fmemopen(3)                                           Library Functions Manual                                          fmemopen(3)

       fmemopen -  open memory as stream

       Standard C library (libc, -lc)

       #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)):

           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 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 indi‐
       cation.  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

       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.

   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 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

   Program source

       #define _GNU_SOURCE
       #include <err.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <string.h>

       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)
               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)

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


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


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

Linux man-pages 6.03                                         2023-02-05                                                 fmemopen(3)