Advanced Programming in the UNIX® Environment

ptg10805159

Section 5.14 Memory Streams 171

trying to create second temp file...

Segmentation fault

The difference in behavior comes from the way the two template strings are

declared. For the first template, the name is allocated on the stack, because we use an

array variable. For the second name, however, we use a pointer.Inthis case, only the

memory for the pointer itself resides on the stack; the compiler arranges for the string to

be stored in the read-only segment of the executable. When themkstempfunction tries

to modify the string, a segmentation fault occurs.

5.14 MemoryStreams

As we’ve seen, the standardI/O library buffers data in memory, so operations such as

character-at-a-time I/O and line-at-a-time I/O aremoreefficient. We’ve also seen that

we can provide our own buffer for the library to use by callingsetbuforsetvbuf.In

Version 4, the Single UNIX Specification added support formemory streams.These are

standardI/O streams for which thereare nounderlying files, although they arestill

accessed withFILEpointers. All I/O is done by transferring bytes to and from buffers

in main memory.As we shall see, even though these streams look like file streams,

several features make them moresuited for manipulating character strings.

Three functions areavailable to create memory streams. The first isfmemopen.

#include <stdio.h>

FILE *fmemopen(void *restrictbuf,size_t size,

const char *restricttype);

Returns: stream pointer if OK,NULLon error

Thefmemopenfunction allows the caller to provide a buffer to be used for the memory

stream: thebufargument points to the beginning of the buffer and thesizeargument

specifies the size of the buffer in bytes. If thebufargument is null, then thefmemopen

function allocates a buffer ofsizebytes. In this case, the buffer will be freed when the

stream is closed.

Thetypeargument controls how the stream can be used. The possible values for

typearesummarized in Figure5.14.

type Description

rorrb open for reading

worwb open for writing

aorab append; open for writing at first null byte

r+orr+borrb+ open for reading and writing

w+orw+borwb+ truncate to 0 length and open for reading and writing

a+ora+borab+ append; open for reading and writing at first null byte

Figure 5.14 Thetypeargument for opening a memory stream

Note that these values correspond to the ones for file-based standardI/O streams, but

thereare some subtle differences. First, whenever a memory stream is opened for