Malloc,字符串指针和Valgrind

Question

我的程序是这样的(main.c):

#include <stdlib.h>
#include <stdio.h>
void main(){
  char *first="hello ";
  char *second="world!";
  char *seq=(char *)malloc((strlen(first)+1)*sizeof(char));
  strcat(strcpy(seq,first),second);
  printf("%s\n",seq);
  free(seq);
}

我用valgrind工具调试,它说($:valgrind --tool = memcheck --leak-check = full --track-originins = yes ./main):

==5118== Memcheck, a memory error detector.
==5118== Copyright (C) 2002-2008, and GNU GPL'd, by Julian Seward et al.
==5118== Using LibVEX rev 1884, a library for dynamic binary translation.
==5118== Copyright (C) 2004-2008, and GNU GPL'd, by OpenWorks LLP.
==5118== Using valgrind-3.4.1, a dynamic binary instrumentation framework.
==5118== Copyright (C) 2000-2008, and GNU GPL'd, by Julian Seward et al.
==5118== For more details, rerun with: -v
==5118== 
==5118== Invalid write of size 1
==5118==    at 0x402575B: strcat (in /usr/lib/valgrind/x86-linux/vgpreload_memcheck.so)
==5118==    by 0x80484EB: main (main.c:7)
==5118==  Address 0x418a02f is 0 bytes after a block of size 7 alloc'd
==5118==    at 0x402522D: malloc (in /usr/lib/valgrind/x86-linux/vgpreload_memcheck.so)
==5118==    by 0x80484C3: main (main.c:6)
==5118== 
==5118== Invalid write of size 1
==5118==    at 0x4025777: strcat (in /usr/lib/valgrind/x86-linux/vgpreload_memcheck.so)
==5118==    by 0x80484EB: main (main.c:7)
==5118==  Address 0x418a034 is 5 bytes after a block of size 7 alloc'd
==5118==    at 0x402522D: malloc (in /usr/lib/valgrind/x86-linux/vgpreload_memcheck.so)
==5118==    by 0x80484C3: main (main.c:6)
==5118== 
==5118== Invalid read of size 1
==5118==    at 0x4025963: strlen (in /usr/lib/valgrind/x86-linux/vgpreload_memcheck.so)
==5118==    by 0x40A0FA4: puts (in /lib/libc-2.10.1.so)
==5118==    by 0x80484F7: main (main.c:8)
==5118==  Address 0x418a02f is 0 bytes after a block of size 7 alloc'd
==5118==    at 0x402522D: malloc (in /usr/lib/valgrind/x86-linux/vgpreload_memcheck.so)
==5118==    by 0x80484C3: main (main.c:6)
==5118== 
==5118== Invalid read of size 1
==5118==    at 0x40ACEFE: _IO_default_xsputn (in /lib/libc-2.10.1.so)
==5118==    by 0x40AA3D0: _IO_file_xsputn@@GLIBC_2.1 (in /lib/libc-2.10.1.so)
==5118==    by 0x40A1020: puts (in /lib/libc-2.10.1.so)
==5118==    by 0x80484F7: main (main.c:8)
==5118==  Address 0x418a02f is 0 bytes after a block of size 7 alloc'd
==5118==    at 0x402522D: malloc (in /usr/lib/valgrind/x86-linux/vgpreload_memcheck.so)
==5118==    by 0x80484C3: main (main.c:6)
hello world!
==5118== 
==5118== ERROR SUMMARY: 17 errors from 4 contexts (suppressed: 13 from 1)
==5118== malloc/free: in use at exit: 7 bytes in 1 blocks.
==5118== malloc/free: 1 allocs, 0 frees, 7 bytes allocated.
==5118== For counts of detected errors, rerun with: -v
==5118== searching for pointers to 1 not-freed blocks.
==5118== checked 47,492 bytes.
==5118== 
==5118== 
==5118== 7 bytes in 1 blocks are definitely lost in loss record 1 of 1
==5118==    at 0x402522D: malloc (in /usr/lib/valgrind/x86-linux/vgpreload_memcheck.so)
==5118==    by 0x80484C3: main (main.c:6)
==5118== 
==5118== LEAK SUMMARY:
==5118==    definitely lost: 7 bytes in 1 blocks.
==5118==      possibly lost: 0 bytes in 0 blocks.
==5118==    still reachable: 0 bytes in 0 blocks.
==5118==         suppressed: 0 bytes in 0 blocks.

谁能告诉我为什么以及如何解决它.

Answer 1

 char *seq=(char *)malloc((strlen(first)+1)*sizeof(char));

你正在为一个大小只是'first'的字符串分配内存.

  strcat(strcpy(seq,first),second);

然后你尝试将第一个和第二个都装进去.那永远不会奏效.strcat没有创造更多的内存,你需要包含在内malloc.

没有必要在纯C中转换malloc的结果.

它也没有必要这样做sizeof(char),因为它保证是1.有些人喜欢在那里有关于类型的说明,以防它改变,有些人认为它是混乱的.