fmemopen(3) Library Functions Manual fmemopen(3)
NAME
fmemopen - open memory as stream
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <stdio.h>
FILE *fmemopen(void buf[.size], 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
DESCRIPTION
The fmemopen() function opens a stream that permits the access speci-
fied 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 po-
sition 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 ini-
tial 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 ap-
pend 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 posi-
tion 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 er-
ror. 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);
RETURN VALUE
Upon successful completion, fmemopen() returns a FILE pointer. Other-
wise, NULL is returned and errno is set to indicate the error.
ATTRIBUTES
For an explanation of the terms used in this section, see attrib-
utes(7).
┌───────────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├───────────────────────────────────────────┼───────────────┼─────────┤
│ fmemopen(), │ Thread safety │ MT-Safe │
└───────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
POSIX.1-2008.
HISTORY
glibc 1.0.x. POSIX.1-2008.
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'.
With glibc 2.22, binary mode (see below) was removed, many longstanding
bugs in the implementation of fmemopen() were fixed, and a new ver-
sioned symbol was created for this interface.
Binary mode
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 bi-
nary mode, the 'b' must be the second character in mode. Thus, for ex-
ample, "wb+" has the desired effect, but "w+b" does not. This is in-
consistent with the treatment of mode by fopen(3).
Binary mode was removed in glibc 2.22; a 'b' specified in mode has no
effect.
NOTES
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).
BUGS
Before glibc 2.22, if size is specified as zero, fmemopen() fails with
the error EINVAL. It would be more consistent if this case success-
fully created a stream that then returned end-of-file on the first at-
tempt 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.
EXAMPLES
The program below uses fmemopen() to open an input buffer, and
open_memstream(3) to open a dynamically sized output buffer. The pro-
gram scans its input string (taken from the program's first command-
line argument) reading integers, and writes the squares of these inte-
gers 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 <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);
}
SEE ALSO
fopen(3), fopencookie(3), open_memstream(3)
Linux man-pages 6.7 2023-10-31 fmemopen(3)
Generated by dwww version 1.16 on Wed Jul 15 09:05:33 CEST 2026.