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C++和C語言在文件操作打開方式上的對應(yīng)

//這是C++文件打開的部分實現(xiàn)fiopen.cpp文件的一個函數(shù),看了就明白了

_CRTIMP2_PURE FILE *__CLRCALL_PURE_OR_CDECL _Fiopen(const _Sysch_t *filename,

ios_base::openmode mode, int prot)

{ // open a file with native name

static const _Sysch_t *mods[] =

{ // fopen mode strings corresponding to valid[i]

_SYSCH("r"), _SYSCH("w"), _SYSCH("w"), _SYSCH("a"),

_SYSCH("rb"), _SYSCH("wb"), _SYSCH("wb"), _SYSCH("ab"),

_SYSCH("r+"), _SYSCH("w+"), _SYSCH("a+"),

_SYSCH("r+b"), _SYSCH("w+b"), _SYSCH("a+b"),

0};

static const int valid[] =

{ // valid combinations of open flags

ios_base::in,

ios_base::out,

ios_base::out | ios_base::trunc,

ios_base::out | ios_base::app,

ios_base::in | ios_base::binary,

ios_base::out | ios_base::binary,

ios_base::out | ios_base::trunc | ios_base::binary,

ios_base::out | ios_base::app | ios_base::binary,

ios_base::in | ios_base::out,

ios_base::in | ios_base::out | ios_base::trunc,

ios_base::in | ios_base::out | ios_base::app,

ios_base::in | ios_base::out | ios_base::binary,

ios_base::in | ios_base::out | ios_base::trunc

| ios_base::binary,

ios_base::in | ios_base::out | ios_base::app

| ios_base::binary,

0};

FILE *fp = 0;

int n;

ios_base::openmode atendflag = mode & ios_base::ate;

ios_base::openmode norepflag = mode & ios_base::_Noreplace;

if (mode & ios_base::_Nocreate)

mode |= ios_base::in; // file must exist

if (mode & ios_base::app)

mode |= ios_base::out; // extension -- app implies out

mode &= ~(ios_base::ate | ios_base::_Nocreate | ios_base::_Noreplace);

for (n = 0; valid[n] != 0 && valid[n] != mode; ++n)

; // look for a valid mode

if (valid[n] == 0)

return (0); // no valid mode

else if (norepflag && mode & (ios_base::out || ios_base::app)

&& (fp = _Xfsopen(filename, _SYSCH("r"), prot)) != 0)

{ // file must not exist, close and fail

fclose(fp);

return (0);

}

else if (fp != 0 && fclose(fp) != 0)

return (0); // can't close after test open

else if ((fp = _Xfsopen(filename, mods[n], prot)) == 0)

return (0); // open failed

if (!atendflag || fseek(fp, 0, SEEK_END) == 0)

return (fp); // no need to seek to end, or seek succeeded

fclose(fp); // can't position at end