I've been out of programming in C for almost 2 years and have recently gotten an assignment in school on using write() and read().
Somewhere in the code I'm receiving the Segmentation Fault error, possibly on the filecopy function is where I'd put my money on. I was trying GDB but I haven't used that since that last time I programmed in C so I turn to here.
The code.
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <fcntl.h>
int main(int argc, char *argv[]) {
void filecopy(int infd, int outfd);
int fd = -1;
char *prog = argv[0];
if(argc == 1)
filecopy(STDIN_FILENO, STDOUT_FILENO);
else
while(--argc > 0) {
if((fd = open(*++argv, O_RDONLY, "rb")) == -1) {
// we don't have fprintf... but we have sprintf =]
char tmp[30];
sprintf(tmp, "%s: can't open %s\0", prog, *argv);
write(STDOUT_FILENO, &tmp, sizeof(tmp));
exit(-1);
} else {
filecopy(fd, STDOUT_FILENO);
close(fd);
}
}
exit(0);
}
void filecopy(int infd, int outfd) {
// char *buf[1]; <-- causes unreadable characters outputted by write
char *buf;
while(read(infd, buf, 1) != -1)
write(outfd, buf, sizeof(buf));
}
The input/output
Thanks!
char *buf; is an uninitialized pointer, writing data through that pointer is
undefined behaviour.
char buf[1024];
ssize_t len;
while((len = read(infd, buf, sizeof buf)) != -1)
write(outfd, buf, len);
would be correct.
Note that char *buf[1]; is a array (of dimension 1) of pointers, that's
different to an array of chars. Using that you would need to do
read(infd, buf[0], somelength), but here again buf[0] would be an
uninitialized pointer and you would have the same problem. That's why declaring
an char array of say 1024 (you can choose another size) is the correct thing
to do.
Also in main use strlen(tmp) and not sizeof(tmp)
char tmp[30];
sprintf(tmp, "%s: can't open %s\0", prog, *argv);
write(STDOUT_FILENO, &tmp, strlen(tmp));
strlen returns you the length of the string which might be smaller than 29 and
if you use sizeof(tmp) you might be writing garbage past the end of the
string. Note also that 0 may be too small for the whole string, I'd use a
larger number or construct the string using snprintf:
snprintf(tmp, sizeof tmp, "%s: can't open %s\0", prog, *argv);
would be more safe.
Last thing:
while(--argc > 0)
if((fd = open(*++argv, O_RDONLY, "rb")) == -1) {
...
While this is correct, I feel that this code is awkward and hard to read. It
would be so much simpler to read if you did:
for(int i = 1; i < argc; ++i)
if((fd = open(argv[i], O_RDONLY, "rb")) == -1) {
...
I've never seen open being called with "rb" as the mode. My man page says:
man 2 open
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
int open(const char *pathname, int flags, mode_t mode);
[...]
The mode argument specifies the file mode bits be applied when a new file is created. This argument must be supplied when
O_CREAT or O_TMPFILE is specified in flags; if neither O_CREAT nor O_TMPFILE is specified, then mode is ignored.
The effective mode is modified by the process's umask in the usual way: in the absence of a default ACL, the mode of the created file is
(mode & ~umask). Note that this mode applies only to future accesses of the newly created file; the open() call that creates a
read-only file may well return a read/write file descriptor.
The following symbolic constants are provided for mode:
S_IRWXU 00700 user (file owner) has read, write, and execute permission
S_IRUSR 00400 user has read permission
S_IWUSR 00200 user has write permission
S_IXUSR 00100 user has execute permission
[...]
As you are neither using O_CREAT nor O_TMPFILE, this parameter will be
ignore and you are passing a char* as a mode_t which is integer in nature.
Hence your call should be:
if((fd = open(argv[i], O_RDONLY, 0)) == -1) {
...
Two adjustments are needed for you filecopy function:
You need to allocate space for your buffer. Right now you are using an uninitialized pointer and passing it to read which is undefined behavior.
