diff --git a/src/formatstring.c b/src/formatstring.c
new file mode 100644
index 0000000..6d4024d
--- /dev/null
+++ b/src/formatstring.c
@@ -0,0 +1,171 @@
+#include "formatstring.h"
+
+#include <stdlib.h>
+#include <limits.h>
+#include <assert.h>
+#include <errno.h>
+#include <string.h>
+
+#define MIN(X, Y) ((X) < (Y) ? (X) : (Y))
+
+void formatstring_error(const char *format, size_t index, const char *message) {
+	fprintf(stderr, "illegal format string:%zu: %s\n\n\t%s\n\t", index + 1, message, format);
+	while (index > 0) {
+		fprintf(stderr, "-");
+		-- index;
+	}
+	fprintf(stderr, "^\n");
+}
+
+#define IS_VAR_NAME(PTR, LEN, NAME) (strncmp((PTR), (NAME), MIN(sizeof (NAME), LEN)) == 0)
+
+#define WRITE_STR(STR, LEN) \
+	{ \
+	const size_t COPY_STR_rem = endptr > bufptr ? endptr - bufptr : 0; \
+	const size_t COPY_STR_cpylen = MIN((LEN), COPY_STR_rem); \
+	memcpy(bufptr, (STR), COPY_STR_cpylen); \
+	bufptr += (LEN); \
+	}
+
+#define HEX_CHARS "0123456789ABCDEF"
+
+#define WRITE_HEX(NUM) \
+	{ \
+	size_t WRITE_HEX_size = sizeof(NUM); \
+	size_t WRITE_HEX_rem  = (NUM); \
+	while (WRITE_HEX_size) { \
+		-- WRITE_HEX_size; \
+		char *WRITE_HEX_outptr = bufptr + 2 * WRITE_HEX_size; \
+		if (WRITE_HEX_outptr < endptr) { \
+			*WRITE_HEX_outptr = HEX_CHARS[(WRITE_HEX_rem >> 4) & 0xF]; \
+		} \
+		++ WRITE_HEX_outptr; \
+		if (WRITE_HEX_outptr < endptr) { \
+			*WRITE_HEX_outptr = HEX_CHARS[WRITE_HEX_rem & 0xF]; \
+		} \
+		WRITE_HEX_rem >>= 8; \
+	} \
+	bufptr += 2 * sizeof(NUM); \
+	}
+
+#define WRITE_CHAR(CH) \
+	if (bufptr < endptr) { \
+		*bufptr = (CH); \
+	} \
+	++ bufptr;
+
+#define WRITE_DEC(NUM) \
+	if ((NUM) == 0) { \
+		WRITE_CHAR('0'); \
+	} else { \
+		size_t WRITE_DEC_isize = 0; \
+		size_t WRITE_DEC_n = (NUM); \
+		while (WRITE_DEC_n) { \
+			WRITE_DEC_n /= 10; \
+			++ WRITE_DEC_isize; \
+		} \
+		WRITE_DEC_n = (NUM); \
+		size_t WRITE_DEC_index = WRITE_DEC_isize; \
+		while (WRITE_DEC_n) { \
+			-- WRITE_DEC_index; \
+			const size_t WRITE_DEC_v = WRITE_DEC_n % 10; \
+			char *WRITE_DEC_outptr = bufptr + WRITE_DEC_index; \
+			if (WRITE_DEC_outptr < endptr) { \
+				*WRITE_DEC_outptr = '0' + WRITE_DEC_v; \
+			} \
+			WRITE_DEC_n /= 10; \
+		} \
+		bufptr += WRITE_DEC_isize; \
+	}
+
+ssize_t formatstring(char *buffer, size_t buffer_size, const char *format, const char *filename,
+                     size_t index, size_t offset, size_t size, const char *ext) {
+	assert(buffer_size == 0 || buffer != NULL);
+	assert(format != NULL);
+	assert(filename != NULL);
+	assert(ext != NULL);
+
+	const char *fmtptr = format;
+	char       *bufptr = buffer_size == 0 ? NULL : buffer;
+	const char *endptr = bufptr + buffer_size;
+	const size_t filename_len = strlen(filename);
+	const size_t ext_len      = strlen(ext);
+
+	while (*fmtptr) {
+		const char *nextptr = fmtptr;
+		while (*nextptr && *nextptr != '{' && *nextptr != '}') {
+			++ nextptr;
+		}
+
+		if (nextptr != fmtptr) {
+			const size_t len = nextptr - fmtptr;
+			WRITE_STR(fmtptr, len);
+			fmtptr = nextptr;
+		}
+
+		if (*fmtptr == '}') {
+			++ fmtptr;
+			if (*fmtptr == '}') {
+				WRITE_CHAR('}');
+				++ fmtptr;
+			} else {
+				formatstring_error(format, fmtptr - format, "expected '}'");
+				errno = EINVAL;
+				return -1;
+			}
+		} else if (*fmtptr == '{') {
+			++ fmtptr;
+			if (*fmtptr == '{') {
+				WRITE_CHAR('{');
+				++ fmtptr;
+			} else {
+				nextptr = fmtptr;
+				while (*nextptr && *nextptr != '}') {
+					++ nextptr;
+				}
+
+				if (fmtptr == nextptr) {
+					formatstring_error(format, fmtptr - format, "expected a variable name");
+					errno = EINVAL;
+					return -1;
+				}
+
+				const size_t len = nextptr - fmtptr;
+				if (IS_VAR_NAME(fmtptr, len, "filename")) {
+					WRITE_STR(filename, filename_len);
+				} else if (IS_VAR_NAME(fmtptr, len, "ext")) {
+					WRITE_STR(ext, ext_len);
+				} else if (IS_VAR_NAME(fmtptr, len, "offset")) {
+					WRITE_HEX(offset);
+				} else if (IS_VAR_NAME(fmtptr, len, "index")) {
+					WRITE_DEC(index);
+				} else if (IS_VAR_NAME(fmtptr, len, "size")) {
+					WRITE_DEC(size);
+				} else {
+					formatstring_error(format, fmtptr - format, "unknown variable name");
+					errno = EINVAL;
+					return -1;
+				}
+				
+				fmtptr = nextptr + 1;
+			}
+		}
+	}
+
+	if (bufptr < endptr) {
+		*bufptr = 0;
+	} else if (buffer_size > 0) {
+		buffer[buffer_size - 1] = 0;
+	}
+	++ bufptr;
+	
+	const size_t strsize = bufptr - buffer;
+
+	if (strsize > SSIZE_MAX) {
+		formatstring_error(format, fmtptr - format, "string too long");
+		errno = ERANGE;
+		return -1;
+	}
+
+	return (ssize_t) strsize;
+}
diff --git a/src/formatstring.h b/src/formatstring.h
new file mode 100644
index 0000000..b03e81c
--- /dev/null
+++ b/src/formatstring.h
@@ -0,0 +1,13 @@
+#ifndef MEDIAEXTRACT_FORMATSTRING_H__
+#define MEDIAEXTRACT_FORMATSTRING_H__
+#pragma once
+
+#include <stddef.h>
+#include <stdio.h>
+
+// {filename}_{offset}.{ext}
+
+ssize_t formatstring(char *buffer, size_t buffer_size, const char *format, const char *filename,
+                     size_t index, size_t offset, size_t size, const char *ext);
+
+#endif