Rename a file and save it as a another one - c

Question

I'm trying to get a filename that exists at the beginning. After that I want to add _blabla.txt to the filename and create the another one.
Here the example. --> example.txt (what I get)
Here the what I want. --> example_blabla.txt (what I want to create)
I tried to use sprintf(filename, "%s%s", argv[1], "_blabla.txt") but the created file's name is example.txt_blabla.txt.
How can I remove extension of first filename from the filename that I created ?
Here is my code:
FILE *fp;
FILE *fp2;
char filename[300];
if (argc != 2 || argv[1] == NULL) {
printf(" You did not enter a file name! \n");
exit(-1);
}
sprintf(filename, "%s%s", argv[1], "_blabla.txt");
fp = fopen(argv[1], "r");
fp2 = fopen(filename, "w");
return 0;

Answer 1

Here are some suggestions:
to locate the last '.' in the string, you can use the function strrchr() from <string.h>.
to avoid undefined behavior on overlong strings, use snprintf instead of sprintf.
to copy a portion of a string with snprintf, use %.*s with an int argument before the string pointer.
Here is a modified version:
#include <stdio.h>
#include <string.h>
int main(int argc, char *argv[]) {
FILE *fp;
FILE *fp2;
char filename[300];
char *p;
int pos;
if (argc != 2) {
printf(" You did not enter a file name!\n");
return -1;
}
p = strrchr(argv[1], '.');
if (p != NULL)
pos = p - argv[1];
else
pos = strlen(argv[1]);
// Using the %.*s precision field to limit the number of characters
// copied from `argv[1]`
snprintf(filename, sizeof filename, "%.*s_blabla.txt", pos, argv[1]);
fp = fopen(argv[1], "r");
if (fp == NULL)
return 1;
fp2 = fopen(filename, "w");
if (fp2 == NULL)
return 1;
/* copy the file */
fclose(fp);
fclose(fp2);
return 0;
}

Answer 2

Append the following statements before and after your sprintf() call as follows:
int i;
char *pChar = NULL;
for (i = strlen(argv[1]) - 1; i >= 0: i--) {
if (argv[1][i] == '.') {
pChar = &argv[1][i];
*pChar = '\0';
break;
}
}
sprintf(filename,"%s%s",argv[1],"_blabla.txt");
if (pChar != NULL)
*pChar = '.';

Answer 3

the following proposed code:
demonstrates a method to extract the root name from a filename then append a set of characters.
demonstrates how to inform the user when the command line parameter is missing.
does not perform a stat() to assure the source file actually exists, That is left to the OP to include.
properly checks for I/O errors
documents why each header file is included
suggest getting the length of the argv[1] parameter, add the length of blabla.txt, add 1, then use the VLA feature of C to appropriately size the array filename[]. Note: strlen() works well for obtaining the length of a NUL terminated char array
And now, the proposed code:
#include <stdio.h> // fopen(), fclose(), perror(), fprintf(), FILE
#include <stdlib.h> // exit(), EXIT_FAILURE
#include <string.h> // strrchr(), strcat()
int main( int argc, char (argv[] )
{
FILE *fp;
FILE *fp2;
char filename[300];
if (argc != 2 )
{
fprintf( stderr, "USAGE: %s fileName\n", argv[0] );
exit( EXIT_FAILURE );
}
strcpy(filename, argv[1] );
char *ptr = strrchr( filename, '.' );
if( ptr )
{
*ptr = '\0';
}
strcat( filename, "_blabla.txt");
fp = fopen(argv[1], "r");
if( ! fp )
{
perror( "fopen for input file failed" );
exit( EXIT_FAILURE );
}
fp2 = fopen(filename, "w");
if( ! fp2 )
{
perror( "fopen for output file failed" );
fclose( fp1 );
exit( EXIT_FAILURE );
}
return 0;
}