You can insert/remove bytes from a file with the following Insert function:
#define TRUNCATE_EXISTS 1
#ifdef TRUNCATE_EXISTS
#define _POSIX_C_SOURCE 200112L
#include <unistd.h>
#include <fcntl.h>
#endif
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <limits.h>
enum {FI_BUFFER_SIZE=256};
static long FileSize(FILE* f) {
/* if function fails, it returns -1 and may have moved the file pointer*/
long ln, sz;
if ((ln=ftell(f))<0 || fseek(f, 0, SEEK_END)<0 || (sz=ftell(f))<0 || fseek(f, ln, SEEK_SET<0))
return -1;
else return sz;
}
static int FileFill(FILE* f,unsigned int length) {
while(length>0) {
if (fputc(' ', f)==EOF) return 0;
--length;
}
return 1;
}
static int MovePositive(FILE* f, unsigned int c_to_insert) {
/* insert c_to_insert space characters in a file at the current file pointer position
the file pointer ftell() is not changed by this function
f must be open in r+b or w+b mode
WARNING: file MUST be open in BINARY mode
*/
unsigned char buffer[FI_BUFFER_SIZE];
long origin=ftell(f), fpos_in_old;
if (origin<0) return 0;
if (fseek(f, 0, SEEK_END)<0
|| (fpos_in_old=ftell(f))<0
|| !FileFill(f, c_to_insert)
|| fseek(f, -c_to_insert, SEEK_CUR)<0) return 0;
while(fpos_in_old>origin) {
size_t size_to_move=(fpos_in_old-origin);
if (size_to_move > FI_BUFFER_SIZE) size_to_move=FI_BUFFER_SIZE;
if (fseek(f, -size_to_move, SEEK_CUR)<0
|| fread(buffer, 1, size_to_move, f)<size_to_move
|| fseek(f, -size_to_move+c_to_insert, SEEK_CUR)<0
|| fwrite(buffer, 1, size_to_move, f)<size_to_move
|| fseek(f, -size_to_move-c_to_insert, SEEK_CUR)<0) return 0;
fpos_in_old-=size_to_move;
}
if (ftell(f)!=origin) return 0; /* sanity check */
FileFill(f, c_to_insert);
return 1;
}
#ifdef TRUNCATE_EXISTS
static int MoveNegative(FILE* f, unsigned int c_to_remove) {
/* remove the c_to_insert bytes preceding the current file pointer position
the file pointer ftell() is not changed by this function
f must be open in r+b or w+b mode
WARNING: file MUST be open in BINARY mode
*/
unsigned char buffer[FI_BUFFER_SIZE];
long origin=ftell(f), old_end;
long fpos_in_old=origin;
if (origin<0 || (old_end=FileSize(f))<0) return 0;
if (c_to_remove > origin) c_to_remove=origin;
while(fpos_in_old<old_end) {
size_t size_to_move=old_end-fpos_in_old;
if (size_to_move > FI_BUFFER_SIZE) size_to_move=FI_BUFFER_SIZE;
if (fread(buffer, 1, size_to_move, f)<size_to_move
|| fseek(f, -size_to_move-c_to_remove, SEEK_CUR)<0
|| fwrite(buffer, 1, size_to_move, f)<size_to_move
|| fseek(f, c_to_remove, SEEK_CUR)<0) return 0;
fpos_in_old+=size_to_move;
}
return fseek(f, origin, SEEK_SET)>=0;
}
#endif
#ifndef TRUNCATE_EXISTS
static int StreamCopy(FILE* in, FILE* out, size_t length) {
unsigned char buffer[FI_BUFFER_SIZE];
while(length > 0) {
size_t size_to_copy=length,read_length;
if (size_to_copy > FI_BUFFER_SIZE) size_to_copy=FI_BUFFER_SIZE;
if (((read_length=fread(buffer, 1, size_to_copy, in))<size_to_copy && ferror(in))
|| fwrite(buffer, 1, read_length, out)<read_length) return 0;
if (read_length < size_to_copy) return feof(in);
length-=read_length;
}
return 1;
}
static int FileCopyErase(const char* filename, unsigned long pos, int c_to_remove) {
FILE* in=NULL,*out=NULL;
char temp_filename[L_tmpnam];
if (!(in=fopen(filename, "rb")) || !tmpnam(temp_filename) || !(out=fopen(temp_filename, "wb"))) {
if (in) fclose(in);return 0;
}
if (!StreamCopy(in, out, pos-c_to_remove)
|| fseek(in, c_to_remove, SEEK_CUR)<0
|| !StreamCopy(in, out, FileSize(in)-pos)) {
fclose(in);fclose(out);
remove(temp_filename);
return 0;
}
fclose(out);fclose(in);
remove(filename);
return rename(temp_filename, filename)>=0;
}
#endif
#ifdef TRUNCATE_EXISTS
static int xtruncate(const char* filename, unsigned long length) {
int fd,r;
if ((fd=open(filename, O_WRONLY))<0) return 0;
r=ftruncate(fd, length);;
close(fd);
return r>=0;
}
#endif
int Insert(const char* filename, unsigned long pos, int c_to_insert) {
/* insert c_to_insert garbage bytes in a file at the position pos
if c_to_insert is negative, it removes the c_to_insert bytes preceding pos in the file
*/
FILE* f;
if (0==c_to_insert) return 1;
f=fopen(filename, "r+b");
if (!f) return 0;
if (c_to_insert > 0) {
int r;
r=fseek(f, pos, SEEK_SET)>=0 && MovePositive(f, c_to_insert);
fclose(f);
return r;
} else {
unsigned int c_to_remove=-c_to_insert;
#ifdef TRUNCATE_EXISTS
long old_length=FileSize(f);
int r;
if (c_to_remove > pos) c_to_remove=pos;
r=fseek(f, pos, SEEK_SET)>=0 && MoveNegative(f, c_to_remove);
fclose(f);
return r && xtruncate(filename, old_length-c_to_remove);
#else
return FileCopyErase(filename, pos, c_to_remove);
#endif
}
}
static int stringToLong(long* presult, const char* str) {
char* p;
long result;
result=strtol(str, &p, 0);
if (*str=='\0' || *p!='\0' || result==LONG_MAX) return 0;
*presult=result;return 1;
}
int main(int argc, char** argv) {
const char* filename;
long position, c_to_insert;
if (argc!=4) {
fprintf(stderr, "%s expects three arguments: <file name> <position> <number of chars to insert>\n", argv[0]);
return 1;
}
filename=argv[1];
if (!stringToLong(&position, argv[2]) || !stringToLong(&c_to_insert, argv[3])) {
fprintf(stderr, "%s: integer expected!\n", argv[0]);
return 1;
}
return Insert(filename, position, c_to_insert)?0:2;
}