[英]How to use malloc and free for Dynamic Memory Allocation
我刚开始学习C,来自php。 我还不了解如何使用malloc和free。
1)在下面的示例代码中,我可以在哪里放置“免费”?
#include <stdio.h>
#include <stdlib.h>
int strLen(char* text) {
int c = 0;
for (int i = 0; text[i] != '\0'; ++i) ++c;
return c;
}
char* reverse(char* text) {
int count = strLen(text);
char* t = malloc(count);
for (int i = count; i > 0; --i) { t[count - i] = text[i-1]; }
t[count] = '\0'; /* Add end of string */
return t;
}
int main (int argc, char** argv) {
if (argc > 1) {
for (int i = 1; i < argc; ++i) {
printf("%d\t%s\t%s\n", i, argv[i], reverse(argv[i]));
}
}
return 0;
}
2)这是valgrind的输出。 错误“大小为1的无效写入”和“大小为1的无效读取”是什么意思?
valgrind ./reverse Text ONe
==3124== Memcheck, a memory error detector
==3124== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==3124== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
==3124== Command: ./reverse Text ONe
==3124==
==3124== Invalid write of size 1
==3124== at 0x40060F: reverse (in /localServer/temp/C/reverse)
==3124== by 0x400654: main (in /localServer/temp/C/reverse)
==3124== Address 0x5203044 is 0 bytes after a block of size 4 alloc'd
==3124== at 0x4C2DB8F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==3124== by 0x4005C6: reverse (in /localServer/temp/C/reverse)
==3124== by 0x400654: main (in /localServer/temp/C/reverse)
==3124==
==3124== Invalid read of size 1
==3124== at 0x4E88CC0: vfprintf (vfprintf.c:1632)
==3124== by 0x4E8F898: printf (printf.c:33)
==3124== by 0x400682: main (in /localServer/temp/C/reverse)
==3124== Address 0x5203044 is 0 bytes after a block of size 4 alloc'd
==3124== at 0x4C2DB8F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==3124== by 0x4005C6: reverse (in /localServer/temp/C/reverse)
==3124== by 0x400654: main (in /localServer/temp/C/reverse)
==3124==
1 Text txeT
2 ONe eNO
==3124==
==3124== HEAP SUMMARY:
==3124== in use at exit: 7 bytes in 2 blocks
==3124== total heap usage: 3 allocs, 1 frees, 1,031 bytes allocated
==3124==
==3124== LEAK SUMMARY:
==3124== definitely lost: 7 bytes in 2 blocks
==3124== indirectly lost: 0 bytes in 0 blocks
==3124== possibly lost: 0 bytes in 0 blocks
==3124== still reachable: 0 bytes in 0 blocks
==3124== suppressed: 0 bytes in 0 blocks
==3124== Rerun with --leak-check=full to see details of leaked memory
==3124==
==3124== For counts of detected and suppressed errors, rerun with: -v
==3124== ERROR SUMMARY: 4 errors from 2 contexts (suppressed: 0 from 0)
谢谢。
在使用反向返回的指针之后,应按如下所述释放循环。
int main (int argc, char** argv) {
char *reverseStrPtr;
if (argc > 1) {
for (int i = 1; i < argc; ++i) {
reverseStrPtr = reverse(argv[i]);
printf("%d\t%s\t%s\n", i, argv[i], reverseStrPtr);
free(reverseStrPtr);
}
}
return 0;
}
同样,valgrind错误基于下面的行,该行是reverse
。
t[count] = '\0'; /* Add end of string */
简而言之,当您分配count-byte个字节时,索引范围为[0, count)
。 因此,尝试访问count等效于访问第(count+1)
位,这是访问冲突。
您需要纠正的是分配的内存量,即(count+1)
个字节。 需要额外的字节来放置终止的'\\0'
字符。
char* reverse(char* text) {
int count = strLen(text);
char* t = malloc(count+1); // +1 for the terminating '\0'
for (int i = count; i > 0; --i) { t[count - i] = text[i-1];}
t[count] = '\0'; /* Add end of string */
return t;
}
您需要保留从reverse(argv[i])
返回的指针,并free
它。
一种方法是将main
电源稍微调整为
int main (int argc, char** argv) {
char* s;
// if (argc > 1) { Comment out the redundant check
for (int i = 1; i < argc; ++i) {
printf("%d\t%s\t%s\n", i, argv[i], s = reverse(argv[i]));
free(s);
}
//}
return 0;
}
表达式内部的分配并不符合每个人的喜好(因为它们可能会让人混淆),但在这样的实例中它们可以很好地工作。 我是我应该习惯C语言中类似的东西。
同样, t[count] = '\\0';
溢出缓冲区-其行为是不确定的 。 您需要malloc(1 + count);
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