#include <stdio.h> FILE *fmemopen(void *buf, size_t size, const char *mode); FILE *open_memstream(char **ptr, size_t *sizeloc); #include <wchar.h> FILE *open_wmemstream(wchar_t **ptr, size_t *sizeloc);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
fmemopen(), open_memstream(), open_wmemstream():
The argument mode is the same as for fopen(3). If mode specifies an append mode, then the initial file position is set to the location of the first null byte ('\0') in the buffer; otherwise the initial file position is set to the start of the buffer. Since glibc 2.9, the letter 'b' may be specified as the second character in mode. This provides "binary" 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.
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. 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(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() function opens a stream for writing to a buffer. The buffer 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 locations pointed to by ptr and sizeloc are updated to contain, respectively, a pointer to the buffer and the current size of the buffer. These values remain valid only as long as the caller performs no further 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.
The stream's file position can be changed with fseek(3) or fseeko(3). Moving the file position past the end of the data already written fills the intervening space with zeros.
The open_wmemstream() is similar to open_memstream(), but operates on wide characters instead of bytes.
$ ./a.out '1 23 43' size=11; ptr=1 529 1849
#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 <file>\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=%ld; ptr=%s\n", (long) size, ptr);
free(ptr);
exit(EXIT_SUCCESS);
}