fmemopen - open memory as stream
Standard C library (
libc,
-lc)
#include <stdio.h>
FILE *fmemopen(void buf[.size], size_t size, const char *mode);
fmemopen():
Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
_GNU_SOURCE
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
size 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);
Upon successful completion,
fmemopen() returns a
FILE pointer.
Otherwise, NULL is returned and
errno is set to indicate the error.
fmemopen() was already available in glibc 1.0.x.
For an explanation of the terms used in this section, see
attributes(7).
Interface |
Attribute |
Value |
fmemopen (), |
Thread safety |
MT-Safe |
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'.
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 glibc 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.
From glibc 2.9 to glibc 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.
Before glibc 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 glibc 2.22, the glibc implementation provides that
behavior.
Before glibc 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.
Before glibc 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.
Before glibc 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.
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
#define _GNU_SOURCE
#include <err.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
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)
err(EXIT_FAILURE, "fmemopen");
out = open_memstream(&ptr, &size);
if (out == NULL)
err(EXIT_FAILURE, "open_memstream");
for (;;) {
s = fscanf(in, "%d", &v);
if (s <= 0)
break;
s = fprintf(out, "%d ", v * v);
if (s == -1)
err(EXIT_FAILURE, "fprintf");
}
fclose(in);
fclose(out);
printf("size=%zu; ptr=%s\n", size, ptr);
free(ptr);
exit(EXIT_SUCCESS);
}
fopen(3),
fopencookie(3),
open_memstream(3)