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)):

           Since glibc 2.10:
               _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:

       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

       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

       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

       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

       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

       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)

       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]);

           in = fmemopen(argv[1], strlen(argv[1]), "r");
           if (in == NULL)

           out = open_memstream(&ptr, &size);
           if (out == NULL)

           for (;;) {
               s = fscanf(in, "%d", &v);
               if (s <= 0)

               s = fprintf(out, "%d ", v * v);
               if (s == -1)


           printf("size=%zu; ptr=%s\n", size, ptr);


       fopen(3), fopencookie(3), open_memstream(3)

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       latest version of this page, can be found at

GNU                                2021-03-22                        FMEMOPEN(3)