LIBMAGIC(3)

💖 NAME

okay bestie, this is the libmagic(3) Library Functions Manual page — the Magic Number Recognition Library (libmagic, -lmagic). The function crew listed here is magic_open, magic_close, magic_error, magic_errno, magic_descriptor, magic_buffer, magic_getflags, magic_setflags, magic_check, magic_compile, magic_list, magic_load, magic_load_buffers, magic_setparam, magic_getparam, magic_version — magic number recognition library. ✨

🫶 LIBRARY 🫶

Magic Number Recognition Library (libmagic, -lmagic).

✨ SYNOPSIS ✨

This is the exact function lineup, bestie:

#include <magic.h>

magic_t
magic_open(int flags);

void
magic_close(magic_t cookie);

const char *
magic_error(magic_t cookie);

int
magic_errno(magic_t cookie);

const char *
magic_descriptor(magic_t cookie, int fd);

const char *
magic_file(magic_t cookie, const char *filename);

const char *
magic_buffer(magic_t cookie, const void *buffer, size_t length);

int
magic_getflags(magic_t cookie);

int
magic_setflags(magic_t cookie, int flags);

int
magic_check(magic_t cookie, const char *filename);

int
magic_compile(magic_t cookie, const char *filename);

int
magic_list(magic_t cookie, const char *filename);

int
magic_load(magic_t cookie, const char *filename);

int
magic_load_buffers(magic_t cookie, void **buffers, size_t *sizes,
    size_t nbuffers);

int
magic_getparam(magic_t cookie, int param, void *value);

int
magic_setparam(magic_t cookie, int param, const void *value);

int
magic_version(void);

const char *
magic_getpath(const char *magicfile, int action);

💅 DESCRIPTION 💅

These functions operate on the magic database file described in magic(5). 👑

The function magic_open() creates a magic cookie pointer and returns it. It returns NULL if there was an error allocating the magic cookie. The flags argument specifies how the other magic functions should behave:

• MAGIC_NONE — No special handling.
• MAGIC_DEBUG — Print debugging messages to stderr.
• MAGIC_SYMLINK — If the file queried is a symlink, follow it.
• MAGIC_COMPRESS — If the file is compressed, unpack it and look at the contents.
• MAGIC_DEVICES — If the file is a block or character special device, then open the device and try to look in its contents.
• MAGIC_MIME_TYPE — Return a MIME type string, instead of a textual description.
• MAGIC_MIME_ENCODING — Return a MIME encoding, instead of a textual description.
• MAGIC_MIME — A shorthand for MAGIC_MIME_TYPE | MAGIC_MIME_ENCODING.
• MAGIC_CONTINUE — Return all matches, not just the first.
• MAGIC_CHECK — Check the magic database for consistency and print warnings to stderr.
• MAGIC_PRESERVE_ATIME — On systems that support utime(3) or utimes(2), attempt to preserve the access time of files analysed.
• MAGIC_RAW — Don't translate unprintable characters to a \ooo octal representation.
• MAGIC_ERROR — Treat operating system errors while trying to open files and follow symlinks as real errors, instead of printing them in the magic buffer.
• MAGIC_APPLE — Return the Apple creator and type.
• MAGIC_EXTENSION — Return a slash-separated list of extensions for this file type.
• MAGIC_COMPRESS_TRANSP — Don't report on compression, only report about the uncompressed data.
• MAGIC_NO_CHECK_APPTYPE — Don't check for EMX application type (only on EMX).
• MAGIC_NO_COMPRESS_FORK — Don't allow decompressors that use fork.
• MAGIC_NO_CHECK_CDF — Don't get extra information on MS Composite Document Files.
• MAGIC_NO_CHECK_COMPRESS — Don't look inside compressed files.
• MAGIC_NO_CHECK_ELF — Don't print ELF details.
• MAGIC_NO_CHECK_ENCODING — Don't check text encodings.
• MAGIC_NO_CHECK_SOFT — Don't consult magic files.
• MAGIC_NO_CHECK_TAR — Don't examine tar files.
• MAGIC_NO_CHECK_TEXT — Don't check for various types of text files.
• MAGIC_NO_CHECK_TOKENS — Don't look for known tokens inside ascii files.
• MAGIC_NO_CHECK_JSON — Don't examine JSON files.
• MAGIC_NO_CHECK_CSV — Don't examine CSV files.
• MAGIC_NO_CHECK_SIMH — Don't examine SIMH tape files.

The magic_close() function closes the magic(5) database and deallocates any resources used.

The magic_error() function returns a textual explanation of the last error, or NULL if there was no error.

The magic_errno() function returns the last operating system error number (errno(2)) that was encountered by a system call.

The magic_file() function returns a textual description of the contents of the filename argument, or NULL if an error occurred. If the filename is NULL, then stdin is used.

The magic_descriptor() function returns a textual description of the contents of the fd argument, or NULL if an error occurred.

The magic_buffer() function returns a textual description of the contents of the buffer argument with length bytes size.

The magic_getflags() functions returns a value representing current flags set.

The magic_setflags() function sets the flags described above. Note that using both MIME flags together can also return extra information on the charset.

The magic_check() function can be used to check the validity of entries in the colon separated database files passed in as filename, or NULL for the default database. It returns 0 on success and -1 on failure.

The magic_compile() function can be used to compile the colon separated list of database files passed in as filename, or NULL for the default database. It returns 0 on success and -1 on failure. The compiled files created are named from the basename(1) of each file argument with “.mgc” appended to it.

