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In this example, a file name is supplied to fopen() to create and open for writing. However, there is no guarantee that the file referenced by file_name does not exist prior to calling fopen(). This may cause an unintended file to be accessed or overwritten.
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FILE * fptr = fopen(file_name, "w");
if (!fptr) {
/* Handle Error */
}
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The ISO/IEC 9899-1999 C standard does not provide a mechanism to determine if an existing file has been opened or a new file has been created. However, the {{open()}} function as defined in the Open Group Base Specifications Issue 6 \[[Open Group 04|AA. C References#Open Group 04]\] is available on many platforms and provides such a mechanism. If the {{O_CREAT}} and {{O_EXCL}} flags are used together, the {{open()}} function fails when the file specified by {{file_name}} already exists. |
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int fd = open(file_name, O_CREAT | O_EXCL | O_WRONLY, new_file_mode);
if (fd == -1) {
/* Handle Error */
}
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The function {{fdopen()}} \[[Open Group 04|AA. C References#Open Group 05]\] can be used in conjunction with {{open()}} to determine if a file is opened or created, and then associate a stream with the file descriptor. |
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FILE *fp;
int fd;
fd = open(file_name, O_CREAT | O_EXCL | O_WRONLY, new_file_mode);
if (fd == -1) {
/* Handle Error */
}
fp = fdopen(fd,"w");
if (fp == NULL) {
/* Handle Error */
}
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