FMEMOPEN(3) | Library Functions Manual | FMEMOPEN(3)

# NAME

`fmemopen` — open a stream that points to the given buffer

# SYNOPSIS

`#include <stdio.h>`

FILE *
`fmemopen`(void *buffer, size_t size, const char *mode);

# DESCRIPTION

The `fmemopen`() function associates a stream with the given buffer and size. The buffer can be either `NULL`, or must be of the given size. If the buffer is `NULL`, a buffer of the given size will be dynamically allocated using malloc(3) and released when fclose(3) is called.

The mode argument has the same meaning as in fopen(3).

The stream treats the buffer as it would treat a file tracking the current position to perform I/O operations. For example, in the beginning the stream points to the beginning of the buffer, unless ‘a’ was specified in the mode argument, and then it points to the first `NUL` byte. If a `NULL` buffer was specified, then the stream will always point at the first byte of the buffer.

The stream also keeps track of the size of the buffer. The size is initialized depending on the mode:

`r/r+`
  Set to the size argument.
`w/w+`
  Set to `0`.
`a/a+`
  Set to the first `NUL` byte, or the size argument if one is not found.

Read or write operations advance the buffer, but not to exceed the given size of the buffer. Trying to read beyond the size of the buffer results in `EOF` returned. `NUL` bytes are read normally. Trying to write beyond the size of the buffer has no effect.

When a stream open for writing is either flushed or closed, a `NUL` byte is written at the current position or at the end of the current size as kept internally.

# RETURN VALUES

Upon successful completion, `fmemopen`() returns a `FILE` pointer. Otherwise, `NULL` is returned and the global variable errno is set to indicate the error.

# ERRORS

[`EINVAL`]
  The size was `0`; or the mode argument is invalid; or the buffer argument is `NULL` and the mode argument does not specify a ‘+’.

The `fmemopen`() function may also fail and set errno for any of the errors specified for the routine malloc(3).

# SEE ALSO

fclose(3), fflush(3), fopen(3), funopen(3), malloc(3), open_memstream(3)

# STANDARDS

The function `fmemopen`() conform to IEEE Std 1003.1-2008 (“POSIX.1”).

# HISTORY

The `fmemopen`() function first appeared in OpenBSD 5.4.

June 1, 2025 | Debian