You need to save the return value of read and pass the value to write
The end result should look something like this.
void filecopy(int infd, int outfd) {
char buf[1024];
size_t bytes_read;
while((bytes_read = read(infd, buf, sizeof buf)) != -1)
write(outfd, buf, bytes_read);
}
Running this through a static analysis tool gives 2 warnings:
1) The uninitialized variable that #Pablo points to
2) a buffer overrun when you sprintf *argv into tmp as *argv can very large (as #Pablo also suggested in his comment re: snprintf)
Related
I am trying to write a struct to a .dat file and when I open it it shows me this:
"1^#^#^#^#^#^#^#^#^#^#^#^#^#^#^#^#^#^#041720881^#^#^#^#^#^#^#^#^#^#^#Denko^#^#^#^#^#^#^#^#^#^#^#^#^#^#^#Brenko^#^#^#^#^#^#^#^#^#^#^#^#^#^#13.07.2000^#^#^#^#^#^#^#^#^#^#
"
It adds random symbols between the actual values. And now when I at least try to read and print some values, it just doesn't work. It's like the buffer is empty. But I followed the instructions and guides I read.
Using fwrite or similar is not an option since I have to work with these specific functions write() and read().
My code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <sys/stat.h>
struct info{
char id[20];
char telefon[20];
char ime[20];
char priimek[20];
char datum[20];
};
int main(int argc, char* argv[]) {
struct info dude =
{
"01",
"041720881",
"Denko",
"Brenko",
"13.07.2000"
};
struct info dude2 =
{
"02",
"041581734",
"Denko",
"Badenko",
"13.07.1990"
};
if(strcmp(argv[1], "-c") == 0){
int fd = open("xpo.dat", O_CREAT| O_APPEND | S_IRWXG, 0666);
if(fd == -1){
perror("Error while creating file");
exit(EXIT_FAILURE);
}
}
else if(strcmp(argv[1], "-o") == 0){
struct stat sizefile;
int fd = open("xpo.dat", O_RDWR);
if(fd == -1){
perror("Error while opening file");
exit(EXIT_FAILURE);
}
fstat(fd,&sizefile);
int wr = write(fd, &dude,sizeof(struct info));
char buf[101];
int sz = read(fd, buf, 100);
buf[sz] = '\0';
if(sz == -1) {
perror("Error while creating file");
exit(EXIT_FAILURE);
}
printf("%s", buf);
int cl = close(fd);
}
return 0;
}
The struct contains 100 chars of data. But you are setting only some of them. When you set ime as Denko, the first six chars are set as 'D', 'e', 'n', 'k', 'o','\0'. The remaining 14 are not initialized (or rather initialized implicitly, see #dave_thompson_085's comment below).
If you want to omit those chars, you cannot write the struct as one block. Either write each field separately, or concatenate the fields into a string and write it instead.
As stated in the comments and in the accepted answer, you have some issues, the why and the what is already talked about and explained.
I would like to add some more information:
And now when I at least try to read and print some values, it just doesn't work. It's like the buffer is empty.
What happens is that you are reading from the end of the file, if you want to read after you write without closing and reopening the file, you can, but you'll need to reposition the offset of the opened file to the beginning using lseek.
Another thing to note is that if you want to write the data as a structure you then need to read it as a structure as well, not as a string.
Taking that in consideration your code could look more or less like this (skipping return value validations, but you should do it, as you know):
//...
else if(strcmp(argv[1], "-o") == 0){
int fd = open("xpo.dat", O_RDWR);
int wr = write(fd, &dude, sizeof dude);
lseek(fd, 0, SEEK_SET); // set the reading position to the beginning of the file
struct info buf;
wr = read(fd, &buf, sizeof buf);
int cl = close(fd);
printf("%s %s %s %s %s", buf.id, buf.telefon, buf.ime, buf.priimek, buf.datum);
}
//...
If you prefer it as a string you can easily concatenate it using something like snprintf or similar.
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My professor asked me to write a simple C program, then asked me to convert using Unix system calls. I have try changing the values around but nothing is working.
