Here’s a link back to the GitHub project page.


A set of basic dynamic string macros for C programs are included with uthash in utstring.h. To use these in your own C program, just copy utstring.h into your source directory and use it in your programs.

#include "utstring.h"

The dynamic string supports operations such as inserting data, concatenation, getting the length and content, substring search, and clear. It’s ok to put binary data into a utstring too. The string operations are listed below.

Some utstring operations are implemented as functions rather than macros.


To download the utstring.h header file, follow the links on to clone uthash or get a zip file, then look in the src/ sub-directory.

BSD licensed

This software is made available under the revised BSD license. It is free and open source.


The utstring macros have been tested on:

  • Linux,

  • Windows, using Visual Studio 2008 and Visual Studio 2010



The dynamic string itself has the data type UT_string. It is declared like,

UT_string *str;

New and free

The next step is to create the string using utstring_new. Later when you’re done with it, utstring_free will free it and all its content.


The utstring_printf or utstring_bincpy operations insert (copy) data into the string. To concatenate one utstring to another, use utstring_concat. To clear the content of the string, use utstring_clear. The length of the string is available from utstring_len, and its content from utstring_body. This evaluates to a char*. The buffer it points to is always null-terminated. So, it can be used directly with external functions that expect a string. This automatic null terminator is not counted in the length of the string.


These examples show how to use utstring.

Sample 1
#include <stdio.h>
#include "utstring.h"

int main() {
    UT_string *s;

    utstring_printf(s, "hello world!" );
    printf("%s\n", utstring_body(s));

    return 0;

The next example demonstrates that utstring_printf appends to the string. It also shows concatenation.

Sample 2
#include <stdio.h>
#include "utstring.h"

int main() {
    UT_string *s, *t;


    utstring_printf(s, "hello " );
    utstring_printf(s, "world " );

    utstring_printf(t, "hi " );
    utstring_printf(t, "there " );

    utstring_concat(s, t);
    printf("length: %u\n", utstring_len(s));
    printf("%s\n", utstring_body(s));

    return 0;

The next example shows how binary data can be inserted into the string. It also clears the string and prints new data into it.

Sample 3
#include <stdio.h>
#include "utstring.h"

int main() {
    UT_string *s;
    char binary[] = "\xff\xff";

    utstring_bincpy(s, binary, sizeof(binary));
    printf("length is %u\n", utstring_len(s));

    utstring_printf(s,"number %d", 10);
    printf("%s\n", utstring_body(s));

    return 0;


These are the utstring operations.



allocate a new utstring


allocate a new utstring (if s is NULL) otherwise clears it


free an allocated utstring


init a utstring (non-alloc)


dispose of a utstring (non-alloc)


printf into a utstring (appends)


insert binary data of length len (appends)


concatenate src utstring to end of dst utstring


clear the content of s (setting its length to 0)


obtain the length of s as an unsigned integer


get char* to body of s (buffer is always null-terminated)


forward search from pos for a substring


reverse search from pos for a substring

New/free vs. init/done

Use utstring_new and utstring_free to allocate a new string or free it. If the UT_string is statically allocated, use utstring_init and utstring_done to initialize or free its internal memory.

Use utstring_find and utstring_findR to search for a substring in a utstring. It comes in forward and reverse varieties. The reverse search scans from the end of the string backward. These take a position to start searching from, measured from 0 (the start of the utstring). A negative position is counted from the end of the string, so, -1 is the last position. Note that in the reverse search, the initial position anchors to the end of the substring being searched for; e.g., the t in cat. The return value always refers to the offset where the substring starts in the utstring. When no substring match is found, -1 is returned.

For example if a utstring called s contains:


Then these forward and reverse substring searches for ABC produce these results:

utstring_find(  s, -9, "ABC", 3 ) = 15
utstring_find(  s,  3, "ABC", 3 ) =  4
utstring_find(  s, 16, "ABC", 3 ) = -1
utstring_findR( s, -9, "ABC", 3 ) = 11
utstring_findR( s, 12, "ABC", 3 ) =  4
utstring_findR( s,  2, "ABC", 3 ) =  0

The preceding examples show "single use" versions of substring matching, where the internal Knuth-Morris-Pratt (KMP) table is internally built and then freed after the search. If your program needs to run many searches for a given substring, it is more efficient to save the KMP table and reuse it.

To reuse the KMP table, build it manually and then pass it into the internal search functions. The functions involved are:

_utstring_BuildTable  (build the KMP table for a forward search)
_utstring_BuildTableR (build the KMP table for a reverse search)
_utstring_find        (forward search using a prebuilt KMP table)
_utstring_findR       (reverse search using a prebuilt KMP table)

This is an example of building a forward KMP table for the substring "ABC", and then using it in a search:

long *KPM_TABLE, offset;
KPM_TABLE = (long *)malloc( sizeof(long) * (strlen("ABC")) + 1));
_utstring_BuildTable("ABC", 3, KPM_TABLE);
offset = _utstring_find(utstring_body(s), utstring_len(s), "ABC", 3, KPM_TABLE );

Note that the internal _utstring_find has the length of the UT_string as its second argument, rather than the start position. You can emulate the position parameter by adding to the string start address and subtracting from its length.


  1. To override the default out-of-memory handling behavior (which calls exit(-1)), override the utstring_oom() macro before including utstring.h. For example,

    #define utstring_oom() do { longjmp(error_handling_location); } while (0)
    #include "utstring.h"