无法使用fopen（）打开文件

Question

I'm writing a program that simulates a platform where users can register. In register function I ask the user for his username and the program should create a new file with username as file name to store his password and other data for other functions. Problem here is that fopen returns NULL which means that it can't be created.

void Register()
{
char name[11];
char password[11];
char aux[11];
system("cls");
FILE *fp=fopen("Users.txt","a+"); //I've created this .txt, just has one user which is admin
if (fp==NULL){
    printf("\nFile cant be opened");
    getchar();
    return;
}
printf("Write a new username (no more than 10 characters, no spaces) ");
fflush(stdin);
fgets(name,sizeof(name),stdin);
getchar();
do{
    if((fgets(aux,sizeof(aux),fp))!=NULL){ //Checks is reading lines
        if ((strcmp(name,aux))==0){ //Username already exists
                printf("\nUsername already exists, try another: ");
                fflush(stdin);
                fgets(name,sizeof(name),stdin);
                rewind(fp); //Takes pointer to the beginning
        }
    }
}while(!(feof(fp))); 
fseek(fp,0,SEEK_END); //Takes pointer to end
fprintf(fp,"%s",name);
fclose(fp);
fp=fopen(name,"w"); /*THIS IS WHERE IT FAILS, RETURNS NULL*/
if (fp==NULL){
    printf("\nFile cant be opened");
    getchar();
    return;
}
printf("\nChoose a password(no more than 10 characters): ");
fflush(stdin);
fgets(password,sizeof(password),stdin);
getchar();
fprintf(fp,"%s\n",name);
fprintf(fp,"%s",password);
fclose(fp);
printf("\nUser successfully registered\nName: %s\nPassword: %s",name,password);
getchar();
}

I've already tried with another method. For example, use strcpy() to copy name to a new array and then strcat() to add ".txt" but it doesn't work either. Like this:

[...]
char aux2[20];
strcpy(aux2,name);
strcat(aux2,".txt");
fp=fopen(aux2,"w"); /*FAILS TOO */
if (fp==NULL){
    printf("\nFile cant be opened");
    getchar();
    return;
}

Can't figure out what is going wrong. Hope you can help me

Answer 1

List of problems:

1. You have not declared fp (at least in shown code) before trying to use it. Change line to:

   FILE *fp=fopen("Users.txt","a+");

2. You have not declared nombre . Add line:

   char nombre[11];//to match size of name

(Or, you may have meant to use name ?)

3. Everywhere it exists in your code, comment out the line:

   //fflush(stdin);

4. Change the line

     fp=fopen(name,"w"); /*THIS IS WHERE IT FAILS, RETURNS NULL*/ 

to:

fp=fopen(name,"w+"); /*THIS IS WHERE IT FAILS, RETURNS NULL*/

Answer 2

Just figured it out, I had to change the line

if((fgets(aux,sizeof(aux),fp))!=NULL){ //Checks is reading lines

And instead use fscanf() just like this:

if((fscanf(fp,"%s",&aux)==1){ //Checks if is reading a string (a line)

Answer 3

the question was not clear about the actual format of the data within the file. So I decided to use a format of name password for each line of the file.

Here is a version of the code that cleanly compiles and performs the desired function.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void Register( void );

void Register( void )
{
    char name[11];
    char password[11];
    char aux[11];

    system("cls");

    printf("Write a new username (no more than 10 characters, no spaces) ");
    if( !fgets(name,sizeof(name),stdin) )
    {
        fprintf( stderr, "fgets for user name failed" );
        exit( EXIT_FAILURE );
    }

    // implied else, fgets successful

    // code should remove trailing new line, if any
    char *newline;
    if( (newline = strchr( name, '\n' ) ) )
    {
        *newline = '\0';
    }

    FILE *fp=fopen("Users.txt","a+"); //I've created this .txt, just has one user which is admin
    if ( !fp)
    {
        perror("fopen for Users.txt for read/write failed");
        getchar();
        exit( EXIT_FAILURE );
    }

    // implied else, fopen successful

    while( fgets(aux,sizeof(aux),fp) )
    { //Checks is reading lines
        // separate out the name field
        char *token = strtok( aux, " " );
        if( token )
        {
            if ((strcmp(name,aux))==0)
            { //Username already exists
                printf("\nUsername already exists, try another: ");
                rewind(fp); //Takes pointer to the beginning
            }
        }
    }

    // name not already in file

    printf("\nChoose a password(no more than 10 characters): ");

    if( !fgets(password,sizeof(password),stdin) )
    {
        perror( "fgets for password failed" );
        fclose( fp ); // cleanup
        exit( EXIT_FAILURE );
    }

    // implied else, fgets successful

    //remove trailing newline
    if( (newline = strchr( password, '\n' ) ) )
    {
        *newline = '\0';
    }

    fprintf(fp,"%s %s\n",name, password);

    fclose(fp);

    printf("\nUser successfully registered\nName: %s\nPassword: %s",name,password);
    getchar();
}  // end function: Register