Provided by: manpages-dev_6.8-2_all bug

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 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 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 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.   Otherwise,  NULL  is
       returned and errno is set to indicate the error.

ATTRIBUTES

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

       ┌───────────────────────────────────────────────────────────────┬───────────────┬─────────┐
       │InterfaceAttributeValue   │
       ├───────────────────────────────────────────────────────────────┼───────────────┼─────────┤
       │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 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.

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 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.

EXAMPLES

       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>

       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)