Requirement:
Write a new C program newcat, which performs exactly as oldcat, but uses the following UNIX system calls for I/O.
int read(int fd, char *buf, int n);
int write(int fd, char *buf, int n);
int open(char *name, int accessmode, int permission);
int close(int fd);
To open a file for read, you can use the symbolic constant O_RDONLY defined in fcntl.h header file to specify the accessmode. Simply pass 0 for permission. That is, the code will appear as follows:
fd = open (filename, O_RDONLY, 0);
You will need the following header files: sys/types.h, unistd.h and fcntl.h
#include <stdio.h>
/* oldcat: Concatenate files */
int main(int argc, char *argv[])
{
void filecopy(FILE *, FILE *); /* prototype for function */
int fd = open(*fp, O_RDONLy,0)
char *prog = argv[0]; /* program name for errors */
if (argc == 1) /* no args; copy standard input */
filecopy(0, 1);
else
while (--argc > 0)
if (fd == -1) {
fprintf(stderr, "%s: can't open %s\n", prog, *argv);
return(-1);
} else {
filecopy(fp, 1);
fclose(fp);
}
return(0);
}
/* filecopy: copy file ifp to ofp */
void filecopy(FILE *ifp, FILE *ofp)
{
int c;
while ((c = getc(ifp)) != EOF)
putc(c, ofp);
}
Is this the write idea? It still won't compile:
#include <stdio.h>
/* oldcat: Concatenate files */
int main(int argc, char *argv[])
{
void filecopy(int ifp, int ifo);
int fd = open(*File,O_RDONLY,0); //is this correct?
char *prog = argv[0];
if (argc == 1) /* no args; copy standard input */
filecopy(0, 1); //is this correct?
else
while (--argc > 0)
if ((fd == -1) //is this correct?{
fprintf(stderr, "%s: can't open %s\n", prog, *argv);
return(-1);
} else {
filecopy(*FILE, 1);//is this correct?
close(*FILE);//is this correct?
}
return(0);
}
/* filecopy: copy file ifp to ofp */
void filecopy(FILE *ifp, FILE *ofp)//NO CLUE HOW THIS SHOULD BE
{
int c;
while (c = read(fd ,&something,1)//What is &ch/&something?
putc(c, ofp);
}
Assuming your oldcat uses the C standard library calls (like fopen), it's a simple matter of mapping those to the UNIX calls.
At a high level:
fopen -> open
fread -> read
fwrite -> write
fclose -> close
For example, when opening your input file with:
FILE *fIn = fopen ("jargon.txt", "r");
you could instead use:
int inFd = open ("jargon.txt", O_RDONLY, 0);
The other calls are very similar, with similar functionality at the C standard library and UNIX system call levels. Details on those calls can usually be obtained from the manpages by entering something like man 2 open into your shell, or by plugging man open into your favourite search engine.
The only "tricky" mapping is if you've used getchar/putchar-style calls to do the actual reading and writing but that too becomes easy when you realise that (for example) reading a character is functionally identical to reading a block of size one:
int c = getc (fIn);
or:
char c;
int numread = read (inFd, &c, 1);
For your added question:
So to open a file: if (fd = open (fp, O_RDONLY, 0); ) == NULL)
Not quite. The fopen function returns NULL on error because it returns a pointer to a FILE structure.
The lower level calls use file descriptors rather than file handles, the former being a small integer value. So, instead of:
FILE *fp = fopen ("nosuchfile", "r");
if (fp == NULL) doSomethingIntelligent();
you would do something like:
int fd = open ("nosuchfile", O_RDONLY, 0);
if (fd == -1) doSomethingIntelligentUsing (errno);
In terms of what you need to change, the following comes off the top of my head (so may not be totally exhaustive but should be a very good start):
Add the required headers.
Stop using FILE* totally, using int instead.
Translate the fopen/fclose calls to open/close. This includes the function name, different parameters and different return types.
Modify filecopy to use file descriptors rather than file handles.
use 1 instead of stdout when calling filecopy (the latter is a FILE *).
As an example of how to do this, the following program testprog.c will read itself and echo each character to standard output:
#include <stdio.h>
#include <errno.h>
#include <fcntl.h>
int main (void) {
int num, ch, inFd;
// Open as read only.
inFd = open ("testprog.c", O_RDONLY, 0);
if (inFd == -1)
printf ("\n**Error %d opening file\n", errno);
// Get and output esach char until EOF/error.
while ((num = read (inFd, &ch, 1) != 0) == 1)
putchar (ch);
// Detect error.
if (num != 0)
printf ("\n**Error %d reading file\n", errno);
// Close file and exit.
close (inFd);
return 0;
}
Please note that documentation of linux sys calls is present in manual called man pages which you can access by using man command in bash shell in a linux system. As UNIX and Linux are quite similar (maybe equivalent) for the syscalls you are interested in you can check the man page for those syscalls in Linux.
