| fopencookie(3) | Library Functions Manual | fopencookie(3) |
fopencookie - opening a custom stream
Standard C library (libc, -lc)
#define _GNU_SOURCE /* See feature_test_macros(7) */ #define _FILE_OFFSET_BITS 64 #include <stdio.h>
FILE *fopencookie(void *restrict cookie, const char *restrict mode,
cookie_io_functions_t io_funcs);
The fopencookie() function allows the programmer to create a custom implementation for a standard I/O stream. This implementation can store the stream's data at a location of its own choosing; for example, fopencookie() is used to implement fmemopen(3), which provides a stream interface to data that is stored in a buffer in memory.
In order to create a custom stream the programmer must:
The fopencookie() function serves a purpose similar to fopen(3): it opens a new stream and returns a pointer to a FILE object that is used to operate on that stream.
The 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.
The mode argument serves the same purpose as for fopen(3). The following modes are supported: r, w, a, r+, w+, and a+. See fopen(3) for details.
The 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
typedef struct {
cookie_read_function_t *read;
cookie_write_function_t *write;
cookie_seek_function_t *seek;
cookie_close_function_t *close;
} cookie_io_functions_t;
The four fields are as follows:
ssize_t read(void *cookie, char *buf, size_t size);
ssize_t write(void *cookie, const char *buf, size_t size);
int seek(void *cookie, off_t *offset, int whence);
int close(void *cookie);
On success fopencookie() returns a pointer to the new stream. On error, NULL is returned.
For an explanation of the terms used in this section, see attributes(7).
| Interface | Attribute | Value |
| fopencookie () | Thread safety | MT-Safe |
GNU.
The program below implements a custom stream whose functionality is similar (but not identical) to that available via fmemopen(3). It implements a stream whose data is stored in a memory buffer. The program writes its command-line arguments to the stream, 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:
$ ./a.out 'hello world' /he/ / w/ /d/ Reached end of file
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).
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <unistd.h>
#define INIT_BUF_SIZE 4
struct memfile_cookie {
char *buf; /* Dynamically sized buffer for data */
size_t allocated; /* Size of buf */
size_t endpos; /* Number of characters in buf */
off_t offset; /* Current file offset in buf */
};
ssize_t
memfile_write(void *c, const char *buf, size_t size)
{
char *new_buff;
struct memfile_cookie *cookie = c;
/* Buffer too small? Keep doubling size until big enough. */
while (size + cookie->offset > cookie->allocated) {
new_buff = realloc(cookie->buf, cookie->allocated * 2);
if (new_buff == NULL)
return -1;
cookie->allocated *= 2;
cookie->buf = new_buff;
}
memcpy(cookie->buf + cookie->offset, buf, size);
cookie->offset += size;
if (cookie->offset > cookie->endpos)
cookie->endpos = cookie->offset;
return size;
}
ssize_t
memfile_read(void *c, char *buf, size_t size)
{
ssize_t xbytes;
struct memfile_cookie *cookie = c;
/* Fetch minimum of bytes requested and bytes available. */
xbytes = size;
if (cookie->offset + size > cookie->endpos)
xbytes = cookie->endpos - cookie->offset;
if (xbytes < 0) /* offset may be past endpos */
xbytes = 0;
memcpy(buf, cookie->buf + cookie->offset, xbytes);
cookie->offset += xbytes;
return xbytes;
}
int
memfile_seek(void *c, off_t *offset, int whence)
{
off_t new_offset;
struct memfile_cookie *cookie = c;
if (whence == SEEK_SET)
new_offset = *offset;
else if (whence == SEEK_END)
new_offset = cookie->endpos + *offset;
else if (whence == SEEK_CUR)
new_offset = cookie->offset + *offset;
else
return -1;
if (new_offset < 0)
return -1;
cookie->offset = new_offset;
*offset = new_offset;
return 0;
}
int
memfile_close(void *c)
{
struct memfile_cookie *cookie = c;
free(cookie->buf);
cookie->allocated = 0;
cookie->buf = NULL;
return 0;
}
int
main(int argc, char *argv[])
{
cookie_io_functions_t memfile_func = {
.read = memfile_read,
.write = memfile_write,
.seek = memfile_seek,
.close = memfile_close
};
FILE *stream;
struct memfile_cookie mycookie;
size_t nread;
char buf[1000];
/* Set up the cookie before calling fopencookie(). */
mycookie.buf = malloc(INIT_BUF_SIZE);
if (mycookie.buf == NULL) {
perror("malloc");
exit(EXIT_FAILURE);
}
mycookie.allocated = INIT_BUF_SIZE;
mycookie.offset = 0;
mycookie.endpos = 0;
stream = fopencookie(&mycookie, "w+", memfile_func);
if (stream == NULL) {
perror("fopencookie");
exit(EXIT_FAILURE);
}
/* Write command-line arguments to our file. */
for (size_t j = 1; j < argc; j++)
if (fputs(argv[j], stream) == EOF) {
perror("fputs");
exit(EXIT_FAILURE);
}
/* Read two bytes out of every five, until EOF. */
for (long p = 0; ; p += 5) {
if (fseek(stream, p, SEEK_SET) == -1) {
perror("fseek");
exit(EXIT_FAILURE);
}
nread = fread(buf, 1, 2, stream);
if (nread == 0) {
if (ferror(stream) != 0) {
fprintf(stderr, "fread failed\n");
exit(EXIT_FAILURE);
}
printf("Reached end of file\n");
break;
}
printf("/%.*s/\n", (int) nread, buf);
}
free(mycookie.buf);
exit(EXIT_SUCCESS);
}
_FILE_OFFSET_BITS should be defined to be 64 in code that uses non-null seek or that takes the address of fopencookie, if the code is intended to be portable to traditional 32-bit x86 and ARM platforms where off_t's width defaults to 32 bits.
fclose(3), fmemopen(3), fopen(3), fseek(3)
| 2023-07-20 | Linux man-pages 6.05.01 |