gets() - fgets() & strcpy() - strncpy()

最新推荐文章于 2025-02-26 11:31:07 发布

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文章讨论了C语言中gets()函数的安全隐患，建议避免使用，因为它可能导致缓冲区溢出。推荐使用fgets()替代，尽管它需要额外处理换行符和长行处理。还提到了非标准的动态分配内存的函数选项。

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Never use gets. It offers no protections against a buffer overflow vulnerability (that is, you cannot tell it how big the buffer you pass to it is, so it cannot prevent a user from entering a line larger than the buffer and clobbering memory).
Avoid using scanf. If not used carefully, it can have the same buffer overflow problems as gets. Even ignoring that, it has other problems that make it hard to use correctly.
Generally you should use fgets instead, although it's sometimes inconvenient (you have to strip the newline, you must determine a buffer size ahead of time, and then you must figure out what to do with lines that are too long–do you keep the part you read and discard the excess, discard the whole thing, dynamically grow the buffer and try again, etc.). There are some non-standard functions available that do this dynamic allocation for you (e.g. getline on POSIX systems, Chuck Falconer's public domain ggets function). Note that ggets has gets-like semantics in that it strips a trailing newline for you.

Like:

char *result = fgets(str, sizeof(str), stdin);
char len = strlen(str);
if(result != NULL && str[len - 1] == '\n')
{
  str[len - 1] = '\0';
}
else
{
  // handle error
}

char *line = NULL;
size_t len = 0;
ssize_t count = getline(&line, &len, stdin);
if(count >= 1 && line[count - 1] == '\n')
{
  line[count - 1] = '\0';
}
else
{
  // Handle error
}

The advantage to getline is it does allocation and reallocation for you, it handles possible embedded NULLs, and it returns the count so you don't have to waste time with strlen . Note that you can't use an array with getline . The pointer must be NULL or free-able.

The strncpy() function was designed with a very particular problem in mind: manipulating strings stored in the manner of original UNIX directory entries. These used a fixed sized array, and a nul-terminator was only used if the filename was shorter than the array.

That's what's behind the two oddities of strncpy():

It doesn't put a nul-terminator on the destination if it is completely filled; and
It always completely fills the destination, with nuls if necessary.

For a "safer strcpy()", you are better off using strncat() like so:

if (dest_size > 0)
{
    dest[0] = '\0';
    strncat(dest, source, dest_size - 1);
}

That will always nul-terminate the result, and won't copy more than necessary.

That's what's behind the two oddities of strncpy():

It doesn't put a nul-terminator on the destination if it is completely filled; and
It always completely fills the destination, with nuls if necessary.

For a "safer strcpy()", you are better off using strncat() like so:

if (dest_size > 0)
{
    dest[0] = '\0';
    strncat(dest, source, dest_size - 1);
}

That will always nul-terminate the result, and won't copy more than necessary.