All the four read, write, open and close linux syscalls are explained in man pages. You can access the manual page for these syscalls by typing below commands in shell:
man 2 read
man 2 write
man 2 open
man 2 close
These should probably guide you to right direction.
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <fcntl.h>
/* newcat: Concatenate files */
int main(int argc, char *argv[])
{
void filecopy(int ifp, int ofp); /* prototype for function */
int fd;
char *prog = argv[0]; /* program name for errors */
if (argc == 1) /* no args; copy standard input */
filecopy(0,1);
else
while (--argc > 0)
fd = open(*++argv , O_RDONLY,0);
if ( fd == -1) {
fprintf(stderr, "%s: can't open %s\n", prog, *argv);
return(-1);
} else {
filecopy(fd, 1);
close(fd);
}
return(0);
}
/* filecopy: copy file ifp to ofp */
void filecopy(int ifp, int ofp)
{
int c;
while (read(ifp,&c,ofp ) != 0)
write(ofp,&c,ofp);
}
So I'm using mmap to then write to another file. But the weird thing is, when my code hits mmap, what it does is clears the file. So I have a file that's populated with random characters (AB, HAA, JAK, etc...). What it's supposed to do is use mmap as read basically and then write that file to the new file. So that first if (argc == 3) is the normal read and write, the second if (argc ==4) is supposed to use mmap. Does anyone have any idea why on Earth this is happening?
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/io.h>
#include <sys/mman.h>
#include <sys/time.h>
#include <sys/resource.h>
int main(int argc, char const *argv[])
{
int nbyte = 512;
char buffer[nbyte];
unsigned char *f;
int bytesRead = 0;
int size;
int totalBuffer;
struct stat s;
const char * file_name = argv[1];
int fd = open (argv[1], O_RDONLY);
int i = 0;
char c;
int fileInput = open(argv[1], O_RDONLY);
int fileOutPut = open(argv[2], O_WRONLY | O_TRUNC | O_CREAT, S_IRUSR | S_IWUSR);
fstat(fileInput, &s);
size = s.st_size;
printf("%d\n", size);
if (argc == 3)
{
printf("size: %d\n", size);
printf("nbyte: %d\n", nbyte);
while (size - bytesRead >= nbyte)
{
read(fileInput, buffer, nbyte);
bytesRead += nbyte;
write(fileOutPut, buffer, nbyte);
}
read(fileInput, buffer, size - bytesRead);
write(fileOutPut, buffer, size - bytesRead);
}
else if (argc == 4)
{
int i = 0;
printf("4 arg\n");
f = (char *) mmap (0, size, PROT_READ, MAP_PRIVATE, fileInput, 0);
/* This is where it is being wipped */
}
close(fileInput);
close(fileOutPut);
int who = RUSAGE_SELF;
struct rusage usage;
int ret;
/* Get the status of the file and print some. Easy to do what "ls" does with fstat system call... */
int status = fstat (fd, & s);
printf("File Size: %d bytes\n",s.st_size);
printf("Number of Links: %d\n",s.st_nlink);
return 0;
}
EDIT: I wanted to mention that the first read and write works perfectly, it is only when you try to do it through the mmap.
If you mean it's clearing your destination file, then yes, that's exactly what your code will do.
It opens the destination with truncation and then, in your argc==4 section, you map the input file but do absolutely nothing to transfer the data to the output file.
You'll need a while loop of some description, similar to the one in the argc==3 case, but which writes the bytes in mapped memory to the fileOutput descriptor.
