[R] Passing string arguments to C code, call to .C, R memory allocation for character vectors

Harold PETITHOMME harold.petithomme at meteo.fr
Fri Jan 25 10:29:50 CET 2008

Hello R-ill world!

My problem : I obtain a segmentation fault when passing a character 
argument to a C function, compiled in a shared object and loaded by 

1. As manuals (and overall "Writing R extensions ") don't seem to 
mention it or I failed to find the info, could someone explain to me 
what is exactly happening when R allocates memory for character vectors? 
Manuals precise that they have to be handled as char ** pointers in C, 
but this does not tell much about effective allocation although it is 
said to be 255 long character pointers for Fortran code.

2. As I understand, R allocates vectors and arrays in Fortran mode, that 
is all dimensions in a single block. This means passing a char * pointer 
to C, which is not true seemingly. Say this pointer is s, is my i-th 
character element s[i] in C or (*s)+LEN*i (LEN=255 as I suppose) or 
something else?

3. My simple R & C code gives me a segmentation fault. Trying to write 
an R character vector in the C function, the fault comes or not, 
depending on the vector dimension or on the element length. I suspect a 
memory protection from R or a faulty adressing in my C function (of 
course, I do as I understand from manuals). Can anybody track the 
error(s)? Note that the 2 tested R versions give different error 
messages (R-2.0.0 and R-2.3.0 on 2 Linux machines, one : Linux host1 
2.4.21-32.ELsmp, other : Linux host2 2.6.9-34.ELsmp)
Here is the code :

R programme :
n = 50

cat("R call to litchar\n")
ret = .C("litchar",noms=character(n),as.integer(n))

C code :
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

void litchar(char **s,int *n)
    int i, j, c;

    for (i=0;i<*n;i++) {
        sprintf(s[i],"%6d ",i); /* writes 'i' in s' i-th element... */
        c = 'a' + (i % 26); /* c is the i-th alphabet letter (set by 
ASCII code) */
        for (j=7 ; j<100 ; j++) s[i][j] = c; /* ... then add the c 
sequence of aaaaa, bbbbb, etc. */
        s[i][j] = 0; /* C strings terminator */

        fprintf(stderr,"s[%6d] : \"%s\" (%p)\n",i,s[i], s[i]); /* writes 
out i, s i-th element and its adress */

Last one : s[i] adresses seem to be not contiguous. How could it be?  
Isn't there a corruption in memory allocation tables?

Many thanks in advance for those who help.
Best regards,

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