C语言 使用 strsep 解析 C 中的字符串（替代方法）

Parsing a string in C with strsep (alternative methods)

提问by Kyrol

I want to parse a string, and I use strsepfunction:

我想解析一个字符串，我使用strsep函数：

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

int main()
{

  char str[] = "Marco:Q:2F7PKC";
  char *token1, *token2, *token3;
  char *r = malloc(30);

  strcpy(r, str);

  token1 = strsep(&r, ":");
  token2 = strsep(&r, ":");
  token3 = strsep(&r, ":");

  printf("tok1 = %s\n", token1);
  printf("tok2 = %s\n", token2);
  printf("tok3 = %s\n", token3);

  free(r);

  return 0;

}

The function do its job well, but If I launch valgrind, the allocated string char * rdoes not freed correctly (definitely lost: 30 bytes in 1 blocks).

该函数可以很好地完成它的工作，但是如果我启动valgrind，则分配的字符串char * r没有正确释放（肯定丢失了：1 个块中的 30 个字节）。

I'd like to know why and if there are alternative way to do the same thing, maybe without call strsep.

我想知道为什么以及是否有其他方法可以做同样的事情，也许不需要调用strsep。

I call valgrind with valgrind --tool=memcheck --leak-check=full --show-reachable=yes ./a.out

我打电话给valgrind valgrind --tool=memcheck --leak-check=full --show-reachable=yes ./a.out

回答by Fred Foo

strsepoverwrites the target of its first (pointer-to-pointer) argument, so you lose the pointer to the malloc'd buffer's base. In fact, if you were do put a printf("%p\n", r);just before the free, you'd find out that you're freeing a null pointer, which has no effect.

strsep覆盖其第一个（指针到指针）参数的目标，因此您丢失了指向malloc'd 缓冲区基址的指针。事实上，如果您确实printf("%p\n", r);在 之前放置了free，您会发现您正在释放一个空指针，这没有任何效果。

The easy solution is to introduce an additional variable to keep that pointer around and freeit when you're done. Idiomatic usage would be

简单的解决方案是引入一个额外的变量来保持该指针，并free在您完成时保留它。惯用的用法是

char *r = strdup("Marco:Q:3F7PKC");
// check for errors

char *tok = r, *end = r;
while (tok != NULL) {
    strsep(&end, ":");
    puts(tok);
    tok = end;
}

free(r);

回答by jrk

I would like to a bit simplify a good reply from Fred Foo:

我想稍微简化一下 Fred Foo 的一个很好的回复：

char *end, *r, *tok;

r = end = strdup("Marco:Q:3F7PKC");
assert(end != NULL);

while ((tok = strsep(&end, ":")) != NULL) {
    printf("%s\n", tok);
}

free(r);

It gives the same result. But it is worth to say that strsep(3)stores next value after delimiter into endvariable and returns current value (into tokvariable).

它给出了相同的结果。但值得一提的是，strsep(3)将分隔符后的下一个值存储到end变量中并返回当前值（到tok变量中）。

回答by Wojtek Surowka

The strsep function updates its first argument (so it points right after the token it found). You need to store the value returned by malloc in a separate variable and free this variable.

strsep 函数更新它的第一个参数（因此它指向它找到的标记之后）。您需要将 malloc 返回的值存储在一个单独的变量中并释放该变量。