I was testing a code from APUE, in chapter 14(Advanced I/O) of memory map file, the fstat() always return the fdin's st_size as zero, and I tried stat() instead, and also get the same result. I list the code below(I have removed the apue.h dependencies):
#include <fcntl.h>
#include <sys/mman.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <sys/stat.h>
#include <unistd.h>
#define COPYINCR (1024*1024*1024) /* 1GB */
int main(int argc, char *argv[]) {
if (argc != 3) {
printf("usage: %s <fromfile> <tofile>", argv[0]);
exit(1);
}
int fdin, fdout;
if ((fdin = open(argv[1], O_RDONLY)) < 0) {
printf("can not open %s for reading", argv[1]);
exit(1);
}
if ((fdout = open(argv[2] /* typo fix */, O_RDONLY | O_CREAT | O_TRUNC)) < 0) {
printf("can not open %s for writing", argv[2]);
exit(1);
}
struct stat sbuf;
if (fstat(fdin, &sbuf) < 0) { /* need size fo input file */
printf("fstat error");
exit(1);
}
// always zero, and cause truncate error (parameter error)
printf("input_file size: %lld\n", (long long)sbuf.st_size);
if (ftruncate(fdout, sbuf.st_size) < 0) { /* set output file size */
printf("ftruncate error");
exit(1);
}
void *src, *dst;
off_t fsz = 0;
size_t copysz;
while (fsz < sbuf.st_size) {
if (sbuf.st_size - fsz > COPYINCR)
copysz = COPYINCR;
else
copysz = sbuf.st_size - fsz;
if (MAP_FAILED == (src = mmap(0, copysz, PROT_READ,
MAP_SHARED, fdin, fsz))) {
printf("mmap error for input\n");
exit(1);
}
if (MAP_FAILED == (dst = mmap(0, copysz,
PROT_READ | PROT_WRITE,
MAP_SHARED, fdout, fsz))) {
printf("mmap error for output\n");
exit(1);
}
memcpy(dst, src, copysz);
munmap(src, copysz);
munmap(dst, copysz);
fsz += copysz;
}
return 0;
}
And then I have tried the Python os.stat, it also get the zero result, why this happened? I have tried these and got the same result on Mac OS (Darwin kernel 13.4) and Ubuntu (kernel 3.13).
UPDATE:
Oh, there was a typo error, I should refer to fdout to argv[2], and the O_TRUNC flag certainly make the fdin to zero. Should I close or delete this question?
The reason why Python's os.stat() also return (stat.st_size == 0) is that I passed the same test file (argv[1]) to test, and the file has been previously truncated to zero (I haven't check its size using ls -lh before passing to os.stat()), and certainly os.stat() return zero.
Do not ask SO questions before you go to bed or in a rush.
Ok, the real problem is double open the same input file, and this does not cause any build or runtime error until the ftruncate().
The first open get a read-only fdin, the second open create a new file (fdout and truncated) to copy from fdin via memory map, and the second open truncated the first file (argv[1]), and cleaned all its content. But the fdin still working with fstat (and certainly), this make me hard to find the reason.
The second part is I always use the same file for testing (generated via dd) and have not checking the size, so the os.stat(/path/to/file) and stat(/path/to/file) also return st_size == 0, this makes me believe that this must be some os-level-prolicy defined the behaviour, and I rushed to Mac OS (using the same typo code), and got the same result (they really consistent on POSIX level, event the bug!), and at last, I came to SO for help.
I wrote a code for writing the content to the mapped buffer which mapped by using the mmap() system call.
After I did some the changes in the mapped buffer,then I called the msync().It should update to the file on disk.
But,It doesn't made any changes to the file on disk.
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include<sys/mman.h>
#include<fcntl.h>
#define FILEMODE S_IRWXU | S_IRGRP | S_IROTH
#define MAX 150
main(int argc,char *argv[])
{
int fd,ret,len;
long int len_file;
struct stat st;
char *addr;
char buf[MAX];
if(argc > 1)
{
if((fd = open(argv[1],O_RDWR | O_APPEND | O_CREAT ,FILEMODE)) < 0)
perror("Error in file opening");
if((ret=fstat(fd,&st)) < 0)
perror("Error in fstat");
len_file = st.st_size;
/*len_file having the total length of the file(fd).*/
if((addr=mmap(NULL,len_file,PROT_READ|PROT_WRITE,MAP_PRIVATE,fd,0)) == MAP_FAILED)
perror("Error in mmap");
len = len_file;
while((fgets(buf,MAX,stdin)) != NULL)
{
strcat(addr+len,buf);
printf( "Val:%s\n",addr ) ; //Checking purpose
len = len + (strlen(buf));
}
if((msync(addr,len,MS_SYNC)) < 0)
perror("Error in msync");
if( munmap(addr,len) == -1)
printf("Error:\n");
printf("addr %p\n",addr);
}
else
{
printf("Usage a.out <filename>\n");
}
}
If you want your changes to be reflected in the on-disk file, you must map the file as MAP_SHARED, not MAP_PRIVATE.
