c - 从使用 fmemopen 创建的流中读取宽字符

Question

我正在尝试从使用 fmemopen 和 char * 创建的流中读取宽字符。

char *s = "foo bar foo";
FILE *f = fmemopen(s,strlen(s),"r");

wchar_t c = getwc(f);

getwc 抛出一个段错误，我使用 GDB 检查过。

我知道这是因为用 fmemopen 打开了流，因为在打开的流上调用 getwc 正常工作。

fmemopen 是否有宽字符版本，或者是否有其他方法可以解决此问题？

Answer 1

第二行应该是 FILE *f = fmemopen(s, strlen(s), "r");。正如发布的那样，fmemopen 具有未定义的行为，可能会返回 NULL，这会导致 getwc() 崩溃。

更改 fmemopen() 行并添加对 NULL 的检查可修复崩溃但不符合 OP 目标。

使用 fmemopen() 打开的流似乎不支持宽方向，至少对于 GNU C 库而言。请注意，fmemopen 未在 C 标准中定义，而是在 POSIX.1-2008 中定义，并且在许多系统(如 OS/X)上不可用。

这是您程序的更正和扩展版本:

#include <errno.h>
#include <stdio.h>
#include <string.h>
#include <wchar.h>

int main(void) {
    const char *s = "foo bar foo";
    FILE *f = fmemopen((void *)s, strlen(s), "r");
    wchar_t c;

    if (f == NULL) {
        printf("fmemopen failed: %s\n", strerror(errno));
        return 1;
    }
    printf("default wide orientation: %d\n", fwide(f, 0));
    printf("selected wide orientation: %d\n", fwide(f, 1));
    while ((c = getwc(f)) != WEOF) {
        printf("read %lc (%d 0x%x)\n", c, c, c);
    }
    return 0;
}

在 Linux 上运行:

default wide orientation: -1
selected wide orientation: -1

无输出，立即返回WEOF。

Linux 手册页中对 fwide(f, 0) 的解释:

SYNOPSIS

#include <wchar.h>
int fwide(FILE *stream, int mode);

When mode is zero, the fwide() function determines the current orientation of stream. It returns a positive value if stream is wide-character oriented, that is, if wide-character I/O is permitted but char I/O is disallowed. It returns a negative value if stream is byte oriented, i.e., if char I/O is permitted but wide-character I/O is disallowed. It returns zero if stream has no orientation yet; in this case the next I/O operation might change the orientation (to byte oriented if it is a char I/O operation, or to wide-character oriented if it is a wide-character I/O operation).

Once a stream has an orientation, it cannot be changed and persists until the stream is closed.

When mode is nonzero, the fwide() function first attempts to set stream's orientation (to wide-character oriented if mode is greater than 0, or to byte oriented if mode is less than 0). It then returns a value denoting the current orientation, as above.

fmemopen() 返回的流是面向字节的，不能更改为面向宽字符。