinfo.so.5.9
7f349e84a000-7f349ea4a000 ---p 00025000 08:01 2137260                    /usr/lib64/libtinfo.so.5.9
7f349ea4a000-7f349ea4e000 r--p 00025000 08:01 2137260                    /usr/lib64/libtinfo.so.5.9
7f349ea4e000-7f349ea4f000 rw-p 00029000 08:01 2137260                    /usr/lib64/libtinfo.so.5.9
7f349ea4f000-7f349ec12000 r-xp 00000000 08:01 2129638                    /usr/lib64/libc-2.17.so
7f349ec12000-7f349ee12000 ---p 001c3000 08:01 2129638                    /usr/lib64/libc-2.17.so
7f349ee12000-7f349ee16000 r--p 001c3000 08:01 2129638                    /usr/lib64/libc-2.17.so
7f349ee16000-7f349ee18000 rw-p 001c7000 08:01 2129638                    /usr/lib64/libc-2.17.so
7f349ee18000-7f349ee1d000 rw-p 00000000 00:00 0
7f349ee1d000-7f349ef1e000 r-xp 00000000 08:01 2129646                    /usr/lib64/libm-2.17.so
7f349ef1e000-7f349f11d000 ---p 00101000 08:01 2129646                    /usr/lib64/libm-2.17.so
7f349f11d000-7f349f11e000 r--p 00100000 08:01 2129646                    /usr/lib64/libm-2.17.so
7f349f11e000-7f349f11f000 rw-p 00101000 08:01 2129646                    /usr/lib64/libm-2.17.so
7f349f11f000-7f349f15b000 r-xp 00000000 08:01 2137399                    /usr/lib64/libreadline.so.6.2
7f349f15b000-7f349f35b000 ---p 0003c000 08:01 2137399                    /usr/lib64/libreadline.so.6.2
7f349f35b000-7f349f35d000 r--p 0003c000 08:01 2137399                    /usr/lib64/libreadline.so.6.2
7f349f35d000-7f349f363000 rw-p 0003e000 08:01 2137399                    /usr/lib64/libreadline.so.6.2
7f349f363000-7f349f365000 rw-p 00000000 00:00 0
7f349f365000-7f349f387000 r-xp 00000000 08:01 2136893                    /usr/lib64/ld-2.17.so
7f349f574000-7f349f57b000 r--s 00000000 08:01 2137112                    /usr/lib64/gconv/gconv-modules.cache
7f349f57b000-7f349f57f000 rw-p 00000000 00:00 0
7f349f583000-7f349f586000 rw-p 00000000 00:00 0
7f349f586000-7f349f587000 r--p 00021000 08:01 2136893                    /usr/lib64/ld-2.17.so
7f349f587000-7f349f588000 rw-p 00022000 08:01 2136893                    /usr/lib64/ld-2.17.so
7f349f588000-7f349f589000 rw-p 00000000 00:00 0
7ffed1ac3000-7ffed1ae4000 rw-p 00000000 00:00 0                          [stack]
7ffed1af3000-7ffed1af5000 r-xp 00000000 00:00 0                          [vdso]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]
Given up

typedef struct lval {
    int type;
    long num;

    /* Count and Pointer to a list of "lval*" */
    struct lval** cell;
    int count;

    /* Error and Symbol types have some string data */
    char *err;
    char *sym;
} lval;

lval *lval_sym(char *sym) {
    lval *v = malloc(sizeof(lval));
    v->type = LVAL_SYM;
    v->sym = malloc(strlen(sym) + 1);
    strcpy(v->sym, sym);
    return v;
}


void lval_del(lval *v) {
    switch (v->type) {
        /* Do nothing special for number type */
        case LVAL_NUM:
            break;

        /* For Err or Sym free the string data */
        case LVAL_ERR:
            free(v->err);
            break;
        case LVAL_SYM:
            free(v->sym);
            break;

        /* If Sexpr then delete all elements inside */
        case LVAL_SEXPR:
            for (int i = 0; i < v->count; i++) {
                lval_del(v->cell[i]);
            }
            /* Also free the memory allocated to contain the pointers */
            free(v->cell);
            break;
    }
    /* Free the memory allocated for the "lval" struct itself */
    free(v);
}

Because in the above code, the struct variable lval * lval_ Members in sym are not released free (V - > sym);, it directly releases the variable itself free (V) . The reason why we don’t enter the switch condition judgment is that V - > There is a bug in the value of the type member

so the essential reason is that the member space of the structure has not been released, and the structure variable itself is directly released

solution : before releasing the memory space of the structure variable itself, the memory space pointed to by the structure member needs to be released first