From 754bbf7a25a8dda49b5d08ef0d0443bbf5af0e36 Mon Sep 17 00:00:00 2001 From: Craig Jennings Date: Sun, 7 Apr 2024 13:41:34 -0500 Subject: new repository --- devdocs/c/io%2Fsetvbuf.html | 85 +++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 85 insertions(+) create mode 100644 devdocs/c/io%2Fsetvbuf.html (limited to 'devdocs/c/io%2Fsetvbuf.html') diff --git a/devdocs/c/io%2Fsetvbuf.html b/devdocs/c/io%2Fsetvbuf.html new file mode 100644 index 00000000..25309406 --- /dev/null +++ b/devdocs/c/io%2Fsetvbuf.html @@ -0,0 +1,85 @@ +

setvbuf

Defined in header <stdio.h> 
int setvbuf( FILE *         stream, char *         buffer, 
+             int mode, size_t size );
+		 (until C99) 
int setvbuf( FILE *restrict stream, char *restrict buffer, 
+             int mode, size_t size );
+		 (since C99) 
#define _IOFBF     /*unspecified*/
+#define _IOLBF     /*unspecified*/
+#define _IONBF     /*unspecified*/
+

Changes the buffering mode of the given file stream stream as indicated by the argument mode. In addition,

+

Parameters

+ + + + +
stream - the file stream to set the buffer to
buffer - pointer to a buffer for the stream to use or null pointer to change size and mode only
mode - buffering mode to use. It can be one of the following values: + + + +
_IOFBF full buffering
_IOLBF line buffering
_IONBF no buffering
size - size of the buffer

Return value

​0​ on success or nonzero on failure.

+

Notes

This function may only be used after stream has been associated with an open file, but before any other operation (other than a failed call to setbuf/setvbuf).

+

Not all size bytes will necessarily be used for buffering: the actual buffer size is usually rounded down to a multiple of 2, a multiple of page size, etc.

+

On many implementations, line buffering is only available for terminal input streams.

+

A common error is setting the buffer of stdin or stdout to an array whose lifetime ends before the program terminates:

+
int main(void) {
+    char buf[BUFSIZ];
+    setbuf(stdin, buf);
+} // lifetime of buf ends, undefined behavior

The default buffer size BUFSIZ is expected to be the most efficient buffer size for file I/O on the implementation, but POSIX fstat often provides a better estimate.

+

Example

+

One use case for changing buffer size is when a better size is known. (This example uses some POSIX function, e.g. fileno. See also SO: #1 and #2).

+
// Make some POSIX functions, such as `int fileno(FILE*)`, visible:
+#define _POSIX_SOURCE
+ 
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/stat.h>
+ 
+int main(void)
+{
+    FILE* fp = fopen("/tmp/test.txt", "w+");
+    if (fp == NULL)
+    {
+        perror("fopen");
+        return EXIT_FAILURE;
+    }
+ 
+    struct stat stats;
+    if (fstat(fileno(fp), &stats) == -1) // POSIX only
+    {
+        perror("fstat");
+        return EXIT_FAILURE;
+    }
+ 
+    printf("BUFSIZ is %d, but optimal block size is %ld\n", BUFSIZ, stats.st_blksize);
+    if (setvbuf(fp, NULL, _IOFBF, stats.st_blksize) != 0)
+    {
+        perror("setvbuf failed"); // POSIX version sets errno
+        return EXIT_FAILURE;
+    }
+ 
+    int ch;
+    while((ch=fgetc(fp)) != EOF); // read entire file: use truss/strace to
+                                  // observe the read(2) syscalls used
+ 
+    fclose(fp);
+    return EXIT_SUCCESS;
+}

Possible output:

+
BUFSIZ is 8192, but optimal block size is 65536

References

See also

+ +
sets the buffer for a file stream
(function)
C++ documentation for setvbuf
+

+ © cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
+ https://en.cppreference.com/w/c/io/setvbuf +

+
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