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FOPENCOOKIE(3) | Linux Programmer's Manual | FOPENCOOKIE(3) |
NAME¶
fopencookie - opening a custom streamSYNOPSIS¶
#define _GNU_SOURCE /* See feature_test_macros(7) */ #include <stdio.h> FILE *fopencookie(void *cookie, const char *mode, cookie_io_functions_t io_funcs);
DESCRIPTION¶
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.- *
- Implement four "hook" functions that are used internally by the standard I/O library when performing I/O on the stream.
- *
- Define a "cookie" data type, a structure that provides bookkeeping information (e.g., where to store data) used by the aforementioned hook functions. The standard I/O package knows nothing about the contents of this cookie (thus it is typed as void * when passed to fopencookie()), but automatically supplies the cookie as the first argument when calling the hook functions.
- *
- Call fopencookie() to open a new stream and associate the cookie and hook functions with that stream.
struct cookie_io_functions_t { cookie_read_function_t *read; cookie_write_function_t *write; cookie_seek_function_t *seek; cookie_close_function_t *close; };
The four fields are as follows:
- cookie_read_function_t *read
- This function implements read operations for the stream.
When called, it receives three arguments:
ssize_t read(void *cookie, char *buf, size_t size);
- cookie_write_function_t *write
- This function implements write operations for the stream.
When called, it receives three arguments:
ssize_t write(void *cookie, const char *buf, size_t size);
- cookie_seek_function_t *seek
- This function implements seek operations on the stream.
When called, it receives three arguments:
int seek(void *cookie, off64_t *offset, int whence);
- SEEK_SET
- The stream offset should be set *offset bytes from the start of the stream.
- SEEK_CUR
- *offset should be added to the current stream offset.
- SEEK_END
- The stream offset should be set to the size of the stream plus *offset.
- Before returning, the seek function should update
*offset to indicate the new stream offset.
- cookie_close_function_t *close
- This function closes the stream. The hook function can do
things such as freeing buffers allocated for the stream. When called, it
receives one argument:
int close(void *cookie);
RETURN VALUE¶
On success fopencookie() returns a pointer to the new stream. On error, NULL is returned.CONFORMING TO¶
This function is a nonstandard GNU extension.EXAMPLE¶
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).
Program source¶
#define _GNU_SOURCE #include <sys/types.h> #include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <string.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; } else { 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, off64_t *offset, int whence) { off64_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 *fp; struct memfile_cookie mycookie; ssize_t nread; long p; int j; 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; fp = fopencookie(&mycookie,"w+", memfile_func); if (fp == 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) { 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) { perror("fseek"); exit(EXIT_FAILURE); } nread = fread(buf, 1, 2, fp); if (nread == -1) { perror("fread"); exit(EXIT_FAILURE); } if (nread == 0) { printf("Reached end of file\n"); break; } printf("/%.*s/\n", nread, buf); } exit(EXIT_SUCCESS); }
SEE ALSO¶
fclose(3), fmemopen(3), fopen(3), fseek(3)COLOPHON¶
This page is part of release 3.44 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.2012-04-30 | Linux |