Revised fmemopen(3) man page

Hello Adhemerval

Since you recently did a reimplementation of fmemopen() (which Paul reminded me of) I've similarly just now done a major rework of the man page. Might you (or Paul, feel free to also chin in!), be willing to take a look at the text below and let me know if anything needs fixing?

Thanks,

Michael

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   fea‐
   ture_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 file ini‐
tial 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 file initial 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 indica‐
tion. A read from the buffer will indicate end-of-file only
when the file current 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. Dis‐ abling buffering with the following call may be useful to detect errors at the time of an output operation:

setbuf(stream, NULL);

Alternatively, the caller can explicitly set buf as the stdio stream buffer, at the same time informing stdio of the buffer's size, using:

setbuffer(stream, buf, size);

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. How‐ ever, Technical Corrigendum 1 adjusts the standard to allow implementation-specific treatment for this case, thus permit‐ ting 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 inter‐ face.

Binary mode
From version 2.9 to 2.21, the glibc implementation of fmemo‐ pen() supported a "binary" mode, enabled by specifying the let‐ ter '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 con‐ sistent 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 file offset 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 fmem‐ open() 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)

I am not the most qualified person to correct syntactic issues or grammar, but regarding the contents it covers current functionalities and previous bugs as I would expect, thanks!

Quoted 254 lines

Hello Adhemerval

Since you recently did a reimplementation of fmemopen() (which Paul reminded me of) I've similarly just now done a major rework of the man page. Might you (or Paul, feel free to also chin in!), be willing to take a look at the text below and let me know if anything needs fixing?

Thanks,

Michael

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   fea‐
   ture_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 file ini‐
tial 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 file initial 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 indica‐
tion. A read from the buffer will indicate end-of-file only
when the file current 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. Dis‐ abling buffering with the following call may be useful to detect errors at the time of an output operation:

setbuf(stream, NULL);

Alternatively, the caller can explicitly set buf as the stdio stream buffer, at the same time informing stdio of the buffer's size, using:

setbuffer(stream, buf, size);

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. How‐ ever, Technical Corrigendum 1 adjusts the standard to allow implementation-specific treatment for this case, thus permit‐ ting 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 inter‐ face.

Binary mode
From version 2.9 to 2.21, the glibc implementation of fmemo‐ pen() supported a "binary" mode, enabled by specifying the let‐ ter '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 con‐ sistent 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 file offset 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 fmem‐ open() 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)