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FMEMOPEN(3) Linux Programmer's Manual FMEMOPEN(3)

NAME

     fmemopen -  open memory as stream

SYNOPSIS

     #include <stdio.h>

     FILE *fmemopen(void *buf, 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
             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 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
     append  mode, if no null byte is found within the buffer, then the ini-
     tial 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  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 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
     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

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

VERSIONS

     fmemopen() was already available in glibc 1.0.x.

ATTRIBUTES

     For   an   explanation   of   the  terms  used  in  this  section,  see
     attributes(7).

     +------------+---------------+---------+
     |Interface   | Attribute     | Value   |
     +------------+---------------+---------+
     |fmemopen(), | Thread safety | MT-Safe |
     +------------+---------------+---------+

CONFORMING TO

     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'.

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).

     With version 2.22, binary mode (see below) was removed, many longstand-
     ing bugs in the implementation of fmemopen() were fixed, and a new ver-
     sioned symbol was created for this interface.

 Binary mode
     From version 2.9 to 2.21, the glibc implementation of  fmemopen()  sup-
     ported  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.

BUGS

     In  versions of glibc before 2.22, if size is specified as zero, fmemo-
     pen() 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 implementa-
     tion 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.

EXAMPLE

     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 <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 '<num>...'\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=%zu; ptr=%s\n", size, ptr);

         free(ptr);
         exit(EXIT_SUCCESS); }