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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 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 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 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.

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

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

       β”Œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”€β”€β”€β”€β”
       β”‚Interface                                                                                                                             β”‚ Attribute     β”‚ Value   β”‚
       β”œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€
       β”‚fmemopen(),                                                                                                                           β”‚ Thread safety β”‚ MT-Safe β”‚
       β””β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”€β”€β”€β”€β”˜

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 longstanding bugs in the implementation of fmemopen() were fixed, and a  new  versioned  symbol  was
       created for this interface.

   Binary mode
       From version 2.9 to 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.

BUGS
       In versions of glibc before 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 version 2.22, the glibc implementation 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 cur‐
       rent 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  fmemo‐
       pen() 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 sub‐
       tracted 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 <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);
       }

SEE ALSO
       fopen(3), fopencookie(3), open_memstream(3)

GNU                                                                            2021-03-22                                                                    FMEMOPEN(3)