Additionally, you cannot extend the file simply by writing beyond the end of the mapping. You must use ftruncate() to extend the file to the new size, then change the mapping to include the new portion of the file. The portable way to change the mapping is to unmap the mapping then recreate it with the new size; on Linux you can instead use mremap().
Your len and len_file variables should be of type size_t, and you should use memcpy() rather than strcat(), since you know exactly the length of the string, exactly where you want to copy it, and you don't want to copy the null-terminator.
The following modification of your code works on Linux (using mremap()) :
#define _GNU_SOURCE
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include<sys/mman.h>
#include<fcntl.h>
#define FILEMODE S_IRWXU | S_IRGRP | S_IROTH
#define MAX 150
int main(int argc,char *argv[])
{
int fd, ret;
size_t len_file, len;
struct stat st;
char *addr;
char buf[MAX];
if (argc < 2)
{
printf("Usage a.out <filename>\n");
return EXIT_FAILURE;
}
if ((fd = open(argv[1],O_RDWR | O_CREAT, FILEMODE)) < 0)
{
perror("Error in file opening");
return EXIT_FAILURE;
}
if ((ret = fstat(fd,&st)) < 0)
{
perror("Error in fstat");
return EXIT_FAILURE;
}
len_file = st.st_size;
/*len_file having the total length of the file(fd).*/
if ((addr = mmap(NULL,len_file,PROT_READ|PROT_WRITE,MAP_SHARED,fd,0)) == MAP_FAILED)
{
perror("Error in mmap");
return EXIT_FAILURE;
}
while ((fgets(buf,MAX,stdin)) != NULL)
{
len = len_file;
len_file += strlen(buf);
if (ftruncate(fd, len_file) != 0)
{
perror("Error extending file");
return EXIT_FAILURE;
}
if ((addr = mremap(addr, len, len_file, MREMAP_MAYMOVE)) == MAP_FAILED)
{
perror("Error extending mapping");
return EXIT_FAILURE;
}
memcpy(addr+len, buf, len_file - len);
printf( "Val:%s\n",addr ) ; //Checking purpose
}
if((msync(addr,len,MS_SYNC)) < 0)
perror("Error in msync");
if (munmap(addr,len) == -1)
perror("Error in munmap");
if (close(fd))
perror("Error in close");
return 0;
}
Note that you've provided a mapping for the file that is exactly the size of the file. If you create the file in your call to open(2), it will have a length of 0, and I wouldn't be surprised if the kernel doesn't bother setting up any kind of memory mapping from a 0 length mapping. (Maybe it does? I've never tried...)
I would suggest using ftruncate(2) to extend the length of your file before performing the mapping. (Note that extending files using ftruncate(2) isn't very portable; not all platforms provide extending functionality and not all filesystem drivers support the extending functionality. See your system's manpage for details.)
You must use the MAP_SHARED mapping for your file modifications to be saved to disk.
Your use of perror(3) isn't quite correct; perror(3) will not terminate your program, so it will continue executing with incorrect assumptions:
if((ret=fstat(fd,&st)) < 0)
perror("Error in fstat");
Should read:
if((ret=fstat(fd,&st)) < 0) {
perror("Error in fstat");
exit(1);
}
(Or exit(EXIT_FAILURE) if you want to be more portable -- I find that a little harder on the eyes but I live in Linux-land.)
strcat(3) expects to find an ASCII NUL character (byte value 0x00, C representation '\0') -- the usual C end-of-string marker -- at the end of the dest string. Your file will not contain an ASCII NUL if you create it in this program -- its length is zero, after all -- and I don't know the consequences of trying to read a zero-byte file via mmap(2). If the file already exists and has data in it, it probably doesn't have an ASCII NUL encoded in the file. strcat(3) is almost certainly the wrong tool to write into your file. (No one wants ASCII NULs in their files anyway.) Try memcpy(3) instead.