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FMEMOPEN(3) Linux Programmer's Manual FMEMOPEN(3)
fmemopen - open memory as stream
#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
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);
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 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.
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.
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);
}
fopen(3), fopencookie(3), open_memstream(3)
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GNU 2016-10-08 FMEMOPEN(3)
Pages that refer to this page: fopen(3), fopencookie(3), open_memstream(3)