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NAME
fmemopen, open_memstream, open_wmemstream - open memory as stream
SYNOPSIS
#include <stdio.h>
FILE *fmemopen(void *buf, size_t size, const char *mode);
FILE *open_memstream(char **ptr, size_t *sizeloc);
#include <wchar.h>
FILE *open_wmemstream(wchar_t **ptr, size_t *sizeloc);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
fmemopen(), open_memstream(), open_wmemstream():
Since glibc 2.10:
_XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
_GNU_SOURCE
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. This buffer must be at least size
bytes long.
The argument mode is the same as for fopen(3). If mode specifies an append mode, then the initial file
position is set to the location of the first null byte ('\0') in the buffer; otherwise the initial file
position is set to the start of the buffer. Since glibc 2.9, the letter 'b' may be specified as the
second character in mode. This provides "binary" 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.
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.
Attempts to write more than size bytes to the buffer result in an error. (By default, such errors will
be visible only when the stdio buffer is flushed. Disabling buffering with setbuf(fp, NULL) may be
useful to detect errors at the time of an output operation. Alternatively, the caller can explicitly set
buf as the stdio stream buffer, at the same time informing stdio of the buffer's size, using
setbuffer(fp, buf, size).)
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 only indicate end-of-file when the file pointer
advances size bytes past the start of the buffer.
If buf is specified as NULL, then fmemopen() dynamically allocates a buffer size bytes long. This is
useful for an application that wants to write data to a temporary buffer and then read it back again.
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() below).
The open_memstream() function opens a stream for writing to a buffer. The buffer is dynamically
allocated (as with malloc(3)), and automatically grows as required. After closing the stream, the caller
should free(3) this buffer.
When the stream is closed (fclose(3)) or flushed (fflush(3)), the locations pointed to by ptr and sizeloc
are updated to contain, respectively, a pointer to the buffer and the current size of the buffer. These
values remain valid only as long as the caller performs no further output on the stream. If further
output is performed, then the stream must again be flushed before trying to access these variables.
A null byte is maintained at the end of the buffer. This byte is not included in the size value stored
at sizeloc.
The stream's file position can be changed with fseek(3) or fseeko(3). Moving the file position past the
end of the data already written fills the intervening space with zeros.
The open_wmemstream() is similar to open_memstream(), but operates on wide characters instead of bytes.
RETURN VALUE
Upon successful completion fmemopen(), open_memstream() and open_wmemstream() return a FILE pointer.
Otherwise, NULL is returned and errno is set to indicate the error.
VERSIONS
fmemopen() and open_memstream() were already available in glibc 1.0.x. open_wmemstream() is available
since glibc 2.4.
CONFORMING TO
POSIX.1-2008. These functions are not specified in POSIX.1-2001, and are 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'.
NOTES
There is no file descriptor associated with the file stream returned by these functions (i.e., fileno(3)
will return an error if called on the returned stream).
BUGS
In glibc before version 2.7, seeking past the end of a stream created by open_memstream() does not
enlarge the buffer; instead the fseek(3) call fails, returning -1.
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.
Furthermore, POSIX.1-2008 does not specify a failure for this case.
Specifying append mode ("a" or "a+") for fmemopen() sets the initial file position to the first null
byte, but (if the file offset 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.
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 file position should be set to the next
byte after the end of the buffer. However, in this case the glibc fmemopen() sets the file position to
-1.
To specify binary mode for fmemopen() 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).
The glibc 2.9 addition of "binary" mode for fmemopen() silently changed the ABI: previously, fmemopen()
ignored 'b' in mode.
EXAMPLE
The program below uses fmemopen() to open an input buffer, and open_memstream() 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 <file>\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=%ld; ptr=%s\n", (long) size, ptr);
free(ptr);
exit(EXIT_SUCCESS);
}
SEE ALSO
fopen(3), fopencookie(3)
COLOPHON
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