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setbuf

Header File

stdio.h

Category

Input/output Routines

Prototype

void setbuf(FILE *stream, char *buf);

Description

Assigns buffering to a stream.

setbuf causes the buffer buf to be used for I/O buffering instead of an automatically allocated buffer. It is used after stream has been opened.

If buf is null, I/O will be unbuffered; otherwise, it will be fully buffered. The buffer must be BUFSIZ bytes long (specified in stdio.h).

stdin and stdout are unbuffered if they are not redirected; otherwise, they are fully buffered. setbuf can be used to change the buffering style used.

Unbuffered means that characters written to a stream are immediately output to the file or device, while buffered means that the characters are accumulated and written as a block.

setbuf produces unpredictable results unless it is called immediately after opening stream or after a call to fseek. Calling setbuf after stream has been unbuffered is legal and will not cause problems.

A common cause for error is to allocate the buffer as an automatic (local) variable and then fail to close the file before returning from the function where the buffer was declared.

Return Value

None.

Example

Copy Code

    #include <stdio.h>
    /* BUFSIZ is defined in stdio.h */
    char outbuf[BUFSIZ];
    int main(void)
    {
       /* attach a buffer to the standard output stream */
       setbuf(stdout, outbuf);
       /* put some characters into the buffer */
       puts("This is a test of buffered output.\n\n");
       puts("This output will go into outbuf\n");
       puts("and won't appear until the buffer\n");
       puts("fills up or we flush the stream.\n");
       /* flush the output buffer */
       fflush(stdout);
       return 0;
    }

Portability

POSIX | Win32 | ANSI C | ANSI C++
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