.\" Copyright (c) 2008, Linux Foundation, written by Michael Kerrisk
.\" <mtk.manpages@gmail.com>
.\"
+.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
+.\" %%%LICENSE_END
.\"
-.TH FOPENCOOKIE 3 2012-04-30 "Linux" "Linux Programmer's Manual"
+.TH FOPENCOOKIE 3 2020-04-11 "Linux" "Linux Programmer's Manual"
.SH NAME
fopencookie \- opening a custom stream
.SH SYNOPSIS
.nf
.BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */"
.B #include <stdio.h>
-
+.PP
.BI "FILE *fopencookie(void *" cookie ", const char *" mode ,
.BI " cookie_io_functions_t " io_funcs );
.fi
.BR fmemopen (3),
which provides a stream interface to data that is stored in a
buffer in memory.
-
+.PP
In order to create a custom stream the programmer must:
.IP * 3
Implement four "hook" functions that are used internally by the
it opens a new stream and returns a pointer to a
.I FILE
object that is used to operate on that stream.
-
+.PP
The
.I cookie
argument is a pointer to the caller's cookie structure
that is to be associated with the new stream.
This pointer is supplied as the first argument when the standard I/O
library invokes any of the hook functions described below.
-
+.PP
The
.I mode
argument serves the same purpose as for
See
.BR fopen (3)
for details.
-
+.PP
The
.I io_funcs
argument is a structure that contains four fields pointing to the
programmer-defined hook functions that are used to implement this stream.
The structure is defined as follows
+.PP
.in +4n
-.nf
-
-struct cookie_io_functions_t {
+.EX
+typedef struct {
cookie_read_function_t *read;
cookie_write_function_t *write;
cookie_seek_function_t *seek;
cookie_close_function_t *close;
-};
-
-.fi
+} cookie_io_functions_t;
+.EE
.in
+.PP
The four fields are as follows:
.TP
.I cookie_read_function_t *read
This function implements read operations for the stream.
When called, it receives three arguments:
-
+.IP
ssize_t read(void *cookie, char *buf, size_t size);
-
+.IP
The
.I buf
and
The
.I read
function should update the stream offset appropriately.
-
+.IP
If
.I *read
-is a NULL pointer,
+is a null pointer,
then reads from the custom stream always return end of file.
.TP
.I cookie_write_function_t *write
This function implements write operations for the stream.
When called, it receives three arguments:
-
+.IP
ssize_t write(void *cookie, const char *buf, size_t size);
-
+.IP
The
.I buf
and
The
.I write
function should update the stream offset appropriately.
-
+.IP
If
.I *write
-is a NULL pointer,
+is a null pointer,
then output to the stream is discarded.
.TP
.I cookie_seek_function_t *seek
This function implements seek operations on the stream.
When called, it receives three arguments:
-
+.IP
int seek(void *cookie, off64_t *offset, int whence);
-
+.IP
The
.I *offset
argument specifies the new file offset depending on which
of the following three values is supplied in
.IR whence :
.RS
-.TP 10
+.TP
.B SEEK_SET
The stream offset should be set
.I *offset
function should update
.I *offset
to indicate the new stream offset.
-
+.IP
As its function result, the
.I seek
function should return 0 on success, and \-1 on error.
-
+.IP
If
.I *seek
-is a NULL pointer,
+is a null pointer,
then it is not possible to perform seek operations on the stream.
.TP
.I cookie_close_function_t *close
The hook function can do things such as freeing buffers allocated
for the stream.
When called, it receives one argument:
-
+.IP
int close(void *cookie);
-
+.IP
The
.I cookie
argument is the cookie that the programmer supplied when calling
.BR fopencookie ().
-
+.IP
As its function result, the
.I close
function should return 0 on success, and
.B EOF
on error.
-
+.IP
If
.I *close
is NULL, then no special action is performed when the stream is closed.
On error, NULL is returned.
.\" .SH ERRORS
.\" It's not clear if errno ever gets set...
+.SH ATTRIBUTES
+For an explanation of the terms used in this section, see
+.BR attributes (7).
+.TS
+allbox;
+lb lb lb
+l l l.
+Interface Attribute Value
+T{
+.BR fopencookie ()
+T} Thread safety MT-Safe
+.TE
.SH CONFORMING TO
This function is a nonstandard GNU extension.
-.SH EXAMPLE
+.SH EXAMPLES
The program below implements a custom stream whose functionality
is similar (but not identical) to that available via
.BR fmemopen (3).
and then seeks through the stream reading two out of every
five characters and writing them to standard output.
The following shell session demonstrates the use of the program:
+.PP
.in +4n
-.nf
-
+.EX
.RB "$" " ./a.out \(aqhello world\(aq"
/he/
/ w/
/d/
Reached end of file
-
-.fi
+.EE
.in
+.PP
Note that a more general version of the program below
could be improved to more robustly handle various error situations
(e.g., opening a stream with a cookie that already has an open stream;
closing a stream that has already been closed).
.SS Program source
\&
-.nf
+.EX
#define _GNU_SOURCE
#include <sys/types.h>
#include <stdio.h>
.seek = memfile_seek,
.close = memfile_close
};
- FILE *fp;
+ FILE *stream;
struct memfile_cookie mycookie;
ssize_t nread;
long p;
mycookie.offset = 0;
mycookie.endpos = 0;
- fp = fopencookie(&mycookie,"w+", memfile_func);
- if (fp == NULL) {
+ stream = fopencookie(&mycookie,"w+", memfile_func);
+ if (stream == NULL) {
perror("fopencookie");
exit(EXIT_FAILURE);
}
/* Write command\-line arguments to our file */
for (j = 1; j < argc; j++)
- if (fputs(argv[j], fp) == EOF) {
+ if (fputs(argv[j], stream) == EOF) {
perror("fputs");
exit(EXIT_FAILURE);
}
/* Read two bytes out of every five, until EOF */
for (p = 0; ; p += 5) {
- if (fseek(fp, p, SEEK_SET) == \-1) {
+ if (fseek(stream, p, SEEK_SET) == \-1) {
perror("fseek");
exit(EXIT_FAILURE);
}
- nread = fread(buf, 1, 2, fp);
+ nread = fread(buf, 1, 2, stream);
if (nread == \-1) {
perror("fread");
exit(EXIT_FAILURE);
}
if (nread == 0) {
- printf("Reached end of file\\n");
+ printf("Reached end of file\en");
break;
}
- printf("/%.*s/\\n", nread, buf);
+ printf("/%.*s/\en", nread, buf);
}
exit(EXIT_SUCCESS);
}
-.fi
+.EE
.SH SEE ALSO
.BR fclose (3),
.BR fmemopen (3),
projects / thirdparty / man-pages.git / blobdiff