BIO_S_FILE(3) | Library Functions Manual | BIO_S_FILE(3)

# NAME

`BIO_s_file`, `BIO_new_file`, `BIO_new_fp`, `BIO_set_fp`, `BIO_get_fp`, `BIO_read_filename`, `BIO_write_filename`, `BIO_append_filename`, `BIO_rw_filename` — FILE BIO

# SYNOPSIS

library “libcrypto”
`#include <openssl/bio.h>`

const BIO_METHOD *
`BIO_s_file`(void);

BIO *
`BIO_new_file`(const char *filename, const char *mode);

BIO *
`BIO_new_fp`(FILE *stream, int flags);

long
`BIO_set_fp`(BIO *b, FILE *fp, int flags);

long
`BIO_get_fp`(BIO *b, FILE **fpp);

long
`BIO_read_filename`(BIO *b, char *name);

long
`BIO_write_filename`(BIO *b, char *name);

long
`BIO_append_filename`(BIO *b, char *name);

long
`BIO_rw_filename`(BIO *b, char *name);

# DESCRIPTION

`BIO_s_file`() returns the BIO file method. As its name implies, it is a wrapper around the stdio FILE structure and it is a source/sink BIO.

Calls to BIO_read(3) and BIO_write(3) read and write data to the underlying stream. BIO_gets(3) and BIO_puts(3) are supported on file BIOs.

BIO_flush(3) on a file BIO calls the fflush(3) function on the wrapped stream.

BIO_reset(3) attempts to change the file pointer to the start of file using `fseek`(stream, 0, 0).

BIO_seek(3) sets the file pointer to position ofs from the start of the file using `fseek`(stream, ofs, 0).

BIO_eof(3) calls feof(3).

Setting the `BIO_CLOSE` flag calls fclose(3) on the stream when the BIO is freed.

`BIO_new_file`() creates a new file BIO with mode mode. The meaning of mode is the same as for the stdio function fopen(3). The `BIO_CLOSE` flag is set on the returned BIO.

`BIO_new_fp`() creates a file BIO wrapping stream. Flags can be: `BIO_CLOSE`, `BIO_NOCLOSE` (the close flag), `BIO_FP_TEXT` (sets the underlying stream to text mode, default is binary: this only has any effect under Win32).

`BIO_set_fp`() sets the file pointer of a file BIO to fp. flags has the same meaning as in `BIO_new_fp`().

`BIO_get_fp`() retrieves the file pointer of a file BIO.

BIO_seek(3) sets the position pointer to offset bytes from the start of file.

BIO_tell(3) returns the value of the position pointer.

`BIO_read_filename`(), `BIO_write_filename`(), `BIO_append_filename`(), and `BIO_rw_filename`() set the file BIO b to use file name for reading, writing, append or read write respectively.

When wrapping stdout, stdin, or stderr, the underlying stream should not normally be closed, so the `BIO_NOCLOSE` flag should be set.

Because the file BIO calls the underlying stdio functions, any quirks in stdio behaviour will be mirrored by the corresponding BIO.

On Windows, `BIO_new_files`() reserves for the filename argument to be UTF-8 encoded. In other words, if you have to make it work in a multi-lingual environment, encode file names in UTF-8.

The following BIO_ctrl(3) cmd constants correspond to macros:

cmd constant | corresponding macro
`BIO_C_FILE_SEEK` | BIO_seek(3)
`BIO_C_FILE_TELL` | BIO_tell(3)
`BIO_C_GET_FILE_PTR` | `BIO_get_fp`()
`BIO_C_SET_FILE_PTR` | `BIO_set_fp`()
`BIO_C_SET_FILENAME` | various, see below
`BIO_CTRL_EOF` | BIO_eof(3)
`BIO_CTRL_FLUSH` | BIO_flush(3)
`BIO_CTRL_GET_CLOSE` | BIO_get_close(3)
`BIO_CTRL_RESET` | BIO_reset(3)
`BIO_CTRL_SET_CLOSE` | BIO_set_close(3)

The meaning of `BIO_C_SET_FILENAME` depends on the flags passed in the BIO_ctrl(3) larg argument:

larg argument | corresponding macro
`BIO_CLOSE` | `BIO_FP_READ` | `BIO_read_filename`()
`BIO_CLOSE` | `BIO_FP_WRITE` | `BIO_write_filename`()
`BIO_CLOSE` | `BIO_FP_APPEND` | `BIO_append_filename`()
`BIO_CLOSE` | `BIO_FP_READ` | `BIO_FP_WRITE` | `BIO_rw_filename`()

# RETURN VALUES

`BIO_s_file`() returns the file BIO method.

`BIO_new_file`() and `BIO_new_fp`() return a file BIO or `NULL` if an error occurred.

When called on a file BIO object, BIO_method_type(3) returns the constant `BIO_TYPE_FILE` and BIO_method_name(3) returns a pointer to the static string "FILE pointer".

`BIO_set_fp`() and `BIO_get_fp`() return 1 for success or 0 for failure (although the current implementation never returns 0).

BIO_seek(3) returns the same value as the underlying fseek(3) function: 0 for success or -1 for failure.

BIO_tell(3) returns the current file position.

`BIO_read_filename`(), `BIO_write_filename`(), `BIO_append_filename`(), and `BIO_rw_filename`() return 1 for success or 0 for failure.

# EXAMPLES

File BIO "hello world":

    BIO *bio_out;
    bio_out = BIO_new_fp(stdout, BIO_NOCLOSE);
    BIO_printf(bio_out, "Hello World\n");

Alternative technique:

    BIO *bio_out;
    bio_out = BIO_new(BIO_s_file());
    if(bio_out == NULL) /* Error ... */
    if(!BIO_set_fp(bio_out, stdout, BIO_NOCLOSE)) /* Error ... */
    BIO_printf(bio_out, "Hello World\n");

Write to a file:

    BIO *out;
    out = BIO_new_file("filename.txt", "w");
    if(!out) /* Error occurred */
    BIO_printf(out, "Hello World\n");
    BIO_free(out);

Alternative technique:

    BIO *out;
    out = BIO_new(BIO_s_file());
    if(out == NULL) /* Error ... */
    if(!BIO_write_filename(out, "filename.txt")) /* Error ... */
    BIO_printf(out, "Hello World\n");
    BIO_free(out);

# SEE ALSO

BIO_new(3), BIO_read(3), BIO_seek(3)

# HISTORY

`BIO_s_file`(), `BIO_set_fp`(), `BIO_get_fp`(), `BIO_read_filename`(), `BIO_write_filename`(), and `BIO_append_filename`() first appeared in SSLeay 0.6.0. `BIO_new_file`() and `BIO_new_fp`() first appeared in SSLeay 0.8.0. All these functions have been available since OpenBSD 2.4.

`BIO_rw_filename`() first appeared in SSLeay 0.9.1 and has been available since OpenBSD 2.6.

# BUGS

BIO_reset(3) and BIO_seek(3) are implemented using fseek(3) on the underlying stream. The return value for fseek(3) is 0 for success or -1 if an error occurred. This differs from other types of BIO which will typically return 1 for success and a non-positive value if an error occurred.

June 8, 2025 | Debian