fmemopen(3) — Linux manual page


fmemopen(3)             Library Functions Manual             fmemopen(3)

NAME         top

       fmemopen -  open memory as stream

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <stdio.h>

       FILE *fmemopen(void buf[.size], size_t size, const char *mode);

   Feature Test Macro Requirements for glibc (see

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

DESCRIPTION         top

       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 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 position advances size bytes past the start of the

       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);

RETURN VALUE         top

       Upon successful completion, fmemopen() returns a FILE pointer.
       Otherwise, NULL is returned and errno is set to indicate the

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       │ Interface                           Attribute     Value   │
       │ fmemopen(),                         │ Thread safety │ MT-Safe │

STANDARDS         top


HISTORY         top

       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 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

       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

       Binary mode was removed in glibc 2.22; a 'b' specified in mode
       has no effect.

NOTES         top

       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         top

       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

       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         top

       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);


SEE ALSO         top

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

Linux man-pages (unreleased)     (date)                      fmemopen(3)

Pages that refer to this page: fopen(3)fopencookie(3)open_memstream(3)