fmemopen

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).

       ┌────────────┬───────────────┬─────────┐
       │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
       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 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
       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)

COLOPHON
       This page is part of release 5.01 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.



GNU                               2019-03-06                       FMEMOPEN(3)