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Defined in header <stdio.h> | ||
| (until C99) | ||
| (since C99) | ||
#define _IOFBF /*unspecified*/ #define _IOLBF /*unspecified*/ | ||
Changes the buffering mode of the given file streamstream as indicated by the argumentmode. In addition,
buffer is a null pointer, resizes the internal buffer tosize.buffer is not a null pointer, instructs the stream to use the user-provided buffer of sizesize beginning atbuffer. The stream must be closed (withfclose) before thelifetime of the array pointed to bybuffer ends. The contents of the array after a successful call tosetvbuf are indeterminate and any attempt to use it is undefined behavior.Contents |
| 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:
| ||||||
| size | - | size of the buffer |
0 on success or nonzero on failure.
This function may only be used afterstream has been associated with an open file, but before any other operation (other than a failed call tosetbuf/setvbuf).
Not allsize 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:
The default buffer sizeBUFSIZ is expected to be the most efficient buffer size for file I/O on the implementation, but POSIXfstat often provides a better estimate.
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");returnEXIT_FAILURE;} struct stat stats;if(fstat(fileno(fp),&stats)==-1)// POSIX only{perror("fstat");returnEXIT_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 errnoreturnEXIT_FAILURE;} int ch;while((ch=fgetc(fp))!=EOF);// read entire file: use truss/strace to// observe the read(2) syscalls used fclose(fp);returnEXIT_SUCCESS;}
Possible output:
BUFSIZ is 8192, but optimal block size is 65536
| sets the buffer for a file stream (function)[edit] | |
C++ documentation forsetvbuf | |
