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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)