[英]How to read a file using boost::iostreams::file_descriptor_source?
[英]istream from file_descriptor_source (boost::iostreams) or file
我需要为我的程序输入执行类似的操作:
stream input;
if (decompressed)
input.open(filepath);
else {
file_descriptor=_popen("decompressor "+filepath,"r");
input.open(file_descriptor);
}
input.read(...)
...
我可以看到一个解决方案 - 在两种情况下都使用_popen,如果它已经解压缩,只需将文件复制到stdout,但这似乎不是很优雅。
有趣的是,与C相比有多难 - 我想标准库错过了它。 现在我迷失在神秘的boost :: iostreams文档中。 如果有人知道如何,示例代码会很棒。
这就是你所追求的:
#include <cstdio>
#include <string>
#include <iostream>
#include <boost/iostreams/device/file_descriptor.hpp>
#include <boost/iostreams/stream.hpp>
namespace io = boost::iostreams;
int main()
{
bool flag = false;
FILE* handle = 0;
if (flag)
{
handle = _popen("dir", "r");
}
else
{
handle = fopen ("main.cpp", "r");
}
io::stream_buffer<io::file_descriptor_source> fpstream (fileno(handle));
std::istream in (&fpstream);
std::string line;
while (in)
{
std::getline (in, line);
std::cout << line << std::endl;
}
return 0;
}
添加到jon-hanson的答案,这是一个演示如何将file_descriptor_source
与管道一起使用的简单示例。
如何建立:
g++ -m32 -DBOOST_IOSTREAMS_NO_LIB -isystem ${BOOST_PATH}/include \
${BOOST_SRC_PATH}/libs/iostreams/src/file_descriptor.cpp blah.cc -o blah
代码:
#include <fcntl.h>
#include <stdio.h>
#include <boost/iostreams/device/file_descriptor.hpp>
#include <boost/iostreams/stream.hpp>
int main( int argc, char* argv[] ) {
// if you just do 'using namespace...', there's a
// namespace collision with the global 'write'
// function used in the child
namespace io = boost::iostreams;
int pipefd[] = {0,0};
pipe( pipefd, 0 ); // If you use O_NONBLOCK, you'll have to
// add some extra checks to the loop so
// it will wait until the child is finished.
if( 0 == fork() ) {
// child
close( pipefd[0] ); // read handle
dup2( pipefd[1], FILENO_STDOUT );
printf( "This\nis\na\ntest\nto\nmake sure that\nit\nis\working as expected.\n" );
return 0; // ya ya, shoot me ;p
}
// parent
close( pipefd[1] ); // write handle
char *buff = new char[1024];
memset( buff, 0, 1024 );
io::stream<io::file_descriptor_source> fds(
io::file_descriptor_source( pipefd[0], io::never_close_handle ) );
// this should work with std::getline as well
while( fds.getline( buff, 1024 )
&& fds.gcount() > 0 // this condition is not enough if you use
// O_NONBLOCK; it should only bail if this
// is false AND the child has exited
) {
printf( "%s,", buff );
}
printf( "\n" );
}
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