FMEMOPEN(3) glibc function FMEMOPEN(3)NAME
fmemopen, open_memstream - open memory as stream
SYNOPSIS
#define _GNU_SOURCE
#include <stdio.h>
FILE *fmemopen(void *buf, size_t size, const char *mode);
FILE *open_memstream(char ** ptr, size_t *sizeloc)
DESCRIPTION
The fmemopen() function opens a stream that permits the access speci‐
fied by mode. The stream allows I/O to be performed on the string or
memory buffer pointed to by buf. This buffer must be at least size
bytes long.
The argument mode is the same as for fopen(3). If mode specifies an
append mode, then the initial file position is set to location of the
first null byte ('\0') in the buffer; otherwise the initial file posi‐
tion is set to the start of the buffer.
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.
Attempts to write more than size bytes to the buffer result in an
error. (By default, such errors will only be visible when the stdio
buffer is flushed. Disabling buffering with setbuf(fp, NULL) may be
useful to detect errors at the time of an output operation. Alterna‐
tively, the caller can explicitly set buf as the stdio stream buffer,
at the same time informing stdio of the buffer's size, using set‐
buffer(fp, buf, size).)
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 only indicate end-of-file when the file pointer
advances size bytes past the start of the buffer.
If buf is specified as NULL, then fmemopen() dynamically allocates a
buffer size bytes long. This is useful for an application that wants
to write data to a temporary buffer and then read it back again. 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() below).
The open_memstream() opens a stream for writing to a buffer. The buf‐
fer is dynamically allocated (as with malloc(3)), and automatically
grows as required. After closing the stream, the caller should free(3)
this buffer.
When the stream is closed (fclose(3)) or flushed (fflush(3)), the loca‐
tions pointed to by ptr and sizeloc are updated to contain, respec‐
tively, a pointer to the buffer and the current size of the buffer.
These values remain valid only as long as the caller performs no fur‐
ther output on the stream. If further output is performed, then the
stream must again be flushed before trying to access these variables.
A null byte is maintained at the end of the buffer. This byte is not
included in the size value stored at sizeloc.
RETURN VALUE
Upon successful completion fmemopen() and open_memstream() return a
FILE pointer. Otherwise, NULL is returned and the global variable
errno is set to indicate the error.
EXAMPLE
The program below uses fmemopen() to open an input buffer, and
open_memstream() to open a dynamically sized output buffer. The pro‐
gram scans its input string (taken from the program's first command-
line argument) reading integers, and writes the squares of these inte‐
gers 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
#define _GNU_SOURCE
#include <assert.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[])
{
FILE *out, *in;
int v, s;
size_t size;
char *ptr;
assert(argc == 2);
in = fmemopen(argv[1], strlen(argv[1]), "r");
if (in == NULL) { perror("fmemopen"); exit(EXIT_FAILURE);}
out = open_memstream(&ptr, &size);
if (out == NULL) { perror("fmemopen"); exit(EXIT_FAILURE);}
for (;;) {
s = fscanf(in, "%d", &v);
if (s <= 0)
break;
s = fprintf(out, "%d ", v * v);
if (s == -1) { perror("fprintf"); exit(EXIT_FAILURE); }
}
fclose(in);
fclose(out);
printf("size=%ld; ptr=%s\n", (long) size, ptr);
free(ptr);
exit(EXIT_SUCCESS);
}
CONFORMING TO
These functions are GNU extensions.
SEE ALSOopen(3)GNU 2005-12-08 FMEMOPEN(3)
