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Showing content from https://en.cppreference.com/w/cpp/algorithm/../ranges/../io/c/setvbuf.html below:

std::setvbuf - cppreference.com

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

• If buffer is a null pointer, resizes the internal buffer to size.
• If buffer is not a null pointer, instructs the stream to use the user-provided buffer of size size beginning at buffer. The stream must be closed (with std::fclose) before the lifetime of the array pointed to by buffer ends. The contents of the array after a successful call to std::setvbuf are indeterminate and any attempt to use it is undefined behavior.

​0​ on success or nonzero on failure.

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 std::setbuf/std::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()
{
    char buf[BUFSIZ];
    std::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.

One use case for changing buffer size is when a better size is known.

#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <sys/stat.h>
 
int main()
{
    std::FILE* fp = std::fopen("/tmp/test.txt", "w+");
    if (!fp)
    {
        std::perror("fopen");
        return EXIT_FAILURE;
    }
 
    struct stat stats;
    if (fstat(fileno(fp), &stats) == -1) // POSIX only
    {
        std::perror("fstat");
        return EXIT_FAILURE;
    }
 
    std::cout << "BUFSIZ is " << BUFSIZ << ", but optimal block size is "
              << stats.st_blksize << '\n';
    if (std::setvbuf(fp, nullptr, _IOFBF, stats.st_blksize) != 0)
    {
        std::perror("setvbuf failed"); // POSIX version sets errno
        return EXIT_FAILURE;
    }
 
    // Read entire file: use truss/strace to observe the read(2) syscalls used
    for (int ch; (ch = std::fgetc(fp)) != EOF;)
    {}
 
    std::fclose(fp);
    return EXIT_SUCCESS;
}

Possible output:

BUFSIZ is 8192, but optimal block size is 65536

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