How to ignore part of the file in c

  

I need to know how to ignore the first rows of a txt in C

If the rows are of variable length (as it seems to be your case) you can not ignore them: you should read them in full.

In C, the files are processed as a data stream: a read pointer is placed in a position and the reading pointer is advanced or retranslated by the contents of the file; It is possible to advance or retract fixed amounts of data (through fseek ) but if the data to be processed is not they have a fixed size the only way to distinguish some data from others is by reading them.

Proposal.

You should read the data file line by line:

char *buffer = NULL;
FILE *archivo = fopen("archivo.txt", "r");

if (archivo)
{
    for (size_t length; archivo && (getline(&buffer, &length, archivo) != -1); )
    {
        // 'buffer' contiene la línea leída.
    }

    fclose(archivo);
}

When the read line contains the data you are looking for, start processing, ignoring the rest, for example:

void procesa_datos(FILE *entrada, char **buffer)
{
    printf("Empezamos a procesar archivo...\n\n"
           "Datos a procesar: %s", *buffer);
    for (size_t length; entrada && (getline(buffer, &length, entrada) != -1); )
        printf("Datos a procesar: %s", *buffer);
}

int main()
{
    char *buffer = NULL;
    FILE *archivo = fopen("archivo.txt", "r");

    if (archivo)
    {
        for (size_t length; archivo && (getline(&buffer, &length, archivo) != -1); )
        {
            if (isalpha(*buffer)) // El primer carácter es una letra o un número?
                printf("Datos a ignorar: %s", buffer);
            else
                procesa_datos(archivo, &buffer);
        }

        fclose(archivo);
    }

    return 0;
}

I have used as a criterion that the first character of the line is a number, you must use the criterion that is convenient for you.