c - 释放 malloc 的内存失败

Question

我有一个相当简单的程序，我使用 snprintf 写入一个字符串，如果需要，我扩展字符串长度以适应我要写入的内容。然后，我尝试添加到该字符串。它确实有效，但是当我释放 malloc 的变量时，它崩溃了。

代码:

#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;
}

输出:

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

现在我不完全是你所谓的 C 专家，所以这个错误可能相当简单，但我无法自己解决这个问题/在网上找到任何有用的东西。

感谢您的帮助!

编辑:

固定码:

#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

在对字符串的第二次复制中，您使用了这段代码，它告诉 snprintf() 缓冲区中有 16 个字节:

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

但是您已经将 str + strlen(str) 作为要写入的地址。显然，您没有从该地址获得 16 个可用字节，您只有 1 个(假设您最初输入的字符串长度超过 16，然后您分配了 len + 1 个字节)。因此，当您告诉 snprintf() 有 16 个时，它会写入超过您分配的内存的末尾，这有时只会在您尝试释放它时显示为问题。