1FMEMOPEN(3) glibc function FMEMOPEN(3)
2
6 fmemopen, open_memstream - open memory as stream
7
9 #define _GNU_SOURCE
10 #include <stdio.h>
11 FILE *fmemopen(void *buf, size_t size, const char *mode);
13 FILE *open_memstream(char ** ptr, size_t *sizeloc)
15
17 The fmemopen() function opens a stream that permits the access speci‐
18 fied by mode. The stream allows I/O to be performed on the string or
19 memory buffer pointed to by buf. This buffer must be at least size
20 bytes long.
21 The argument mode is the same as for fopen(3). If mode specifies an
23 append mode, then the initial file position is set to location of the
24 first null byte ('\0') in the buffer; otherwise the initial file posi‐
25 tion is set to the start of the buffer.
26 When a stream that has been opened for writing is flushed (fflush(3))
28 or closed (fclose(3)), a null byte is written at the end of the buffer
29 if there is space. The caller should ensure that an extra byte is
30 available in the buffer (and that size counts that byte) to allow for
31 this.
32 Attempts to write more than size bytes to the buffer result in an
34 error. (By default, such errors will only be visible when the stdio
35 buffer is flushed. Disabling buffering with setbuf(fp, NULL) may be
36 useful to detect errors at the time of an output operation. Alterna‐
37 tively, the caller can explicitly set buf as the stdio stream buffer,
38 at the same time informing stdio of the buffer's size, using set‐
39 buffer(fp, buf, size).)
40 In a stream opened for reading, null bytes ('\0') in the buffer do not
42 cause read operations to return an end-of-file indication. A read from
43 the buffer will only indicate end-of-file when the file pointer
44 advances size bytes past the start of the buffer.
45 If buf is specified as NULL, then fmemopen() dynamically allocates a
47 buffer size bytes long. This is useful for an application that wants
48 to write data to a temporary buffer and then read it back again. The
49 buffer is automatically freed when the stream is closed. Note that the
50 caller has no way to obtain a pointer to the temporary buffer allocated
51 by this call (but see open_memstream() below).
52 The open_memstream() opens a stream for writing to a buffer. The buf‐
54 fer is dynamically allocated (as with malloc(3)), and automatically
55 grows as required. After closing the stream, the caller should free(3)
56 this buffer.
57 When the stream is closed (fclose(3)) or flushed (fflush(3)), the loca‐
59 tions pointed to by ptr and sizeloc are updated to contain, respec‐
60 tively, a pointer to the buffer and the current size of the buffer.
61 These values remain valid only as long as the caller performs no fur‐
62 ther output on the stream. If further output is performed, then the
63 stream must again be flushed before trying to access these variables.
64 A null byte is maintained at the end of the buffer. This byte is not
66 included in the size value stored at sizeloc.
67
69 Upon successful completion fmemopen() and open_memstream() return a
70 FILE pointer. Otherwise, NULL is returned and the global variable
71 errno is set to indicate the error.
72
75 The program below uses fmemopen() to open an input buffer, and
76 open_memstream() to open a dynamically sized output buffer. The pro‐
77 gram scans its input string (taken from the program's first command-
78 line argument) reading integers, and writes the squares of these inte‐
79 gers to the output buffer. An example of the output produced by this
80 program is the following:
81 $ ./a.out "1 23 43"
83 size=11; ptr=1 529 1849
84 #define _GNU_SOURCE
86 #include <assert.h>
87 #include <string.h>
88 #include <stdio.h>
89 #include <stdlib.h>
90 int main(int argc, char *argv[])
92 {
93 FILE *out, *in;
94 int v, s;
95 size_t size;
96 char *ptr;
97 assert(argc == 2);
99 in = fmemopen(argv[1], strlen(argv[1]), "r");
101 if (in == NULL) { perror("fmemopen"); exit(EXIT_FAILURE);}
102 out = open_memstream(&ptr, &size);
104 if (out == NULL) { perror("fmemopen"); exit(EXIT_FAILURE);}
105 for (;;) {
107 s = fscanf(in, "%d", &v);
108 if (s <= 0)
109 break;
110 s = fprintf(out, "%d ", v * v);
112 if (s == -1) { perror("fprintf"); exit(EXIT_FAILURE); }
113 }
114 fclose(in);
115 fclose(out);
116 printf("size=%ld; ptr=%s\n", (long) size, ptr);
117 free(ptr);
118 exit(EXIT_SUCCESS);
119 }
120
123 These functions are GNU extensions.
124
126 open(3), feature_test_macros(7)
127GNU 2005-12-08 FMEMOPEN(3)