ptg10805159172 StandardI/O Library Chapter 5
append, the current file position is set to the first null byte in the buffer.Ifthe buffer
contains no null bytes, then the current position is set to one byte past the end of the
buffer.When a stream is not opened for append, the current position is set to the
beginning of the buffer.Because the append mode determines the end of the data by
the first null byte, memory streams aren’t well suited for storing binary data (which
might contain null bytes beforethe end of the data).
Second, if thebufargument is a null pointer, it makes no sense to open the stream
for only reading or only writing. Because the buffer is allocated byfmemopenin this
case, there is no way to find the buffer ’s address, so to open the stream only for writing
means we could never read what we’ve written. Similarly, to open the stream only for
reading means we can only read the contents of a buffer into which we can never write.
Third, a null byte is written at the current position in the stream whenever we
increase the amount of data in the stream’s buffer and callfclose,fflush,fseek,
fseeko,orfsetpos.Example
It’s instructive to look at how writes to a memory stream operate on a buffer we
provide. Figure5.15 shows a sample program that seeds the buffer with a known
pattern to see how writes to the stream behave.
#include "apue.h"#define BSZ 48
int
main()
{
FILE *fp;
char buf[BSZ];
memset(buf, ’a’, BSZ-2);
buf[BSZ-2] = ’\0’;
buf[BSZ-1] = ’X’;
if ((fp = fmemopen(buf, BSZ, "w+")) == NULL)
err_sys("fmemopen failed");
printf("initial buffer contents: %s\n", buf);
fprintf(fp, "hello, world");
printf("before flush: %s\n", buf);
fflush(fp);
printf("after fflush: %s\n", buf);
printf("len of string in buf = %ld\n", (long)strlen(buf));
memset(buf, ’b’, BSZ-2);
buf[BSZ-2] = ’\0’;
buf[BSZ-1] = ’X’;
fprintf(fp, "hello, world");
fseek(fp, 0, SEEK_SET);
printf("after fseek: %s\n", buf);
printf("len of string in buf = %ld\n", (long)strlen(buf));