открыть память как поток (open memory as stream)
Имя (Name)
fmemopen - open memory as stream
Синопсис (Synopsis)
#include <stdio.h>
FILE *fmemopen(void *buf, size_t size, const char *mode);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
fmemopen():
Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
_GNU_SOURCE
Описание (Description)
The fmemopen() function opens a stream that permits the access
specified by mode. The stream allows I/O to be performed on the
string or memory buffer pointed to by buf.
The mode argument specifies the semantics of I/O on the stream,
and is one of the following:
r The stream is opened for reading.
w The stream is opened for writing.
a Append; open the stream for writing, with the initial
buffer position set to the first null byte.
r+ Open the stream for reading and writing.
w+ Open the stream for reading and writing. The buffer
contents are truncated (i.e., '\0' is placed in the first
byte of the buffer).
a+ Append; open the stream for reading and writing, with the
initial buffer position set to the first null byte.
The stream maintains the notion of a current position, the
location where the next I/O operation will be performed. The
current position is implicitly updated by I/O operations. It can
be explicitly updated using fseek(3), and determined using
ftell(3). In all modes other than append, the initial position
is set to the start of the buffer. In append mode, if no null
byte is found within the buffer, then the initial position is
size+1.
If buf is specified as NULL, then fmemopen() allocates a buffer
of size bytes. This is useful for an application that wants to
write data to a temporary buffer and then read it back again.
The initial position is set to the start of the buffer. The
buffer is automatically freed when the stream is closed. Note
that the caller has no way to obtain a pointer to the temporary
buffer allocated by this call (but see open_memstream(3)).
If buf is not NULL, then it should point to a buffer of at least
len bytes allocated by the caller.
When a stream that has been opened for writing is flushed
(fflush(3)) or closed (fclose(3)), a null byte is written at the
end of the buffer if there is space. The caller should ensure
that an extra byte is available in the buffer (and that size
counts that byte) to allow for this.
In a stream opened for reading, null bytes ('\0') in the buffer
do not cause read operations to return an end-of-file indication.
A read from the buffer will indicate end-of-file only when the
current buffer position advances size bytes past the start of the
buffer.
Write operations take place either at the current position (for
modes other than append), or at the current size of the stream
(for append modes).
Attempts to write more than size bytes to the buffer result in an
error. By default, such errors will be visible (by the absence
of data) only when the stdio buffer is flushed. Disabling
buffering with the following call may be useful to detect errors
at the time of an output operation:
setbuf(stream, NULL);
Возвращаемое значение (Return value)
Upon successful completion, fmemopen() returns a FILE pointer.
Otherwise, NULL is returned and errno is set to indicate the
error.
Версии (Versions)
fmemopen() was already available in glibc 1.0.x.
Атрибуты (Attributes)
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│fmemopen(), │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
Стандарты (Conforming to)
POSIX.1-2008. This function is not specified in POSIX.1-2001,
and is not widely available on other systems.
POSIX.1-2008 specifies that 'b' in mode shall be ignored.
However, Technical Corrigendum 1 adjusts the standard to allow
implementation-specific treatment for this case, thus permitting
the glibc treatment of 'b'.
Примечание (Note)
There is no file descriptor associated with the file stream
returned by this function (i.e., fileno(3) will return an error
if called on the returned stream).
With version 2.22, binary mode (see below) was removed, many
longstanding bugs in the implementation of fmemopen() were fixed,
and a new versioned symbol was created for this interface.
Binary mode
From version 2.9 to 2.21, the glibc implementation of fmemopen()
supported a "binary" mode, enabled by specifying the letter 'b'
as the second character in mode. In this mode, writes don't
implicitly add a terminating null byte, and fseek(3) SEEK_END is
relative to the end of the buffer (i.e., the value specified by
the size argument), rather than the current string length.
An API bug afflicted the implementation of binary mode: to
specify binary mode, the 'b' must be the second character in
mode. Thus, for example, "wb+" has the desired effect, but "w+b"
does not. This is inconsistent with the treatment of mode by
fopen(3).
Binary mode was removed in glibc 2.22; a 'b' specified in mode
has no effect.
Ошибки (баги) (Bugs)
In versions of glibc before 2.22, if size is specified as zero,
fmemopen() fails with the error EINVAL. It would be more
consistent if this case successfully created a stream that then
returned end-of-file on the first attempt at reading; since
version 2.22, the glibc implementation provides that behavior.
In versions of glibc before 2.22, specifying append mode ("a" or
"a+") for fmemopen() sets the initial buffer position to the
first null byte, but (if the current position is reset to a
location other than the end of the stream) does not force
subsequent writes to append at the end of the stream. This bug
is fixed in glibc 2.22.
In versions of glibc before 2.22, if the mode argument to
fmemopen() specifies append ("a" or "a+"), and the size argument
does not cover a null byte in buf, then, according to
POSIX.1-2008, the initial buffer position should be set to the
next byte after the end of the buffer. However, in this case the
glibc fmemopen() sets the buffer position to -1. This bug is
fixed in glibc 2.22.
In versions of glibc before 2.22, when a call to fseek(3) with a
whence value of SEEK_END was performed on a stream created by
fmemopen(), the offset was subtracted from the end-of-stream
position, instead of being added. This bug is fixed in glibc
2.22.
The glibc 2.9 addition of "binary" mode for fmemopen() silently
changed the ABI: previously, fmemopen() ignored 'b' in mode.
Примеры (Examples)
The program below uses fmemopen() to open an input buffer, and
open_memstream(3) to open a dynamically sized output buffer. The
program scans its input string (taken from the program's first
command-line argument) reading integers, and writes the squares
of these integers to the output buffer. An example of the output
produced by this program is the following:
$ ./a.out '1 23 43'
size=11; ptr=1 529 1849
Program source
#define _GNU_SOURCE
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#define handle_error(msg) \
do { perror(msg); exit(EXIT_FAILURE); } while (0)
int
main(int argc, char *argv[])
{
FILE *out, *in;
int v, s;
size_t size;
char *ptr;
if (argc != 2) {
fprintf(stderr, "Usage: %s '<num>...'\n", argv[0]);
exit(EXIT_FAILURE);
}
in = fmemopen(argv[1], strlen(argv[1]), "r");
if (in == NULL)
handle_error("fmemopen");
out = open_memstream(&ptr, &size);
if (out == NULL)
handle_error("open_memstream");
for (;;) {
s = fscanf(in, "%d", &v);
if (s <= 0)
break;
s = fprintf(out, "%d ", v * v);
if (s == -1)
handle_error("fprintf");
}
fclose(in);
fclose(out);
printf("size=%zu; ptr=%s\n", size, ptr);
free(ptr);
exit(EXIT_SUCCESS);
}
Смотри также (See also)
fopen(3), fopencookie(3), open_memstream(3)
