int result;
char *buffer;
XpmImage image;
...
/* xpm image creation */
...
result = XpmCreateBufferFromXpmImage(&buffer, &image, NULL);
...

result is always -3 (XpmNoMemory)

XpmWriteFileFromXpmImage() with the same arguments works fine.

I could track down the problem to this code segment of WriteColors() in src/CrBufFrI.c (near line 314):
        strcpy(s, "\",\n");
        l = s + 3 - buf;
        if( *data_size                   >= UINT_MAX-l ||
            *data_size + l               <= *used_size ||
           (*data_size + l - *used_size) <= sizeof(buf))
                return(XpmNoMemory);

The last comparison is always true (under normal circumstances) and therefore this function returns XpmNoMemory. Is this check really necessary?

The call of XpmCreateBufferFromXpmImage() worked as expected after removing the third comparison.