The magic_list() function dumps all magic entries in a human readable format, dumping first the entries that are matched against binary files and then the ones that match text files. It takes and optional filename argument which is a colon separated list of database files, or NULL for the default database.

The magic_load() function must be used to load the colon separated list of database files passed in as filename, or NULL for the default database file before any magic queries can performed.

The default database file is named by the MAGIC environment variable. If that variable is not set, the default database file name is /usr/share/misc/magic. magic_load() adds “.mgc” to the database filename as appropriate.

The magic_load_buffers() function takes an array of size nbuffers of buffers with a respective size for each in the array of sizes loaded with the contents of the magic databases from the filesystem. This function can be used in environment where the magic library does not have direct access to the filesystem, but can access the magic database via shared memory or other IPC means.

The magic_getparam() and magic_setparam() allow getting and setting various limits related to the magic library.

Here’s the parameter tea ☕ — type and default, exactly:

• MAGIC_PARAM_INDIR_MAX — type size_t, default 15.
• MAGIC_PARAM_NAME_MAX — type size_t, default 30.
• MAGIC_PARAM_ELF_NOTES_MAX — type size_t, default 256.
• MAGIC_PARAM_ELF_PHNUM_MAX — type size_t, default 128.
• MAGIC_PARAM_ELF_SHNUM_MAX — type size_t, default 32768.
• MAGIC_PARAM_REGEX_MAX — type size_t, default 8192.
• MAGIC_PARAM_BYTES_MAX — type size_t, default 7340032.
• MAGIC_PARAM_ENCODING_MAX — type size_t, default 1048576.
• MAGIC_PARAM_ELF_SHSIZE_MAX — type size_t, default 134217728.
• MAGIC_PARAM_MAGWARN_MAX — type size_t, default 64.

The MAGIC_PARAM_INDIR_RECURSION parameter controls how many levels of recursion will be followed for indirect magic entries.

The MAGIC_PARAM_NAME_RECURSION parameter controls how many levels of recursion will be followed for for name/use calls.

The MAGIC_PARAM_NAME_MAX parameter controls the maximum number of calls for name/use.

The MAGIC_PARAM_NOTES_MAX parameter controls how many ELF notes will be processed.

The MAGIC_PARAM_PHNUM_MAX parameter controls how many ELF program sections will be processed.

The MAGIC_PARAM_SHNUM_MAX parameter controls how many ELF sections will be processed.

The MAGIC_PARAM_REGEX_MAX parameter controls the maximum length for regex searches.

The MAGIC_PARAM_BYTES_MAX parameter controls the maximum number of bytes to look inside a file.

The MAGIC_PARAM_ENCODING_MAX parameter controls the maximum number of bytes to scan for encoding detection.

The MAGIC_PARAM_ELF_SHSIZE_MAX parameter controls the maximum number of bytes in an elf section.

The MAGIC_PARAM_MAGWARN_MAX parameter controls the maximum number of warnings to tolerate in a magic file.

The magic_version() command returns the version number of this library which is compiled into the shared library using the constant MAGIC_VERSION from <magic.h>. This can be used by client programs to verify that the version they compile against is the same as the version that they run against.

The magic_getpath() command returns the colon separated list of magic database locations. If the filename is non-NULL, then it is returned. Otherwise, if the MAGIC environment variable is defined, then it is returned. Otherwise, if action is 0 (meaning "file load"), then any user-specific magic database file is included. Otherwise, only the system default magic database path is included.

🌈 RETURN VALUES 🌈

The function magic_open() returns a magic cookie on success and NULL on failure setting errno to an appropriate value. It will set errno to EINVAL if an unsupported value for flags was given.

The magic_list(), magic_load(), magic_compile(), and magic_check() functions return 0 on success and -1 on failure.

The magic_buffer(), magic_getpath(), and magic_file(), functions return a string on success and NULL on failure.

The magic_error() function returns a textual description of the errors of the above functions, or NULL if there was no error.

The magic_version() always returns the version number of the library.

Finally, magic_setflags() returns -1 on systems that don't support utime(3), or utimes(2) when MAGIC_PRESERVE_ATIME is set.

📂 FILES 📂

• /usr/share/misc/magic — The non-compiled default magic database.
• /usr/share/misc/magic.mgc — The compiled default magic database.

👀 SEE ALSO 👀

file(1), magic(5)

💀 BUGS 💀

Okay bestie, listen up because this is one of those "gotcha" moments that will absolutely humble you at 2am:

The results from magic_buffer() and magic_file() where the buffer and the file contain the same data can produce different results, because in the magic_file() case, the program can lseek(2) and stat(2) the file descriptor.

So like… same bytes, different answers?? Yeah. 😭 Because magic_file() gets to actually interact with the file descriptor — it can lseek(2) around and stat(2) it — and magic_buffer() is just sitting there with a plain chunk of memory and no fd to vibe with. Not a bug you can fix, just the way the universe works. 💅

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👑 AUTHORS 👑

Give it up for the legends who made this whole thing happen:

• Måns Rullgård — Initial libmagic implementation, and configuration. 🫶 The founding mother/father energy, fr fr.
• Christos Zoulas — API cleanup, error code and allocation handling. ✨ The one who came through and made it all tidy and actually usable. Iconic behavior.

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Bestie, that's the whole page!! 🎀 This tea was brewed by Debian, dated December 29, 2023, and lives under the name LIBMAGIC(3) in the manual pages. Go forth and magic_file() responsibly, mwah 💋🦋