如果您知道要拦截的库调用，您可以使用通过LD_PRELOAD 加载的共享对象来插入调用。
shortread.c:
#include <sys/types.h>
#include <dlfcn.h>
#define MAX_FDS 1024
static int short_read_array[ MAX_FDS ];
// #define these to match your system's values
// (need to be really careful with header files since
// getting open() declared would make things very
// difficult - just try this with open( const char *, int, ...);
// declared to see what I mean...)
#define O_RDONLY 0
#define O_WRONLY 1
#define O_RDWR 2
// note that the mode bits for read/write are
// not a bitwise-or - they are distinct values
#define MODE_BITS 3
// it's much easier to *NOT* even deal with the
// fact that open() is a varargs function
// but that means probably having to do some
// typedef's and #defines to get this to compile
// typedef some function points to make things easier
typedef int ( *open_ptr_t )( const char *name, int flags, mode_t mode );
typedef ssize_t ( *read_ptr_t )( int fd, void *buf, size_t bytes );
typedef int ( *close_ptr_t )( int fd );
// function points to the real IO library calls
static open_ptr_t real_open = NULL;
static read_ptr_t real_read = NULL;
static close_ptr_t real_close = NULL;
// this will return non-zero if 'filename' is a file
// to cause short reads on
static int shortReadsOnFd( const char *filename )
{
// add logic here based on the file name to
// return non-zero if you want to do
// short reads on this file
//
// return( 1 );
return( 0 );
}
// interpose on open()
int open( const char *filename, int flags, mode_t mode )
{
static pthread_mutex_t open_mutex = PTHREAD_MUTEX_INITIALIZER;
int fd;
pthread_mutex_lock( &open_mutex );
if ( NULL == real_open )
{
real_open = dlsym( RTLD_NEXT, "open" );
}
pthread_mutex_unlock( &open_mutex );
fd = real_open( filename, flags, mode );
if ( ( -1 == fd ) || ( fd >= MAX_FDS ) )
{
return( fd );
}
int mode_bits = flags & MODE_BITS;
// if the file can be read from, check if this is a file
// to do short reads on
if ( ( O_RDONLY == mode_bits ) || ( O_RDWR == mode_bits ) )
{
short_read_array[ fd ] = shortReadsOnFd( filename );
}
return( fd );
}
ssize_t read( int fd, void *buffer, size_t bytes )
{
static pthread_mutex_t read_mutex = PTHREAD_MUTEX_INITIALIZER;
if ( ( fd < MAX_FDS ) && ( short_read_array[ fd ] ) )
{
// read less bytes than the caller asked for
bytes /= 2;
if ( 0 == bytes )
{
bytes = 1;
}
}
pthread_mutex_lock( &read_mutex );
if ( NULL == real_read )
{
real_read = dlsym( RTLD_NEXT, "read" );
}
pthread_mutex_unlock( &read_mutex );
return( real_read( fd, buffer, bytes ) );
}
int close( int fd )
{
static pthread_mutex_t close_mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_mutex_lock( &close_mutex );
if ( NULL == real_close )
{
real_close = dlsym( RTLD_NEXT, "close" );
}
pthread_mutex_unlock( &close_lock );
if ( fd < MAX_FDS )
{
short_read_array[ fd ] = 0;
}
return( real_close( fd ) );
}
用类似的东西编译：
gcc -shared [-m32|-m64] shortread.c -o libshortread.so
然后：
export LD_PRELOAD=/path/to/libshortread.so
对这样的 LD_PRELOAD 非常小心 - 进程树中的所有进程都将被强制加载库。如果必须加载 64 位库，则 32 位进程将无法运行，强制尝试加载 32 位库的 64 位进程也将无法运行。您可以在上面的源代码中添加一个 init 函数，该函数会删除 LD_PRELOAD 环境变量（或将其设置为无害的变量）以在一定程度上对其进行控制。
如果任何应用程序将O_DIRECT 标志用于open()，您可能还需要小心。修改正在读取的字节数可能会破坏某些 Linux 文件系统和/或实现的直接 IO，因为可能仅支持页面大小的 IO 操作。
而且这段代码只处理read()。您可能还需要处理creat()。还有pread()、readat()、aio_read() 和lio_listio()，（甚至可能还有一些我现在想不起来的其他人）虽然这当然不太可能。并注意处理大文件的 32 位进程。自从我处理这些问题以来已经有一段时间了，但我记得那会变得很难看。
另外需要注意的是fopen() 和fread() 等调用可能不会调用open() 和read() 库调用，可能会直接发出相关的系统调用。在这种情况下，您将无法轻松修改这些调用的行为。对整个基于 STDIO 的调用家族进行干预，这些调用可以读取诸如 fgets() 之类的数据，这可能是一件非常困难的事情。
如果您知道您的应用程序是单线程的，则可以删除互斥锁。